include ../_util-fns

.alert.is-important
  :marked
    The Component Router is in alpha release. This is the recommended Angular 2 router and supersedes
    the earlier *deprecated beta* and *v2* routers.

:marked
  The Angular ***Component Router*** enables navigation from one [view](./glossary.html#view) to the next
  as users perform application tasks.

  We cover the router's primary features in this chapter, illustrating them through the evolution
  of a small application that we can [run live](/resources/live-examples/router/ts/plnkr.html).
.l-sub-section
  img(src='/resources/images/devguide/plunker-separate-window-button.png' alt="pop out the window" align="right" style="margin-right:-20px")
  :marked
    To see the URL changes in the browser address bar,
    pop out the preview window by clicking the blue 'X' button in the upper right corner.

.l-main-section
:marked
  ## Overview

  The browser is a familiar model of application navigation.
  We enter a URL in the address bar and the browser navigates to a corresponding page.
  We click links on the page and the browser navigates to a new page.
  We click the browser's back and forward buttons and the browser navigates
  backward and forward through the history of pages we've seen.

  The Angular ***Component Router*** ("the router") borrows from this model.
  It can interpret a browser URL as an instruction
  to navigate to a client-generated view and pass optional parameters along to the supporting view component
  to help it decide what specific content to present.
  We can bind the router to links on a page and it will navigate to
  the appropriate application view when the user clicks a link.
  We can navigate imperatively when the user clicks a button, selects from a drop box,
  or in response to some other stimulus from any source. And the router logs activity
  in the browser's history journal so the back and forward buttons work as well.

  We'll learn many router details in this chapter which covers

  * Setting the [base href](#base-href)
  * Importing from the [router library](#import)
  * [configuring the router](#route-config)
  * the [link parameters array](#link-parameters-array) that propels router navigation
  * navigating when the user clicks a data-bound [RouterLink](#router-link)
  * navigating under [program control](#navigate)
  * embedding critical information in the URL with [route parameters](#route-parameters)
  * add [child routes](#child-routing-component) under a feature section
  * setting an [index route](#index) as the default
  * confirming or canceling navigation with [guards](#guards)
    * [CanActivate](#can-activate-guard) to prevent navigation to a route
    * [CanDeactivate](#can-deactivate-deactivate) to prevent navigation away from the current route
  * passing optional information in [query parameters](#query-parameters)
  * choosing the "HTML5" or "hash" [URL style](#browser-url-styles)

  We proceed in phases marked by milestones building from a simple two-pager with placeholder views
  up to a modular, multi-view design with child routes.

  But first, an overview of router basics.

.l-main-section
:marked
  ## The Basics
  Let's begin with a few core concepts of the Component Router.
  Then we can explore the details through a sequence of examples.

:marked
  ### *&lt;base href>*
  Most routing applications should add a `<base>` element to the **`index.html`** as the first child in the  `<head>` tag
  to tell the router how to compose navigation URLs.

  If the `app` folder is the application root, as it is for our sample application,
  set the `href` value *exactly* as shown here.
+makeExample('router/ts/index.1.html','base-href', 'index.html (base href)')(format=".")

:marked
  ### Router imports
  The Angular Component Router is an optional service that presents a particular component view for a given URL.
  It is not part of the Angular 2 core. It is in its own library package, `@angular/router`.
  We import what we need from it as we would from any other Angular package.

+makeExample('router/ts/app/app.component.1.ts','import-router', 'app/app.component.ts (import)')(format=".")
.l-sub-section
  :marked
    We cover other options in the [details below](#browser-url-styles).
:marked
  ### Configuration
  The application will have one *`router`*. When the browser's URL changes, the router looks for a corresponding **`Route`**
  from which it can determine the component to display.

  A router has no routes until we configure it.
  The preferred way is to bootstrap our application with an array of routes using the **`provideRouter`** function.

  In the following example, we configure our application with three route definitions.
+makeExample('router/ts/app/app.routes.1.ts','route-config','app/app.routes.ts')(format='.')

.l-sub-section
  :marked
    The `RouterConfig` is an array of *routes* that describe how to navigate.
    Each *Route* maps a URL `path` to a component.

    The `:id` in the third route is a token for a route parameter. In a URL such as `/hero/42`, "42"
    is the value of the `id` parameter. The corresponding `HeroDetailComponent`
    will use that value to find and present the hero whose `id` is 42.
    We'll learn more about route parameters later in this chapter.

    We pass the configuration array to the `provideRouter()` function which returns
    (among other things) a configured *Router* [service provider](dependency-injection.html#!#injector-providers).

    Finally, we export this provider in the `APP_ROUTER_PROVIDERS` array
    so we can simplify registration of router dependencies later in `main.ts`.
    We don't have any other providers to register right now. But we will.
:marked
  Next we open `main.ts` where we must register our router providers in the `bootstrap` method.
+makeExample('router/ts/app/main.ts','','app/main.ts')(format='.')
:marked
  ### Router Outlet
  Given this configuration, when the browser URL for this application becomes `/heroes`,
  the router matches that URL to the `Route` path `/heroes` and displays the `HeroListComponent`
  in a **`RouterOutlet`** that we've placed in the host view's HTML.
code-example(format="", language="html").
  &lt;!-- Routed views go here -->
  &lt;router-outlet>&lt;/router-outlet>
:marked
  ### Router Links
  Now we have routes configured and a place to render them, but
  how do we navigate? The URL could arrive directly from the browser address bar.
  But most of the time we navigate as a result of some user action such as the click of
  an anchor tag.

  We add a **`RouterLink`** directive to the anchor tag and bind it to a template expression that
  returns an array of route link parameters (the **link parameters array**). The router ultimately resolves that array
  into a URL and a component view.

  We see such bindings in the following `AppComponent` template:
+makeExample('router/ts/app/app.component.1.ts', 'template')(format=".")
.l-sub-section
 :marked
   We're adding two anchor tags with `RouterLink` directives.
   We bind each `RouterLink` to an array containing the path of a route.
   '/crisis-center' and '/heroes' are the paths of the `Routes` we configured above.

   We'll learn to write more complex link expressions &mdash; and why they are arrays &mdash;
   [later](#link-parameters-array) in the chapter.
:marked
  ### Let's summarize

  The application is provided with a configured router.
  The component has a `RouterOutlet` where it can display views produced by the router.
  It has `RouterLink`s that users can click to navigate via the router.

  Here are the key *Component Router* terms and their meanings:
table
  tr
    th Router Part
    th Meaning
  tr
    td <code>Router</code>
    td.
      Displays the application component for the active URL.
      Manages navigation from one component to the next.
  tr
    td <code>RouterConfig</code>
    td.
      Contains an array of Routes, each mapping a URL path to a component.
  tr
    td <code>Route</code>
    td.
      Defines how the router should navigate to a component based on a URL pattern. 
      Most routes consist of a path and a component type.
  tr
    td <code>RouterOutlet</code>
    td.
      The directive (<code>&lt;router-outlet></code>) that marks where the router should display a view.
  tr
    td <code>RouterLink</code>
    td.
      The directive for binding a clickable HTML element to
      a route. Clicking an anchor tag with a <code>routerLink</code> directive
      that is bound to a <i>Link Parameters Array</i> triggers a navigation.
  tr
    td <code><i>Link Parameters Array</i></code>
    td.
      An array that the router interprets into a routing instruction.
      We can bind a <code>RouterLink</code> to that array or pass the array as an argument to
      the <code>Router.navigate</code> method.
  tr
    td <code><i>Routing Component</i></code>
    td.
      An Angular component with a *RouterOutlet* that displays views based on router navigations.
:marked
  We've barely touched the surface of the router and its capabilities.

  The following detail sections describe a sample routing application
  as it evolves over a sequence of milestones.
  We strongly recommend taking the time to read and understand this story.

.l-main-section
:marked
  ## The Sample Application
  We have an application in mind as we move from milestone to milestone.

.l-sub-section
  :marked
    While we make incremental progress toward the ultimate sample application, this chapter is not a tutorial.
    We discuss code and design decisions pertinent to routing and application design.
    We gloss over everything in between.

    The full source is available in the [live example](/resources/live-examples/router/ts/plnkr.html).
:marked
  Our client is the Hero Employment Agency.
  Heroes need work and The Agency finds Crises for them to solve.

  The application has two main feature areas:
  1. A *Crisis Center* where we maintain the list of crises for assignment to heroes.
  1. A *Heroes* area where we maintain the list of heroes employed by The Agency.

  Run the [live example](/resources/live-examples/router/ts/plnkr.html).
  It opens in the *Crisis Center*.  We'll come back to that.

  Click the *Heroes* link. We're presented with a list of Heroes.
figure.image-display
  img(src='/resources/images/devguide/router/hero-list.png' alt="Hero List" width="250")
:marked
  We select one and the application takes us to a hero editing screen.
figure.image-display
  img(src='/resources/images/devguide/router/hero-detail.png' alt="Crisis Center Detail" width="250")
:marked
  Our changes take effect immediately. We click the "Back" button and the
  app returns us to the Heroes list.

  We could have clicked the browser's back button instead.
  That would have returned us to the Heroes List as well.
  Angular app navigation updates the browser history as normal web navigation does.

  Now click the *Crisis Center* link. We go to the *Crisis Center* and its list of ongoing crises.
figure.image-display
  img(src='/resources/images/devguide/router/crisis-center-list.png' alt="Crisis Center List" )
:marked
  We select one and the application takes us to a crisis editing screen.
figure.image-display
  img(src='/resources/images/devguide/router/crisis-center-detail.png' alt="Crisis Center Detail")
:marked
   This is a bit different from the *Hero Detail*. *Hero Detail* saves the changes as we type.
   In *Crisis Detail* our changes are temporary until we either save or discard them by pressing the "Save" or "Cancel" buttons.
   Both buttons navigate back to the *Crisis Center* and its list of crises.

   Suppose we click a crisis, make a change, but ***do not click either button***.
   Maybe we click the browser back button instead. Maybe we click the "Heroes" link.

   Do either. Up pops a dialog box.
figure.image-display
  img(src='/resources/images/devguide/router/confirm-dialog.png' alt="Confirm Dialog" width="300")
:marked
  We can say "OK" and lose our changes or click "Cancel" and continue editing.

  The router supports a `CanDeactivate` guard that gives us a chance to clean-up
  or ask the user's permission before navigating away from the current view.

  Here we see an entire user session that touches all of these features.
<a id="full-app-demo"></a>
figure.image-display
  img(src='/resources/images/devguide/router/router-anim.gif' alt="App in action" )
:marked
  Here's a diagram of all application routing options:
figure.image-display
  img(src='/resources/images/devguide/router/complete-nav.png' alt="Navigation diagram" )
:marked
  This app illustrates the router features we'll cover in this chapter

  * navigating to a component (*Heroes* link to "Heroes List")
  * including a route parameter (passing the Hero `id` while routing to the "Hero Detail")
  * child routes (the *Crisis Center* has its own routes)
  * the `CanActivate` guard (checking route access)
  * the `CanDeactivate` guard (ask permission to discard unsaved changes)

<a id="getting-started"></a>
.l-main-section
:marked
  ## Milestone #1: Getting Started with the Router

  Let's begin with a simple version of the app that navigates between two empty views.
figure.image-display
  img(src='/resources/images/devguide/router/router-1-anim.gif' alt="App in action" )

<a id="base-href"></a>
:marked
  <a id="base-href"></a>
  ### Set the *&lt;base href>*
  The Component Router uses the browser's
  [history.pushState](https://developer.mozilla.org/en-US/docs/Web/API/History_API#Adding_and_modifying_history_entries)
  for navigation. Thanks to `pushState`, we can make our in-app URL paths look the way we want them to
  look, e.g. `localhost:3000/crisis-center`. Our in-app URLs can be indistinguishable from server URLs.

  Modern HTML 5 browsers were the first to support `pushState` which is why many people refer to these URLs as
  "HTML 5 style" URLs.

  We must **add a [&lt;base href> element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base) tag**
  to the `index.html` to make `pushState` routing work.
  The browser also needs the base `href` value to prefix *relative* URLs when downloading and linking to
  css files, scripts, and images.

  Add the base element just after the  `<head>` tag.
  If the `app` folder is the application root, as it is for our application,
  set the `href` value in **`index.html`** *exactly* as shown here.

+makeExample('router/ts/index.1.html','base-href', 'index.html (base href)')(format=".")
.l-sub-section
  :marked
    HTML 5 style navigation is the Component Router default.
    Learn why "HTML 5" style is preferred, how to adjust its behavior, and how to switch to the
    older hash (#) style if necessary in the [Browser URL Styles](#browser-url-styles) appendix below.

:marked
.l-sub-section
  :marked
    #### Live example note
    We have to get tricky when we run the live example because the host service sets
    the application base address dynamically. That's why we replace the `<base href...>` with a
    script that writes a `<base>` tag on the fly to match.
  code-example(format="")
    &lt;script>document.write('&lt;base href="' + document.location + '" />');&lt;/script>
  :marked
    We should only need this trick for the live example, not production code.

:marked
  ### Configure the routes for the Router
  We teach our router how to navigate by configuring it with routes.
  We recommend creating a separate `app.routes.ts` file dedicated to this purpose.
.l-sub-section
  :marked
    Defining configuration in a separate file paves the way for a future
    in which we load routing configuration immediately but *delay
    loading the components themselves* until the user needs them.

    Such *asynchronous routing* can make our application launch more quickly.
    We'll cover asynchronous routing in a future chapter update.
:marked
  Here is our first configuration.

+makeExample('router/ts/app/app.routes.2.ts','', 'app/app.routes.ts')(format=".")

h4#import Import from the Component Router library
:marked
  We begin by importing some symbols from the router library. 
  
  The Component Router is in its own `@angular/router` package.
  It's not part of the Angular 2 core. 
  The router is an optional service because not all applications need routing and,
  depending on your requirements, you may need a different routing library.

a#route-config
h4#define-routes Define routes
:marked
  A router must be configured with a list of route definitions.

  Our first configuration defines an array of two routes with simple paths leading to the
  `CrisisListComponent` and `HeroListComponent` components.

  Each definition translates to a [Route](../api/router/index/Route-class.html) object which has a
  `path`, the URL path segment for this route, and a
  `component`, the component associated with this route.

  The router draws upon its registry of such route definitions when the browser URL changes
  or when our code tells the router to navigate along a route path.

  In plain English, we might say of the first route:
  * *When the browser's location URL changes to match the path segment `/crisis-center`, create or retrieve an instance of
  the `CrisisListComponent` and display its view.*

  * *When the application requests navigation to the path `/crisis-center`, create or retrieve an instance of
  the `CrisisListComponent`, display its view, and update the browser's address location and history with the URL
  for that path.* 

h4#provideRouter Call <i>provideRouter</i>
:marked
  We pass the route configuration to the `provideRouter` function which returns an array containing the configured 
  `Router` service provider ... and some other, unseen providers that the routing library requires.

:marked
  We add the `provideRouter` array to an `APP_ROUTER_PROVIDERS` array and export it.

  We could add *additional* service providers to `APP_ROUTER_PROVIDERS` &mdash; 
  providers that are specific to our routing configuration.
  We don't have any yet. We will have some later in this chapter.

.l-sub-section
  :marked
    Learn about *providers* in the [Dependency Injection](dependency-injection.html#!#injector-providers) chapter.

h4#register-providers Register routing in bootstrap
:marked
  Our app launches from the `main.ts` file in the `/app` folder.
  It's short and not much different from the default `main.ts`.

  The important difference: we import the `APP_ROUTER_PROVIDERS` array
  and pass it as the second parameter of the `bootstrap` function.
+makeExample('router/ts/app/main.1.ts','all', 'main.ts')(format=".")
:marked
  Providing the router providers at the root makes the Router available everywhere in our application.
.alert.is-important
  :marked
    We must register router providers in `bootstrap`. 
    We cannot wait to do it in `AppComponent`.

h3#shell The <i>AppComponent</i> shell
:marked
  The root `AppComponent` is the application shell. It has a title at the top, a navigation bar with two links,
  and a *Router Outlet* at the bottom where the router swaps views on and off the page. Here's what we mean:
figure.image-display
  img(src='/resources/images/devguide/router/shell-and-outlet.png' alt="Shell" width="300" )

a#shell-template
:marked
  The corresponding component template looks like this:
+makeExample('router/ts/app/app.component.1.ts','template')(format=".")

h3#router-outlet <i>RouterOutlet</i>
:marked
  `RouterOutlet` is a component from the router library.
  The router displays views within the bounds of the `<router-outlet>` tags.

.l-sub-section
  :marked
    A template may hold exactly one ***unnamed*** `<router-outlet>`.
    The router supports multiple *named* outlets, a feature we'll cover in future. 

h3#router-link <i>RouterLink</i> binding
:marked
  Above the outlet, within the anchor tags, we see [Property Bindings](template-syntax.html#property-binding) to
  the `RouterLink` directive that look like `[routerLink]="[...]"`. We imported `RouterLink` from the router library.

  The template expression to the right of the equals (=) returns a *link parameters array*.

  A link parameters array holds the ingredients for router navigation:
  * the *path* of the route to the destination component 
  * optional route and query parameters that go into the route URL

  The arrays in this example each have a single string parameter, the path of a route that
  we configured earlier. We don't have route parameters yet.
.l-sub-section
  :marked
    Learn more about the link parameters array in the [appendix below](#link-parameters-array).

h3#router-directives <i>ROUTER_DIRECTIVES</i>
:marked
  `RouterLink` and `RouterOutlet` are directives in the `ROUTER_DIRECTIVES` collection. 
  Remember to add them to the `directives` array of the `@Component` metadata.
+makeExample('router/ts/app/app.component.1.ts','directives')(format=".")
:marked
  The current state of `app.component.ts` looks like this:
+makeExample('router/ts/app/app.component.1.ts','', 'app/app.component.ts')(format=".")

:marked
  ### "Getting Started" wrap-up

  We've got a very basic, navigating app, one that can switch between two views
  when the user clicks a link.

  We've learned how to
  * load the router library
  * add a nav bar to the shell template with anchor tags and `routerLink` directives
  * added a `router-outlet` to the shell template where views will be displayed
  * configure the router with `provideRouter`
  * set the router to compose "HTML 5" browser URLs.

  The rest of the starter app is mundane, with little interest from a router perspective.
  Here are the details for readers inclined to build the sample through to this milestone.

  Our starter app's structure looks like this:
.filetree
  .file router-sample
  .children
    .file app
      .children
        .file app.component.ts
        .file app.routes.ts
        .file crisis-list.component.ts
        .file hero-list.component.ts
        .file main.ts
    .file node_modules ...
    .file typings ...
    .file index.html
    .file package.json
    .file styles.css
    .file tsconfig.json
    .file typings.json
:marked
  Here are the files discussed in this milestone
+makeTabs(
  `router/ts/app/app.component.1.ts,
  router/ts/app/app.routes.2.ts,
  router/ts/app/main.1.ts,
  router/ts/app/hero-list.component.ts,
  router/ts/app/crisis-list.component.ts,
  router/ts/index.html`,
  ',,all,,',
  `app.component.ts,
  app.routes.ts,
  main.ts,
  hero-list.component.ts,
  crisis-list.component.ts,
  index.html`)

h2#heroes-feature Milestone #2: The Heroes Feature
.l-main-section
:marked
  We've seen how to navigate using the `RouterLink` directive.

  Now we'll learn some new tricks such as how to
  * organize our app into *feature areas*
  * navigate imperatively from one component to another
  * pass information in route parameters

  To demonstrate, we'll build out the *Heroes* feature.

  ### The Heroes "feature area"

  A typical application has multiple *feature areas*, each an island of functionality
  with its own workflow(s), dedicated to a particular business purpose.

  We could continue to add files to the `app/` folder.
  That's unrealistic and ultimately not maintainable.
  We think it's better to put each feature area in its own folder.

  Our first step is to **create a separate `app/heroes/` folder**
  and add *Hero Management* feature files there.

  We won't be creative about it. Our example is pretty much a
  copy of the code and capabilities in the "[Tutorial: Tour of Heroes](../tutorial/index.html)".

  Here's how the user will experience this version of the app
figure.image-display
  img(src='/resources/images/devguide/router/router-2-anim.gif' alt="App in action" )
:marked
  ### Add Heroes functionality

  We delete the placeholder `hero-list.component.ts` that's in
  the `app/` folder.

  We create a new `hero-list.component.ts` in the `app/heroes/`
  folder and copy over the contents of the final `heroes.component.ts` from the tutorial.
  We copy the `hero-detail.component.ts` and the `hero.service.ts` files
  into the `heroes/` folder.

  When we're done organizing, we have three *Hero Management* files:

.filetree
  .file app/heroes
  .children
    .file hero-detail.component.ts
    .file hero-list.component.ts
    .file hero.service.ts

:marked
  We provide the `HeroService` in the application root `AppComponent`
  so that it is available everywhere in the app.

  Now it's time for some surgery to bring these files and the rest of the app
  into alignment with our application router.

  ### *Hero* feature routing requirements

  The new Heroes feature has two interacting components, the list and the detail.
  The list view is self-sufficient; we navigate to it, it gets a list of heroes and displays them.
  It doesn't need any outside information.

  The detail view is different. It displays a particular hero. It can't know which hero on its own.
  That information must come from outside.

  In our example, when the user selects a hero from the list, we navigate to the detail view to show that hero.
  We'll tell the detail view which hero to display by including the selected hero's id in the route URL.

  ### *Hero* feature route configuration

  We recommend giving each feature area its own route configuration file. 
  
  Create a new `hero.routes.ts` in the `heroes` folder like this:
+makeExample('router/ts/app/heroes/heroes.routes.ts','', 'app/heroes/heroes.routes.ts')(format=".")
:marked
  We use the same techniques we learned for `app.routes.ts`.

  We import the two components from their new locations in the `app/heroes/` folder, define the two hero routes.
  and add them to an exported `HeroesRoutes` array.

  ### Route definition with a parameter
  The route to `HeroDetailComponent` has a twist.
+makeExample('router/ts/app/heroes/heroes.routes.ts','hero-detail-route')(format=".")
:marked
  Notice the `:id` token in the path. That creates a slot in the path for a **Route Parameter**.
  In this case, we're expecting the router to insert the `id` of a hero into that slot.

  If we tell the router to navigate to the detail component and display "Magneta", we expect hero `id` (15) to appear in the
  browser URL like this:
code-example(format="." language="bash").
  localhost:3000/hero/15
:marked
  If a user enters that URL into the browser address bar, the router should recognize the
  pattern and go to the same "Magneta" detail view.
.l-sub-section
  :marked
    #### Route parameter or query parameter?
    Embedding the route parameter token, `:id`, in the route definition path is a good choice for our scenario
    because the `id` is *required* by the `HeroDetailComponent` and because
    the value `15` in the path clearly distinguishes the route to "Magneta" from
    a route for some other hero.

    A [query parameter](#query-parameter) might be a better choice if we were passing an *optional* value to `HeroDetailComponent`.

h3#merge-hero-routes Merge hero routes into application routes
:marked
  Our application doesn't know about our hero routes yet. 
  We'll need to merge them into the application routes we defined in `app.routes.ts`.

  Update `app.routes.ts` as follows:
+makeExample('router/ts/app/app.routes.3.ts', '', 'app/app.routes.ts (v.2)')(format=".")
:marked
  We replace the `HeroListComponent` import with an `HeroesRoutes` import.

  We *flatten* the `HeroesRoutes` into the `routes` array with the ES6 *spread operator* (`...`).

  As a result, the `app.routes.ts` no longer has specific knowledge of the hero feature, its components, or its route details.
  It won't change as we evolve the hero feature with more components and different routes.
  That's a key benefit of creating a separate route configuration for each feature area.

h3#navigate Navigate to hero detail imperatively
:marked
  *We won't navigate to the detail component by clicking a link* 
  so we won't be adding a new `RouterLink` anchor tag to the shell.

  Instead, when the user *clicks* a hero in the list, we'll *command* the router
  to navigate to the hero detail view for the selected hero.

  We'll adjust the `HeroListComponent` to implement these tasks, beginning with its constructor
  which acquires the router service and the `HeroService` by dependency injection:
+makeExample('router/ts/app/heroes/hero-list.component.1.ts','ctor', 'app/heroes/hero-list.component.ts (Constructor)')(format=".")
:marked
  We make a few changes to the template:
+makeExample('router/ts/app/heroes/hero-list.component.1.ts','template')(format=".")
:marked
  The template defines an `*ngFor` repeater such as [we've seen before](displaying-data.html#ngFor).
  There's a `(click)` [EventBinding](template-syntax.html#event-binding) to the component's `onSelect` method
  which we implement as follows:
+makeExample('router/ts/app/heroes/hero-list.component.1.ts','select')(format=".")
:marked
  It calls the router's **`navigate`** method with a **Link Parameters Array**.
  This array is similar to the  *link parameters array* we met [earlier](#shell-template) in an anchor tag while
  binding to the `RouterLink` directive. This time we see it in code rather than in HTML.

h3#route-parameters Setting the route parameters in the list view
:marked
  We're navigating to the `HeroDetailComponent` where we expect to see the details of the selected hero.
  We'll need *two* pieces of information: the destination and the hero's `id`.

  Accordingly, the *link parameters array* has *two* items:  the **path** of the destination route and a **route parameter** that specifies the
  `id` of the selected hero.
+makeExample('router/ts/app/heroes/hero-list.component.1.ts','link-parameters-array')(format=".")
:marked
  The router composes the appropriate two-part destination URL from this array:
code-example(format="." language="bash").
  localhost:3000/hero/15
h3#get-route-parameter Getting the route parameter in the details view
:marked
  How does the target `HeroDetailComponent` learn about that `id`?
  Certainly not by analyzing the URL!  That's the router's job.

  The router extracts the route parameter (`id:15`) from the URL and supplies it to
  the `HeroDetailComponent` via the **ActivatedRoute** service.

a#hero-detail-ctor
:marked
  As usual, we write a constructor that asks Angular to inject services
  that the component requires and reference them as private variables.
+makeExample('router/ts/app/heroes/hero-detail.component.ts','ctor', 'app/heroes/hero-detail.component.ts (Constructor)')(format=".")
:marked
  Later, in the `ngOnInit` method,
  we use the `ActivatedRoute` service to retrieve the parameters for our route.
  Since our parameters are provided as an `Observable`, we _subscribe_ to them for the `id` parameter by name and
  tell the `HeroService` to fetch the hero with that `id`. We'll keep a reference to this `Subscription` so we can
  tidy things up later.
+makeExample('router/ts/app/heroes/hero-detail.component.ts','ngOnInit')(format=".")
.l-sub-section
  :marked
    Angular calls the `ngOnInit` method shortly after creating an instance of the `HeroDetailComponent`.

    We put the data access logic in the `ngOnInit` method rather than inside the constructor
    to improve the component's testability.
    We explore this point in greater detail in the [OnInit appendix](#onInit) below.

:marked
  Eventually, we'll navigate somewhere else.
  The router will remove this component from the DOM and destroy it.
  We need to clean up after ourselves before that happens.
  Specifically, we **must unsubscribe** before Angular destroys the component.
  *Failure to do so could create a memory leak.*

  We unsubscribe from our `Observable` in the `ngOnDestroy` method.
+makeExample('router/ts/app/heroes/hero-detail.component.ts','ngOnDestroy')(format=".")

.l-sub-section
  :marked
    Learn about the `ngOnInit` and `ngOnDestroy` methods in the 
    [Lifecycle Hooks](lifecycle-hooks.html) chapter.

h4#reuse Observable <i>params</i> and component re-use
:marked
  In this example, we subscribe to the route params `Observable`.
  That implies that the route params can change during the lifetime of this component.

  They might. By default, the router reuses a component instance when it re-navigates to the same component type 
  without visiting a different component first. The parameters can change between each re-use.

  Suppose a parent component navigation bar had "forward" and "back" buttons 
  that scrolled through the list of heroes. 
  Each click navigated imperatively to the `HeroDetailComponent` with the next or previous `id`.

  We don't want the router to remove the current `HeroDetailComponent` instance from the
  DOM only to re-create it for the next `id`.
  That could be visibly jarring.
  Better to simply re-use the same component instance and update the parameter.

  But `ngOnInit` is only called once per instantiation. 
  We need a way to detect when the route parameters change from _within the same instance_.
  The observable `params` property handles that beautifully.

h4#snapshot <i>Snapshot</i>: the no-observable alternative
:marked
  This application won't reuse the `HeroDetailComponent`. 
  We always return to the hero list to select another hero to view.
  There's no way to navigate from hero detail to hero detail
  without visiting the list component in between.
  That means we get a new `HeroDetailComponent` instance every time.

  Suppose we know for certain that `HeroDetailComponent` will *never, never, ever*
  be re-used. We'll always re-create the component each time we navigate to it.

  The router offers a *Snapshot* alternative that gives us the initial value of the route parameters.
  We don't need to subscribe. We don't have to unsubscribe in `ngDestroy`.
  It's much simpler to write and read:
+makeExample('router/ts/app/heroes/hero-detail.component.2.ts','snapshot')(format=".")
.l-sub-section
  :marked
    **Remember:** we only get the _initial_ value of the parameters with this technique.
    Stick with the observable `params` approach if there's even a chance that we might navigate 
    to this component multiple times in a row. 
    We are leaving the observable `params` strategy in place just in case.

h3#nav-to-list Navigating back to the list component
:marked
  The `HeroDetailComponent` has a "Back" button wired to its `gotoHeroes` method that navigates imperatively
  back to the `HeroListComponent`.

  The router `navigate` method takes the same one-item *link parameters array*
  that we bound to the application shell's *Heroes* `[routerLink]` directive.
  It holds the **path to the `HeroListComponent`**:
+makeExample('router/ts/app/heroes/hero-detail.component.1.ts','gotoHeroes')(format=".")
:marked
  ### Heroes App Wrap-up

  We've reached the second milestone in our router education.

  We've learned how to
  * organize our app into *feature areas*
  * navigate imperatively from one component to another
  * pass information along in route parameters and subscribe to them in our component

  After these changes, the folder structure looks like this:
.filetree
  .file router-sample
  .children
    .file app
    .children
      .file heroes
      .children
        .file hero-detail.component.ts
        .file hero-list.component.ts
        .file hero.service.ts
        .file heroes.routes.ts
      .file app.component.ts
      .file app.routes.ts
      .file crisis-list.component.ts
      .file main.ts
    .file node_modules ...
    .file typings ...
    .file index.html
    .file package.json
    .file styles.css
    .file tsconfig.json
    .file typings.json
:marked
  <a id="heroes-app-code"></a>
  ### The Heroes App code
  Here are the relevant files for this version of the sample application.
+makeTabs(
  `router/ts/app/app.component.1.ts,
   router/ts/app/app.routes.3.ts,
   router/ts/app/heroes/hero-list.component.1.ts,
   router/ts/app/heroes/hero-detail.component.1.ts,
   router/ts/app/heroes/hero.service.ts,
   router/ts/app/heroes/heroes.routes.ts`,
   null,
  `app.component.ts,
  app.routes.ts,
  hero-list.component.ts,
  hero-detail.component.ts,
  hero.service.ts,
  heroes.routes.ts`)
:marked

<a id="crisis-center-feature"></a>
.l-main-section
:marked
  ## Milestone #3: The Crisis Center
  The *Crisis Center* is a fake view at the moment. Time to make it useful.

  The new *Crisis Center* begins as a virtual copy of the *Heroes* feature.
  We create a new `app/crisis-center` folder, copy the Hero files,
  and change every mention of "hero" to "crisis".

  A `Crisis` has an `id` and `name`, just like a `Hero`
  The new `CrisisListComponent` displays lists of crises.
  When the user selects a crisis, the app navigates to the `CrisisDetailComponent`
  for display and editing of the crisis name.

  Voilà, instant feature module!

  There's no point to this exercise unless we can learn something.
  We do have new ideas and techniques in mind:

  * We'd like our route URLs to branch in to child route trees that reflect the component treese in our feature areas.

  * The application should navigate to the *Crisis Center* by default.

  * The router should prevent navigation away from the detail view while there are pending changes.

  * The user should be able to cancel unwanted changes.

  * The router should block access to certain features until the user logs-in.

  * Our `CrisisService` is only needed within the *Crisis Center* feature area.
  We should limit access to it to that feature area.

  * Changes to a sub-module such as *Crisis Center* shouldn't provoke changes to the `AppComponent` or 
  any other feature's component.
  We need to [*separate our concerns*](https://blog.8thlight.com/uncle-bob/2014/05/08/SingleReponsibilityPrinciple.html).

  We'll address all of these issues in the *Crisis Center*
  starting with the introduction of **child routes**

.l-sub-section
  :marked
    We'll leave *Heroes* in its less-than-perfect state to
    serve as a contrast with what we believe to be a superior *Crisis Center* design.

:marked
  ### A Crisis Center with child routes

  We'll organize the *Crisis Center* to conform to the following recommended pattern for Angular applications.
  * each feature area in its own folder
  * each area with its own area root component
  * each area root component with its own router-outlet and child routes
  * area routes rarely (if ever) cross
  
  If we had many feature areas, their component trees might look like this:

figure.image-display
  img(src='/resources/images/devguide/router/component-tree.png' alt="Component Tree" )

h3#child-routing-component Child Routing Component
:marked
  Add the following `crisis-center.component.ts` to the `crisis-center` folder:
+makeExample('router/ts/app/crisis-center/crisis-center.component.1.ts', 'minus-imports', 'crisis-center/crisis-center.component.ts (minus imports)')(format='.')
:marked
  The `CrisisCenterComponent` is much like the `AppComponent` shell.

  * It is the root of the *Crisis Center* area
  just as `AppComponent` is the root of the entire application.

  * It is a shell for the crisis management feature area
  just as the `AppComponent` is a shell to manage the high-level workflow.

  * It is dead simple &mdash; simpler even than the `AppComponent` template.
  It has no content, no links, just a `<router-outlet>` for the *Crisis Center* child views.

  Unlike `AppComponent` (and most other components), it **lacks a selector**. 
  It doesn't need one. We don't *embed* this component in a parent template. 
  We *navigate* to it from the outside, via the router.
.l-sub-section
  :marked
    We *can* give it a selector. There's no harm in it.
    Our point is that we don't *need* one because we only *navigate* to it.

:marked
  ### Service isolation

  The`CrisisService` is neither needed nor wanted outside the *Crisis Center* domain.
  Instead of registering it with the root `AppComponent` providers &mdash;
  which makes it visible everywhere &mdash;
  we register the `CrisisService` in the component's providers array.
+makeExample('router/ts/app/crisis-center/crisis-center.component.1.ts', 'providers')(format='.')
:marked
  This limits the scope of the `CrisisService` to the *Crisis Center* component and its sub-component tree.
  No component outside of the *Crisis Center* can access it.

  There's a practical benefit to restricting its scope in this way.

  First we can evolve the service independently of the rest of the application
  without fear of breaking what should be unrelated modules.
  
  Second, we can delay loading this service into memory until we need it.
  We can remove it from the application launch bundle, 
  reducing the size of the initial payload and improving performance.
  We can load it optionally, asynchronously with the other *Crisis Center* components
  if and when the user begins that workflow.

.l-sub-section
  :marked
    We'll describe asynchronous module loading in a future update.
:marked
  ### Child Route Configuration
  The `CrisisCenterComponent` is a *Routing Component* like the `AppComponent`.
  It has its own `RouterOutlet` and its own child routes.

  We create a `crisis-center.routes.ts` file as we did the `heroes.routes.ts` file.
  But this time we define **child routes** *within* the parent `/crisis-center` route.
+makeExample('router/ts/app/crisis-center/crisis-center.routes.1.ts', 'routes', 'app/crisis-center/crisis-center.routes.ts (Routes)' )(format='.')  
:marked
  Notice that the parent `/crisis-center` route has a `children` property
  with an array of two routes.
  These two routes navigate to the two *Crisis Center* child components, 
  `CrisisListComponent` and `CrisisDetailComponent`.

  There are some *important differences* in the treatment of these routes.

  First, the router displays the components of these child routes in the `RouterOutlet`
  of the `CrisisCenterComponent`, not in the `RouterOutlet` of the `AppComponent` shell.

  Second, the child paths *extend* the path of their parent route.

  Normally paths that begin with `/` refer to the root of the application.
  Here they are appended to the path to the `CrisisCenterComponent`. 
  
  To write an URL that navigates to the `CrisisListComponent`, we'd append its child route path,  `/`,
  to `/crisis-center`.

  To write an URL that navigates to the `CrisisDetailComponent`, we'd append the child route path,  `/`,
  followed by the crisis id, yielding something like:

code-example(format="").
  localhost:3000/crisis-center/2

:marked
  Here's the complete `crisis-center.routes.ts` with its imports.
+makeExample('router/ts/app/crisis-center/crisis-center.routes.1.ts', '', 'app/crisis-center/crisis-center.routes.ts' )(format='.')  

h3#merge-crisis-routes Merge crisis routes into the application routes
:marked
  As with hero routes, we must update the router configuration at the top of the application
  by merging the crisis routes into the app routes:
+makeExample('router/ts/app/app.routes.4.ts', '', 'app/app.routes.ts' )(format='.')
:marked
  We used the spread operator again (...) to insert the crisis routes array. 

a#index
h3#default-route Setting default routes
:marked
  When the application launches, the initial URL in the browser bar is something like:
code-example(format="").
  localhost:3000
:marked
  That doesn't match any of our configured routes which means that our application won't display any component when it's launched.
  The user must click one of the navigation links to trigger a navigation and display something.

  We want the application to display the list of crises as it would if we pasted `localhost:3000/crisis-center/` into the address bar.
  This is our *default* route.

  We can arrange for that behavior in several ways. 
  One way is to add `index: true` to each route on the path to the default component.

  In our example, we'll add `index: true` to two routes:
  1. The parent route for the `CrisisCenterComponent`
  1. The child route for the `CrisisListComponent`

  The updated route definitions look like this:
+makeExample('router/ts/app/crisis-center/crisis-center.routes.2.ts', 'routes', 'app/crisis-center/crisis-center.routes.ts (Routes v.2)' )(format='.')

.l-main-section
h2#guards Route Guards
:marked
  At the moment, *any* user can navigate *anywhere* in the application *anytime*.

  That's not always the right thing to do.
  * Perhaps the user is not authorized to navigate to the target component.
  * Maybe the user must login (*authenticate*) first.
  * Maybe we should fetch some data before we display the target component.
  * We might want to save pending changes before leaving a component.
  * We might ask the user if it's OK to discard pending changes rather than save them.

  We can add ***guards*** to our route configuration to handle these scenarios.

  A guard's return value controls the router's behavior:
  * if it returns `true`, the navigation process continues
  * if it returns `false`, the navigation process stops and the user stays put
.l-sub-section
  :marked
    The guard can also tell the router to navigate elsewhere, effectively canceling the current navigation.
:marked
  The guard *might* return its boolean answer synchronously. 
  But in many cases, the guard can't produce an answer synchronously.
  The guard could ask the user a question, save changes to the server, or fetch fresh data.
  These are all asynchronous operations.

  Accordingly, a routing guard can return an `Observable<boolean>` and the
  router will wait for the observable to resolve to `true` or `false. 

  The router supports two kinds of guards:
  
  1. [CanActivate](../api/router/index/CanActivate-interface.html) to mediate navigation *to* a route. 
  
  2. [CanDeactivate](../api/router/index/CanDeactivate-interface.html) to mediate navigation *away* from the current route.

.l-sub-section
  :marked
    We'll examine other router guards in a future update to this chapter.
:marked
  We can have multiple guards at every level of a routing hierarchy.
  The router checks the `CanDeactive` guards first, from deepest child route to the top.
  Then it checks the `CanActivate` guards from the top down to the deepest child route.
  If _any_ guard returns false, pending guards that have not completed will be canceled, 
  and the entire navigation is canceled.

  Let's look at some examples.

.l-main-section
// :marked
  <a id="lifecycle-hooks"></a>
  ## Router Lifecycle Hooks

  TODO: Pausing activation

h3#can-activate-guard <i>CanActivate</i>: requiring authentication
:marked
  Applications often restrict access to a feature area based on who the user is.
  We could permit access only to authenticated users or to users with a specific role.
  We might block or limit access until the user's account is activated.

  The `CanActivate` guard is the tool to manage these navigation business rules.

  #### Add a crisis admin feature

  We intend to extend the Crisis Center with some new *administrative* features.
  Those features aren't defined yet. So we add the following placeholder component.

+makeExample('router/ts/app/crisis-center/crisis-admin.component.ts', '', 'crisis-admin.component.ts')(format=".")
:marked
  Next, we add a child route to the `crisis-center.routes` with the path, `/admin`.
+makeExample('router/ts/app/crisis-center/crisis-center.routes.3.ts', 'admin-route-no-guard', 'crisis-center.routes.ts (admin route)')(format=".")
:marked
  And we add a link to the `AppComponent` shell that users can click to get to this feature.
+makeExample('router/ts/app/app.component.4.ts', 'template', 'app/app.component.ts (template)')(format=".")  
:marked
  #### Guard the admin feature
  Currently every route within our *Crisis Center* is open to everyone. 
  The new *admin* feature should be accessible only to authenticated users.

  We could hide the link until the user logs in. But that's tricky and difficult to maintain. 

  Instead we'll write a `CanActivate` guard to redirect anonymous users to the login page when they try to reach the admin component.
  
  This is a general purpose guard &mdash; we can imagine other features that require authenticated users &mdash;
  so we create an `auth.guard.ts` in the application root folder.  
  
  At the moment we're interested in seeing how guards work so our first version does nothing useful.
  It simply logs to console and `returns` true immediately, allowing navigation to proceed:
+makeExample('router/ts/app/auth.guard.1.ts', '', 'app/auth.guard.ts')(format=".")  
:marked
  Next we open `crisis-center.routes.ts `, import the `AuthGuard` class, and 
  update the admin route with a `CanActivate` guard property that references it:
+makeExample('router/ts/app/crisis-center/crisis-center.routes.ts', 'admin-route', 'crisis-center.routes.ts (guarded admin route)')(format=".")
  Our admin feature is now protected by the guard, albeit protected poorly. 
:marked
  #### Teach *AuthGuard* to authenticate
  Let's make our `AuthGuard` at least pretend to authenticate.

  The `AuthGuard` should call an application service that can login a user and retain information about the current user.
  Here's a demo `AuthService`:
+makeExample('router/ts/app/auth.service.ts', '', 'app/auth.service.ts')(format=".")
:marked
  Although it doesn't actually log in, it has what we need for this discussion. 
  It has an `isLoggedIn` flag to tell us whether the user is authenticated.
  Its `login` method simulates an API call to an external service by returning an observable that resolves successfully after a short pause.

  Let's revise our `AuthGuard` to call it.
+makeExample('router/ts/app/auth.guard.ts', '', 'app/auth.guard.ts (v.2)')(format=".")
:marked
  Notice that we *inject* the `AuthService` and the `Router` in the constructor. 
  We haven't provided the `AuthService` yet but it's good to know that we can inject helpful services into our routing guards.

  This guard returns a synchronous boolean result. 
  If the user is logged in, it returns true and the navigation continues.

  If the user is not logged in, we tell the router to navigate to a login page &mdash; a page we haven't created yet.
  This secondary navigation automatically  cancels the current navigation; we return `false` just to be clear about that.

  #### Add the *LoginComponent*
  We need a `LoginComponent` for the user to log in to the app. 
  There is nothing new about this component or the way we wire it into the router configuration.
  Here is the pertinent code, offered without comment:
+makeTabs(
   `router/ts/app/login.component.ts,
   router/ts/app/login.routes.ts,
   router/ts/app/app.routes.5.ts
  `,
  null,
   `app/login.component.ts,
   app/login.routes.ts,
   app/app.routes.ts
  `)

h3#can-deactivate-guard <i>CanDeactivate</i>: handling unsaved changes
:marked
  Back in the "Heroes" workflow, the app accepts every change to a hero immediately without hesitation or validation.

  In the real world, we might have to accumulate the users changes.
  We might have to validate across fields. We might have to validate on the server.
  We might have to hold changes in a pending state until the user confirms them *as a group* or
  cancels and reverts all changes.

  What do we do about unapproved, unsaved changes when the user navigates away?
  We can't just leave and risk losing the user's changes; that would be a terrible experience.

  We'd like to pause and let the user decide what to do.
  If the user cancels, we'll stay put and allow more changes.
  If the user approves, the app can save.

  We still might delay navigation until the save succeeds.
  If we let the user move to the next screen immediately and
  the save failed (perhaps the data are ruled invalid), we would have lost the context of the error.

  We can't block while waiting for the server &mdash; that's not possible in a browser.
  We need to stop the navigation while we wait, asynchronously, for the server
  to return with its answer.

  We need the `CanDeactivate` guard.

  ### Cancel and Save

  Our sample application doesn't talk to a server.
  Fortunately, we have another way to demonstrate an asynchronous router hook.

  Users update crisis information in the `CrisisDetailComponent`.
  Unlike the `HeroDetailComponent`, the user changes do not update the
  crisis entity immediately. We update the entity when the user presses the *Save* button.
  We discard the changes if the user presses he *Cancel* button.

  Both buttons navigate back to the crisis list after save or cancel.
+makeExample('router/ts/app/crisis-center/crisis-detail.component.1.ts', 'cancel-save', 'crisis-detail.component.ts (excerpt)')(format=".")
:marked
  What if the user tries to navigate away without saving or canceling?
  The user could push the browser back button or click the heroes link.
  Both actions trigger a navigation.
  Should the app save or cancel automatically?

  We'll do neither. Instead we'll ask the user to make that choice explicitly
  in a confirmation dialog box that *waits asynchronously for the user's
  answer*.
.l-sub-section
  :marked
    We could wait for the user's answer with synchronous, blocking code.
    Our app will be more responsive ... and can do other work ...
    by waiting for the user's answer asynchronously. Waiting for the user asynchronously
    is like waiting for the server asynchronously.
:marked
  The `DialogService` (injected in the `AppComponent` for app-wide use) does the asking.

  It returns a [promise](http://exploringjs.com/es6/ch_promises.html) that
  *resolves* when the user eventually decides what to do: either
  to discard changes and navigate away (`true`) or to preserve the pending changes and stay in the crisis editor (`false`).

  We will take the result of that promise and convert it to an `Observable` for our guard to use.

<a id="CanDeactivate"></a>
:marked
  We create a `Guard` that will check for the presence of a `canDeactivate` function in our component, in this
  case being `CrisisDetailComponent`. We don't need to know the details of how our `CrisisDetailComponent` confirms deactivation.
  This makes our guard reusable, which is an easy win for us.
+makeExample('router/ts/app/interfaces.ts', '', 'interfaces.ts')
:marked
  Looking at our `CrisisDetailComponent`, we have implemented our confirmation workflow for unsaved changes.

+makeExample('router/ts/app/crisis-center/crisis-detail.component.1.ts', 'cancel-save-only', 'crisis-detail.component.ts (excerpt)')
:marked
  Notice that the `canDeactivate` method *can* return synchronously;
  it returns `true` immediately if there is no crisis or there are no pending changes.
  But it can also return a promise or an `Observable` and the router will wait for that
  to resolve to truthy (navigate) or falsey (stay put).

:marked
  We add the `Guard` to our crisis detail route in `crisis-center.routes.ts` using the `canDeactivate` array.
+makeExample('router/ts/app/crisis-center/crisis-center.routes.4.ts', '', 'crisis-center.routes.ts')

:marked
  We also need to add the `Guard` to our main `APP_ROUTER_PROVIDERS` so the `Router` can inject it during the navigation process.
+makeExample('router/ts/app/app.routes.ts', '', 'app.routes.ts')

:marked
  Now we have given our user a safeguard against unsaved changes.

  **Two critical points**
  1. The router interface is optional. We don't inherit from a base class. We simply implement the interface method or not.

  1. We rely on the router to call the guard. We don't worry about all the ways that the user
  could navigate away. That's the router's job.
  We simply write this class and let the router take it from there.

  The relevant *Crisis Center* code for this milestone is

+makeTabs(
   `router/ts/app/app.component.ts,
   router/ts/app/auth.guard.ts,
   router/ts/app/crisis-center/crisis-center.component.ts,
   router/ts/app/crisis-center/crisis-center.routes.ts,
   router/ts/app/crisis-center/crisis-list.component.1.ts,
   router/ts/app/crisis-center/crisis-detail.component.1.ts,
   router/ts/app/crisis-center/crisis.service.ts,
   router/ts/app/interfaces.ts
  `,
  null,
   `app.component.ts,
   auth.guard.ts,
   crisis-center.component.ts,
   crisis-center.routes.ts,
   crisis-list.component.ts,
   crisis-detail.component.ts,
   crisis.service.ts,
   interfaces.ts
  `)


<a id="query-parameters"></a>
<a id="query-parameter"></a>
.l-main-section
:marked
  ## Milestone #4: Query Parameters

  We use [*route parameters*](#route-parameters) to specify a *required* parameterized value *within* the route URL
  as we do when navigating to the `HeroDetailComponent` in order to view-and-edit the hero with *id:15*.
code-example(format="." language="bash").
  localhost:3000/hero/15
:marked
  Sometimes we wish to add *optional* information to a route request.
  For example, the `HeroListComponent` doesn't need help to display a list of heroes.
  But it might be nice if the previously-viewed hero were pre-selected when returning from the `HeroDetailComponent`.
figure.image-display
  img(src='/resources/images/devguide/router/selected-hero.png' alt="Selected hero")
:marked
  That becomes possible if we can include hero Magneta's `id` in the URL when we
  return from the `HeroDetailComponent`, a scenario we'll pursue in a moment.

  Optional information takes other forms. Search criteria are often loosely structured, e.g., `name='wind*'`.
  Multiple values are common &mdash; `after='12/31/2015' & before='1/1/2017'` &mdash; in no particular order &mdash;
   `before='1/1/2017' & after='12/31/2015'` &mdash; in a variety of formats &mdash; `during='currentYear'` .

  These kinds of parameters don't fit easily in a URL *path*. Even if we could define a suitable URL token scheme,
  doing so greatly complicates the pattern matching required to translate an incoming URL to a named route.

  The **URL query string** is the ideal vehicle for conveying arbitrarily complex information during navigation.
  The query string isn't involved in pattern matching and affords enormous flexiblity of expression.
  Almost anything serializable can appear in a query string.

  The Component Router supports navigation with query strings as well as route parameters.
  We define query string parameters in the *route parameters object* just like we do with route parameters.

  <a id="route-or-query-parameter"></a>
  ### Route Parameters or Query Parameters?

  There is no hard-and-fast rule. In general,

  *prefer a route parameter when*
  * the value is required.
  * the value is necessary to distinguish one route path from another.

  *prefer a query parameter when*
  * the value is optional.
  * the value is complex and/or multi-variate.

  <a id="route-parameters-object"></a>
  ### Route parameter
  When navigating to the `HeroDetailComponent` we specified the `id` of the hero-to-edit in the
  *route parameter* and made it the second item of the [*link parameters array*](#link-parameters-array).

+makeExample('router/ts/app/heroes/hero-list.component.1.ts','link-parameters-array')(format=".")
:marked
  The router embedded the `id` value in the navigation URL because we had defined it
  as a route parameter with an  `:id` placeholder token in the route `path`:
+makeExample('router/ts/app/heroes/heroes.routes.ts','hero-detail-route')(format=".")
:marked
  When the user clicks the back button, the `HeroDetailComponent` constructs another *link parameters array*
  which it uses to navigate back to the `HeroListComponent`.
+makeExample('router/ts/app/heroes/hero-detail.component.1.ts','gotoHeroes')(format=".")
:marked
  This array lacks a route parameter because we had no reason to send information to the `HeroListComponent`.

  Now we have a reason. We'd like to send the id of the current hero with the navigation request so that the
  `HeroListComponent` can highlight that hero in its list.

  We do that with a `NavigationExtras` object with `queryParams`.
  We also defined a junk parameter (`foo`) that the `HeroListComponent` should ignore.
  Here's the revised navigation statement:
+makeExample('router/ts/app/heroes/hero-detail.component.ts','gotoHeroes-navigate')(format=".")
:marked
  The application still works. Clicking "back" returns to the hero list view.

  Look at the browser address bar.
.l-sub-section
  img(src='/resources/images/devguide/plunker-separate-window-button.png' alt="pop out the window" align="right" style="margin-right:-20px")
  :marked
    When running in plunker, pop out the preview window by clicking the blue 'X' button in the upper right corner.
:marked
 It should look something like this, depending on where you run it:
code-example(format="." language="bash").
  localhost:3000/heroes?id=15&foo=foo
:marked
  The `id` value appears in the query string (`?id=15&foo=foo`), not in the URL path.
  The path for the "Heroes" route doesn't have an `:id` token.

// .alert.is-helpful
  :marked
    The router replaces route path tokens with corresponding values from the route parameters object.
    **Every parameter _not_ consumed by a route path goes in the query string.**
:marked
  ### Query parameters in the *ActivatedRoute* service

  The list of heroes is unchanged. No hero row is highlighted.

.l-sub-section
  :marked
    The [live example](/resources/live-examples/router/ts/plnkr.html) *does* highlight the selected
    row because it demonstrates the final state of the application which includes the steps we're *about* to cover.
    At the moment we're describing the state of affairs *prior* to those steps.
:marked
  The `HeroListComponent` isn't expecting any parameters at all and wouldn't know what to do with them.
  Let's change that.

  When navigating from the `HeroListComponent` to the `HeroDetailComponent`
  we subscribed the route params `Observable` and made it available to the `HeroDetailComponent`
  in the `ActivatedRoute` service. We injected that service in the constructor of the `HeroDetailComponent`.

  This time we'll be navigating in the opposite direction, from the `HeroDetailComponent` to the `HeroListComponent`.
  This time we'll inject the `Router` service in the constructor of the `HeroListComponent`.

  First we extend the router import statement to include the `ActivatedRoute` service symbol;
+makeExample('router/ts/app/heroes/hero-list.component.ts','import-router', 'hero-list.component.ts (import)')(format=".")
:marked
  Then we use the `routerState` to access the globally available query parameters `Observable` so we can subscribe
  and extract the `id` parameter as the `selectedId`:
+makeExample('router/ts/app/heroes/hero-list.component.ts','ctor', 'hero-list.component.ts (constructor)')(format=".")
.l-sub-section
  :marked
    All route/query parameters are strings.
    The (+) in front of the `params['id']` expression is a JavaScript trick to convert the string to an integer.
:marked
  We add an `isSelected` method that returns true when a hero's id matches the selected id.
+makeExample('router/ts/app/heroes/hero-list.component.ts','isSelected', 'hero-list.component.ts (constructor)')(format=".")
:marked
  Finally, we update our template with a [Class Binding](template-syntax.html#class-binding) to that `isSelected` method.
  The binding adds the `selected` CSS class when the method returns `true` and removes it when `false`.
  Look for it within the repeated `<li>` tag as shown here:
+makeExample('router/ts/app/heroes/hero-list.component.ts','template', 'hero-list.component.ts (template)')(format=".")
:marked
  When the user navigates from the heroes list to the "Magneta" hero and back, "Magneta" appears selected:
figure.image-display
  img(src='/resources/images/devguide/router/selected-hero.png' alt="Selected List" )
:marked
  The `foo` query string parameter is harmless and continues to be ignored.

  ### Child Routers and Query Parameters

  We can define query parameters for child routers too.

  The technique is precisely the same.
  In fact, we made exactly the same changes to the *Crisis Center* feature.
  Confirm the similarities in these *Hero* and *CrisisCenter* components,
  arranged side-by-side for easy comparison:
+makeTabs(
    `router/ts/app/heroes/hero-list.component.ts,
    router/ts/app/crisis-center/crisis-list.component.ts,
    router/ts/app/heroes/hero-detail.component.ts,
    router/ts/app/crisis-center/crisis-detail.component.ts
    `,
    null,
    `hero-list.component.ts,
    crisis-list.component.ts,
    hero-detail.component.ts,
    crisis-detail.component.ts
    `)
:marked
  When we navigate back from a `CrisisDetailComponent` that is showing the *Asteroid* crisis,
  we see that crisis properly selected in the list like this:
figure.image-display
  img(src='/resources/images/devguide/router/selected-crisis.png' alt="Selected crisis" )
:marked
  **Look at the browser address bar again**. It's *different*. It looks something like this:
code-example(format="." language="bash").
  localhost:3000/crisis-center/;id=3;foo=foo
:marked
  The query string parameters are no longer separated by "?" and "&".
  They are **separated by semicolons (;)**
  This is *matrix URL* notation &mdash; something we may not have seen before.
.l-sub-section
  :marked
    *Matrix URL* notation is an idea first floated
    in a [1996 proposal](http://www.w3.org/DesignIssues/MatrixURIs.html) by the founder of the web, Tim Berners-Lee.

    Although matrix notation never made it into the HTML standard, it is legal and
    it became popular among browser routing systems as a way to isolate parameters
    belonging to parent and child routes. The Angular Component Router is such a system.

    The syntax may seem strange to us but users are unlikely to notice or care
    as long as the URL can be emailed and pasted into a browser address bar
    as this one can.


<a id="final-app"></a>
.l-main-section
:marked
  ## Wrap Up
  We've covered a lot of ground in this chapter and the application is too big to reprint here.
  Please visit the [live example](/resources/live-examples/router/ts/plnkr.html) and 
  where you can download the final source code.

.l-main-section
:marked
  ## Appendices
  The balance of this chapter is a set of appendices that
  elaborate some of the points we covered quickly above.

  The appendix material isn't essential. Continued reading is for the curious.


.l-main-section
<a id="link-parameters-array"></a>
:marked
  ## Appendix: Link Parameters Array
  We've mentioned the *Link Parameters Array* several times. We've used it several times.

  We've bound the `RouterLink` directive to such an array like this:
+makeExample('router/ts/app/app.component.3.ts', 'h-anchor')(format=".")
:marked
  We've written a two element array when specifying a route parameter like this
+makeExample('router/ts/app/heroes/hero-list.component.1.ts', 'nav-to-detail')(format=".")
:marked
  These two examples cover our needs for an app with one level routing.
  The moment we add a child router, such as the *Crisis Center*, we create new link array possibilities.

  Recall that we specified a default child route for *Crisis Center* so this simple `RouterLink` is fine.
+makeExample('router/ts/app/app.component.3.ts', 'cc-anchor-w-default')(format=".")
:marked
  Let's parse it out.
  * The first item in the array identifies the parent route ('/crisis-center').
  * There are no parameters for this parent route so we're done with it.
  * There is no default for the child route so we need to pick one.
  * We decide to go to the `CrisisListComponent` whose route path is '/' but we don't need to explicitly add it
  * Voila! `['/crisis-center']`.

  Let's take it a step further.
  This time we'll build a link parameters array that navigates from the root of the application
  down to the "Dragon Crisis".

  * The first item in the array identifies the parent route ('/crisis-center').
  * There are no parameters for this parent route so we're done with it.
  * The second item identifies the child route for details about a particular crisis ('/:id').
  * The details child route requires an `id` route parameter
  * We add `id` of the *Dragon Crisis* as the third item in the array (`1`)

  It looks like this!
+makeExample('router/ts/app/app.component.3.ts', 'Dragon-anchor')(format=".")
:marked
  If we wanted to, we could redefine our `AppComponent` template with *Crisis Center* routes exclusively:
+makeExample('router/ts/app/app.component.3.ts', 'template')(format=".")
:marked
  In sum, we can write applications with one, two or more levels of routing.
  The link parameters array affords the flexibility to represent any routing depth and
  any legal sequence of route paths, (required) router parameters and (optional) route parameter objects.

<a id="onInit"></a>
.l-main-section
:marked
  ## Appendix: Why use an *ngOnInit* method

  We implemented an `ngOnInit` method in many of our Component classes.
  We did so, for example, in the [HeroDetailComponent](#hero-detail-ctor).
  We might have put the `ngOnInit` logic inside the constructor instead. We didn't for a reason. The reason is *testability*.

  A constructor that has major side-effects can be difficult to test because it starts doing things as soon as
  we create a test instance. In this case, it might have made a request to a remote server, something it shouldn't
  do under test. It may even be impossible to reach the server in the test environment.

  The better practice is to limit what the constructor can do. Mostly it should stash parameters in
  local variables and perform simple instance configuration.

  Yet we want an instance of this class to get the hero data from the `HeroService` soon after it is created.
  How do we ensure that happens if not in the constructor?

  Angular detects when a component has certain lifecycle methods like
  [ngOnInit](../api/core/OnInit-interface.html) and
  [ngOnDestroy](../api/core/OnDestroy-interface.html) and calls
  them
  at the appropriate moment.

  Angular will call `ngOnInit` when we navigate to the `HeroDetailComponent`, we'll get the `id` from the `ActivatedRoute`
  params and ask the server for the hero with that `id`.

  We too can call that `ngOnInit` method in our tests if we wish ... after taking control of the injected
  `HeroService` and (perhaps) mocking it.

<a name="browser-url-styles"></a>
<a id="location-strategy"></a>
.l-main-section
:marked
  ## Appendix: *LocationStrategy* and browser URL styles

  When the router navigates to a new component view, it updates the browser's location and history
  with a URL for that view.
  This is a strictly local URL. The browser shouldn't send this URL to the server
  and should not reload the page.

  Modern HTML 5 browsers support
  [history.pushState](https://developer.mozilla.org/en-US/docs/Web/API/History_API#Adding_and_modifying_history_entries),
  a technique that changes a browser's location and history without triggering a server page request.
  The router can compose a "natural" URL that is indistinguishable from
  one that would otherwise require a page load.

  Here's the *Crisis Center* URL in this "HTML 5 pushState" style:
code-example(format=".", language="bash").
  localhost:3002/crisis-center/
:marked
  Older browsers send page requests to the server when the location URL changes ...
  unless the change occurs after a "#" (called the "hash").
  Routers can take advantage of this exception by composing in-application route
  URLs with hashes.  Here's a "hash URL" that routes to the *Crisis Center*
code-example(format=".", language="bash").
  localhost:3002/src/#/crisis-center/
:marked
  The Angular Component Router supports both styles with two `LocationStrategy` providers:
  1. `PathLocationStrategy` - the default "HTML 5 pushState" style.
  1. `HashLocationStrategy` - the "hash URL" style.

  The router's `provideRouter` function sets the `LocationStrategy` to the `PathLocationStrategy`,
  making it the default strategy.
  We can switch to the `HashLocationStrategy` with an override during the bootstrapping process if we prefer it.
.l-sub-section
  :marked
    Learn about "providers" and the bootstrap process in the
    [Dependency Injection chapter](dependency-injection#bootstrap)
:marked
  ### Which Strategy is Best?
  We must choose a strategy and we need to make the right call early in the project.
  It won't be easy to change later once the application is in production
  and there are lots of application URL references in the wild.

  Almost all Angular 2 projects should use the default HTML 5 style.
  It produces URLs that are easier for users to understand.
  And it preserves the option to do **server-side rendering** later.

  Rendering critical pages on the server is a technique that can greatly improve
  perceived responsiveness when the app first loads.
  An app that would otherwise take ten or more seconds to start
  could be rendered on the server and delivered to the user's device
  in less than a second.

  This option is only available if application URLs look like normal web URLs
  without hashes (#) in the middle.

  Stick with the default unless you have a compelling reason to
  resort to hash routes.

  ### HTML 5 URLs and the  *&lt;base href>*
  While the router uses the "[HTML 5 pushState](https://developer.mozilla.org/en-US/docs/Web/API/History_API#Adding_and_modifying_history_entries)"
  style by default, we *must* configure that strategy with a **base href**

  The preferred way to configure the strategy is to add a
  [&lt;base href> element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base) tag
  in the `<head>` of the `index.html`.
+makeExample('router/ts/index.1.html','base-href')(format=".")
:marked
  Without that tag, the browser may not be able to load resources
  (images, css, scripts) when "deep linking" into the app.
  Bad things could happen when someone pastes an application link into the
  browser's address bar or clicks such a link in an email link.

  Some developers may not be able to add the `<base>` element, perhaps because they don't have
  access to `<head>` or the `index.html`.

  Those developers may still use HTML 5 URLs by taking two remedial steps:

  1. Provide the router with an appropriate `APP_BASE_HREF` value.
  1. Use **absolute URLs** for all web resources: css, images, scripts, and template html files.

.l-sub-section
  :marked
    Learn about the [APP_BASE_HREF](../api/router/APP_BASE_HREF-let.html)
    in the API Guide.
:marked
  ### *HashLocationStrategy*
  We can go old-school with the `HashLocationStrategy` by
  providing it as the router's `LocationStrategy` during application bootstrapping.

  First, import the `provide` symbol for Dependency Injection and the
  `Location` and `HashLocationStrategy` symbols from the router.

  Then *override* the default strategy defined in `provideRouter` by
  providing the `HashLocationStrategy` later in the `AppComponent` providers array argument:
+makeExample('router/ts/app/main.2.ts','', 'main.ts (hash URL strategy)')
