angular-cn/public/docs/ts/latest/guide/router.jade

include ../../../../_includes/_util-fns

: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

  * loading the [router library](#load-library) and setting the [base href](#base-href)
  * [configuring a 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)
  * creating a [child router](#child-router) with its own routes
  * setting a [default route](#default)
  * confirming or canceling navigation with [router lifecycle hooks](#lifecycle-hooks)
  * 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.
  ### Setup
  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 
  within the Angular npm bundle.
  We make it available by loading its script in our `index.html`, after
  the Angular core script.
+makeExample('router/ts/index.html','router-lib')(format=".")
:marked
  ### *&lt;base href>*
  Most routing applications should add a `<base>` element just after 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.html','base-href')(format=".")
.l-sub-section
  :marked
    We cover other options in the [details below](#browser-url-styles).
:marked
  ### Configuration
  When the browser's URL changes, the router looks for a corresponding **`RouteDefinition`**
  from which it can determine the component to display.

  A router has no route definitions until we configure it.
  The preferred way to simultaneously create a router and add its routes is with a **`@RouteConfig`** [decorator](glossary.html#decorator)
  applied to the router's host component.

  In this example, we configure the top-level `AppComponent` with three route definitions
+makeExample('router/ts/app/app.component.2.ts', 'route-config', 'app.component.ts (excerpt)')(format=".")
:marked

.l-sub-section
  :marked
    There are several flavors of `RouteDefinition`.
    The most common by far is the named **`Route`** which maps a URL path to a component

    The `name` field is the route name which **must** be spelled in **PascalCase** 
    to avoid potential confusion with the route `path`.

    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.
:marked
  ### Router Outlet
  Now we know how the router gets its configuration.
  When the browser URL for this application becomes `/heroes`,
  the router matches that URL to the `RouteDefintion` named *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 string name of a route definition.
   'CrisisCenter' and 'Heroes' are the names 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 `@RouterConfig` configuration tied the `AppComponent` to a router configured with routes.
  The component has a `RouterOutlet` where it can display views produced by the router.
  It has `RouterLinks` that users can click to navigate via the router.

  The `AppComponent` has become a ***Routing Component***, a component that can route.

  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>@RouteConfig</code>
    td.
      Configures a router with <code>RouteDefinitions</code>, each mapping a URL path to a component.
  tr
    td <code>RouteDefinition</code>
    td.
      Defines how the router should navigate to a component based on a URL pattern.
  tr
    td <code>Route</code>
    td.
      The most common form of <code>RouteDefinition</code> consisting of a path, a route name,
      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 <i>Link Parameters Array</i></code>
    td.
      An array that the router inteprets 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 <i>Routing Component</i></code>
    td.
      An Angular component with an attached router.
: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 `routerCanDeactivate` lifecycle hook 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 `routerCanDeactivate` lifecycle hook (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" )
:marked
  <a id="load-library"></a>
  ### Load the Component Router library
  The Component Router is not part of the Angular 2 core. It is in its own library.
  The router is an optional service because not all applications need routing and,
  depending on your requirements, you may need a different routing library.

  The Component Router library is part of the Angular npm bundle.
  We make it available by loading its script in our `index.html`, right after
  the Angular core script.
<a id="base-href"></a>
+makeExample('router/ts/index.html','router-lib')(format=".")
: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 *exactly* as shown here.

+makeExample('router/ts/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 be 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
  ### Booting with the router service providers
  Our app launches from the `main.ts` file in the `/app` folder so let's start there.
  It's short and all of it is relevant to routing.
+makeExample('router/ts/app/main.1.ts','all', 'main.ts')(format=".")
:marked
  We import our root `AppComponent` and Angular's `bootstrap` function as expected.

  We also import `ROUTER_PROVIDERS` from the router library.
  The router is a service implemented by a collection of *Dependency Injection* providers, most of which are identified in the
  `ROUTER_PROVIDERS` array.

  We're booting Angular with `AppComponent` as our app's root component and
  registering providers, as we often do, in the providers array in the second parameter of the `bootstrap` function.
  Providing the router providers at the root makes the Component Router available everywhere in our application.
.l-sub-section
  :marked
    Learn about providers, the `provide` function, and injected services in the
    [Dependency Injection chapter](dependency-injection.html).
:marked
  ### The *AppComponent* shell
  The root `AppComponent` is the application shell. It has 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" )
:marked
  <a id="shell-template"></a>
  The corresponding component template looks like this:
+makeExample('router/ts/app/app.component.1.ts','template')(format=".")
:marked
  ### *RouterOutlet*
  `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>`.

<a id="router-link"></a>
:marked
  ### *RouterLink* binding
  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 name of the route that prescribes the destination component and a path for the URL
  * the optional route and query parameters that go into the route URL

  The arrays in this example each have a single string parameter, the name of a `Route` that
  we'll configure for this application with `@RouteConfig()`. We don't need to set route parameters yet.
.l-sub-section
  :marked
    Learn more about the link parameters array in the [appendix below](#link-parameters-array).
<a id="route-config"></a>
:marked
  ### *@RouteConfig()*
  A router holds a list of route definitions. The list is empty for a new router. We must configure it.

  A router also needs a **Host Component**, a point of origin for its navigations.

  It's natural to combine the creation of a new router, its configuration, and its assignment to a host component
  in a single step. That's the purpose of the `@RouteConfig` decorator which we put to good use here:
+makeExample('router/ts/app/app.component.1.ts','route-config')(format=".")
:marked
  The `@RouteConfig` decorator creates a new router.
  We applied the decorator to `AppComponent` which makes that the router's host component.
  The argument to `@RouteConfig()` is an array of **Route Definitions**.

  We're supplying two definitions:
+makeExample('router/ts/app/app.component.1.ts','route-defs')(format=".")
:marked
  Each definition translates to a [Route](https://angular.io/docs/ts/latest/api/router/Route-class.html) which has a
  * `path` - the URL path segment for this route
  * `name` - the name of the route
  * `component` - the component associated with this route.

  The router draws upon its registry of route definition when
  1. the browser URL changes
  2. we tell the router to go to a named route

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

  1. *When the application requests navigation to a route **named** `CrisisCenter`, compose a browser URL
  with the path segment `/crisis-center`, update the browser's address location and history, create or retrieve an instance of
  the `CrisisListComponent`, and display that component's list view.*

  ### "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 `@RouterConfig`
  * 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 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/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, 
  main.ts,
  hero-list.component.ts,
  crisis-list.component.ts, 
  index.html`)
:marked

<a id="heroes-feature"></a>
.l-main-section
:marked
  ## Milestone #2: The Heroes Feature

  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 along in route parameters (`RouteParams`)

  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 also 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 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.

  ### New route definition with route parameter

  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.

  With that plan in mind, we return to the `app.component.ts` to  make changes to the router's configuration

  First, we import the two components from their new locations in the `app/heroes/` folder:
+makeExample('router/ts/app/app.component.2.ts','hero-import')(format=".")
:marked
  Then we update the `@RouteConfig` route definitions :
+makeExample('router/ts/app/app.component.2.ts','route-config')(format=".")
:marked
  The `CrisisCenter` and `Heroes` definitions didn't change.
  While we moved `hero-list.component.ts` to a new location in the `app/heroes/` folder, that only affects the `import` statement;
  it doesn't affect its route definition.

  We added a new route definition for the `HeroDetailComponent` &mdash; and this definition has a twist.
+makeExample('router/ts/app/app.component.2.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`.

<a id="navigate"></a>
:marked
  ### Navigate to the detail imperatively

  *We don't navigate to the detail component by clicking a link*.
  We won't be adding a new anchor tag to the shell navigation bar.

  Instead, we'll *detect* when the user selects a hero from the list and *command* the router
  to present the hero detail view of 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')(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.
  
  <a id="route-parameters"></id>
  ### Setting the route parameters object

  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 **name** of the destination route and a **route parameters object** 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
:marked
  ### Getting the route parameter

  <a id="hero-detail-ctor"></a>
  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 **RouteParams** service.

  As usual, we write a constructor that asks Angular to inject that service among the other services
  that the component require and reference them as private variables.
+makeExample('router/ts/app/heroes/hero-detail.component.1.ts','ctor')(format=".")
:marked
  Later, in the `ngOnInit` method,
  we ask the `RouteParams` service for the `id` parameter by name and
  tell the `HeroService` to fetch the hero with that `id`.
+makeExample('router/ts/app/heroes/hero-detail.component.1.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
  ### Navigating back to the list component
  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 wrote for the `[routerLink]` directive binding.
  It holds the **name of the `HeroListComponent` route**: 
+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 (`RouteParams`)

  After these changes, the folder structure looks like this:
.filetree
  .file router-sample
  .children
    .file app
    .children
      .file app.component.ts
      .file crisis-list.component.ts
      .file heroes
      .children
        .file hero-detail.component.ts
        .file hero-list.component.ts
        .file hero.service.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.2.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`,
   null,
  `app.component.ts,
  hero-list.component.ts,
  hero-detail.component.ts,
  hero.service.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:

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

  * The user should be able to cancel unwanted changes.

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

  There are also a few lingering annoyances in the *Heroes* implementation that we can cure in the *Crisis Center*.

  * We currently register every route of every view at the highest level of the application.
  If we expand the *Crisis Center* with a 100 new views, we'll make 100 changes to the
  `AppComponent` route configuration. If we rename a *Crisis Center* component or change a route definition,
  we'll be changing the `AppComponent` too.

  * If we followed *Heroes* lead, we'd be adding the `CrisisService` to the providers in `app.component.ts`.
  Then both `HeroService` and `CrisisService` would be available everywhere although
  they're only needed in their respective feature modules. That stinks.

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

  We'll fix all of these problems and add the new routing features to *Crisis Center*.

  The most significant fix is the introduction of a **child *Routing Component***
  and its **child router**

  We'll leave *Heroes* in its less-than-perfect state to
  serve as a contrast with what we hope is a superior *Crisis Center*.

  ### A free-standing Crisis Center Feature Module
  The *Crisis Center* is one of two application workflows.
  Users navigate between them depending on whether they are managing crises or heroes.

  The `CrisisCenter` and `Heroes` components are children of the root `AppComponent`.

  Unfortunately, they and their related files are physically commingled in the same folder with the `AppComponent`.
  We'd prefer to separate them in their own *feature areas* so they can operate and evolve independently.
  Someday we might re-use one or the other in a different application.
  Someday we might load one of them dynamically only when the user chose to enter its workflow.

  Some might call it [yagni](http://martinfowler.com/bliki/Yagni.html) to even think about such things.
  But we're right to be nervous about the way *Heroes* and *Crisis Center* artifacts are
  bubbling up to the root `AppComponent` and blending with each other.
  That's a [code smell](http://martinfowler.com/bliki/CodeSmell.html).

  Isolating feature area modules from each other looks good to us.
.l-sub-section
  :marked
    It's looking good as a general pattern for Angular applications.
  figure.image-display
    img(src='/resources/images/devguide/router/component-tree.png' alt="Component Tree" )
  :marked
    * each feature area in its own module folder
    * each area with its own root component
    * each area root component with its own router-outlet and child routes
    * area routes rarely (if ever) cross

:marked
  We'll make the *Crisis Center* stand on its own and leave the *Heroes* as it is
  so we can compare the effort, results, and consequences.
  Then each of us can decide which path to prefer (as if we didn't already know).

  <a id="child-router"></id>
  ### Child Routing Component
  We create a new `app/crisis-center` folder and add `crisis-center.component.ts` to it with the following contents:
+makeExample('router/ts/app/crisis-center/crisis-center.component.1.ts', 'minus-imports', 'crisis-center/crisis-center.component.ts (minus imports)')
:marked
  The `CrisisCenterComponent` parallels the `AppComponent`.

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

  This `CrisisCenterComponent` is a shell for crisis management
  just as the `AppComponent` is a shell to manage the high-level workflow.

  `AppComponent` has a `@RouteConfig` decorator that defines the top-level routes.
  `CrisisCenterComponent` has a `@RouteConfig` decorator that defines *Crisis Center* child routes.

  The `CrisisCenterComponent` template 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.

  It has no selector either. It doesn't need one. We don't *embed* this component in a parent template. We *navigate* to it
  from the outside, via a parent router (more on that soon).

  ### Service isolation
  We add the `CrisisService` to the component's providers array
  instead of registering it with the `bootstrap` function in `main.ts`.
+makeExample('router/ts/app/crisis-center/crisis-center.component.1.ts', 'providers')
:marked
  This step limits the scope of that service to the *Crisis Center* component and its sub-component tree.
  No component outside of the *Crisis Center* needs access to the `CrisisService`.
  By restricting its scope, we feel confident that we can evolve it independently without fear of breaking
  unrelated application modules &mdash; modules that *shouldn't have access to it anyway*.

  ### Child Route Configuration
  The `CrisisCenterComponent` is a *Routing Component* like the `AppComponent`.

  The `@RouteConfig` decorator that adorns the `CrisisCenterComponent` class defines routes in much the same way
  that we did earlier.
+makeExample('router/ts/app/crisis-center/crisis-center.component.1.ts', 'route-config', 'app/crisis-center/crisis-center.component.ts (routes only)' )(format=".")
:marked
  The two routes terminate in the two *Crisis Center* child components, `CrisisListComponent` and `CrisisDetailComponent`.
  
  There is an *important difference* in the treatment of the root `AppComponent` paths and these paths. 
  Normally paths that begin with `/` refer to the root of the application.
  Here they refer to the **root of the child component!**.
  
  The Component Router composes the final route by concatenating route paths beginning with the ancestor paths to this child router.
  In our example, there is one ancestor path: "crisis-center".
  The final route to the `CrisisDetailComponent` displaying the crisis whose `id` is 2 would be something like:
code-example(format="").
  localhost:3000/crisis-center/2
:marked
  We cannot know this simply by looking at the `CrisisCenterComponent` alone.
  We can't tell that it is a *child* routing component.
  We can't tell that its routes are child routes; they are indistinguiable from top level application routes.

  Such ignorance is intentional. The *Crisis Center* shouldn't know that it is the child of anything.
  Today it is a child component one level down. 
  Tomorrow it might be the top level component of its own application. 
  Next month it might be re-purposed in a different application.
  The *Crisis Center* itself is indifferent to these possibilities.

  *We* make it a child component of our application by reconfiguring the routes of the top level `AppComponent`.
:marked
  ### Parent Route Configuration
  Here is the revised route configuration for the parent `AppComponent`:
+makeExample('router/ts/app/app.component.ts', 'route-config', 'app/app.component.ts (routes only)' )
:marked
  The last two *Hero* routes haven't changed.
  
  The first *Crisis Center* route has changed &mdash; *significantly* &mdash; and we've formatted it to draw attention to the differences:
+makeExample('router/ts/app/app.component.ts', 'route-config-cc')(format=".")
:marked
  Notice that the **path ends with a slash and three trailing periods (`/...`)**.

  That means this is an incomplete route (a ***non-terminal route***).  The finished route will be some combination of
  the parent `/crisis-center/` route and a route from the **child router** that belongs to the designated component.

  All is well.
  The parent route's designated component is the `CrisisCenterComponent` which is a *Routing Component* with its own router and routes.

  <a id="default"></a>
  ### Default route
  The other important change is the addition of the `useAsDefault` property.
  Its value is `true` which makes *this* route the *default* route.
  When the application launches, in the absence of any routing information from the browser's URL, the router
  will default to the *Crisis Center*. That's our plan.

  ### Routing to the Child

  We've set the top level default route to go to the `CrisisCenterComponent`.
  The final route will be a combination of `/crisis-center/` 
  and one of the child `CrisisCenterComponent` router's two routes. Which one?

  It could be either of them. In the absence of additional information, the router can't decide and must throw an error.
  
  We've tried the sample application and it didn't fail. We must have done something right.

  Look at the end of the child `CrisisCenterComponent`s first route.
+makeExample('router/ts/app/crisis-center/crisis-center.component.1.ts', 'default-route', 'app/crisis-center/crisis-center.component.ts (default route)')(format=".")
:marked
  We see `useAsDefault: true` once again. 
  That tells the router to compose the final URL using the path from the default *child* route.
  Concatenate the base URL with the parent path, `/crisis-center/`, and the child path, `/`. 
  Remove superfluous slashes. We get:
code-example(format="").
  localhost:3000/crisis-center/

.l-main-section
:marked
  ### Handling Unsaved Changes
  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'd like to pause and let the user decide what to do. Perhaps we'll cancel the
  navigation, stay put, and make more changes.

  We need the router's cooperation to pull this off. We need router lifecycle hooks.

  <a id="lifecycle-hooks"></a>
  ### Router Lifecycle Hooks
  Angular components have [lifecycle hooks](lifecycle-hooks.html). For example, Angular calls the hook methods of the
  [OnInit](../api/core/OnInit-interface.html) and [OnDestroy]((../api/core/OnDestroy-interface.html)
  interfaces when it creates and destroys components.

  The router calls similar hook methods,
  [routerCanActivate](../api/router/CanActivate-var.html) and [routerCanDeactivate](../api/router/CanDeactivate-interface.html),
  before it navigates *to* a component or *away* from a component.

  If a *`can...`* method returns `true`, the navigation proceeds. If it returns `false`, the
  router cancels the navigation and stays on the current view.

  The router lifecycle hooks *supplement* the component lifecycle hooks.
  We still need the component hooks but the router hooks do what the component hooks cannot.
  
  For example, the component hooks can't stop component creation or destruction.
  Because they are synchronous, they can't pause view navigation to wait for an asynchronous process to finish.

  Imagine we have unsaved changes. The user starts to navigate away.
  We shouldn't lose the user's changes; that would be a terrible experience. So we try to save those changes to the server.
  
  If the save fails for any reason (perhaps the data are ruled invalid), what do we do?

  If we let the user move to the next screen, we 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.

  The router hook methods can pause asynchronously, return promises, and cancel navigation if necessary.

  ### 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://www.html5rocks.com/en/tutorials/es6/promises/) 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`).

<a id="CanDeactivate"></a>
<a id="routerCanDeactivate"></a>
:marked
  We execute the dialog inside the router's `routerCanDeactivate` lifecycle hook method.
+makeExample('router/ts/app/crisis-center/crisis-detail.component.1.ts', 'routerCanDeactivate', 'crisis-detail.component.ts (excerpt)')
:marked
  Notice that the `routerCanDeactivate` 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 and the router will wait for that promise to resolve to truthy (navigate) or falsey (stay put).

  **Two critical points**
  1. The router hook 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 hook. We don't worry about all the ways that the user
  could navigate away. That's the router's job.
  We simply write this method and let the router take it from there.
  
  The relevant *Crisis Center* code for this milestone is
  
+makeTabs(
   `router/ts/app/crisis-center/crisis-center.component.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
  `,
  null,
   `crisis-center.component.ts,
   crisis-list.component.ts,
   crisis-detail.component.ts,
   crisis.service.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 parameters object
  When navigating to the `HeroDetailComponent` we specified the `id` of the hero-to-edit in the 
  *route parameters object* 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/app.component.2.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 parameters object 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 route parameters object in the same manner as before. 
  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 *RouteParams* 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` 
  the router picked up the route parameter object and made it available to the `HeroDetailComponent`
  in the `RouteParams` 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 `RouteParams` service in the constructor of the `HeroListComponent`.
  
  First we extend the router import statement to include the `RouteParams` service symbol;
+makeExample('router/ts/app/heroes/hero-list.component.ts','import-route-params', 'hero-list.component.ts (import)')(format=".")
:marked
  Then we extend the constructor to inject the `RouteParams` service 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 parameters are strings. 
    The (+) in front of the `routeParameters.get` 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
  As we end our chapter, we take a parting look at
  the entire application.

  We can always try the [live example](/resources/live-examples/router/ts/plnkr.html) and download the source code from there.

  Our final project folder structure looks like this:
.filetree
  .file router-sample
  .children
    .file app
    .children
      .file crisis-center/...
      .file heroes/...
      .file app.component.ts
      .file dialog.service.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
  The pertinent top level application files are
+makeTabs(
  `router/ts/app/app.component.ts,
   router/ts/app/main.ts,
   router/ts/app/dialog.service.ts,
   router/ts/index.html
  `,
  null,
  `app.component.ts,
   main.ts,
   dialog.service.ts,
   index.html
  `)
:marked
  <a id="crisis-center-structure-and-code"></id>
  ### Crisis Center
  The *Crisis Center* feature area within the `crisis-center` folder follows:
.filetree
  .file app
  .children
    .file crisis-center
    .children
      .file crisis-center.component.ts
      .file crisis-detail.component.ts
      .file crisis-list.component.ts
      .file crisis.service.ts
:marked
+makeTabs(
   `router/ts/app/crisis-center/crisis-center.component.ts,
   router/ts/app/crisis-center/crisis-list.component.ts,
   router/ts/app/crisis-center/crisis-detail.component.ts,
   router/ts/app/crisis-center/crisis.service.ts
  `,
  null,
   `crisis-center.component.ts,
   crisis-list.component.ts,
   crisis-detail.component.ts,
   crisis.service.ts,
  `)
:marked
  ### Heroes
  The *Heroes* feature area within the `heroes` folder is next:
.filetree
  .file app
  .children
    .file heroes
    .children
      .file hero-detail.component.ts
      .file hero-list.component.ts
      .file hero.service.ts
:marked
+makeTabs(
   `router/ts/app/heroes/hero-list.component.ts,
   router/ts/app/heroes/hero-detail.component.ts,
   router/ts/app/heroes/hero.service.ts
  `,
  null,
   `hero-list.component.ts,
   hero-detail.component.ts,
   hero.service.ts
  `)
:marked


.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
  ## 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
  *If we had not specified a default route*, our single item array would fail
  because we didn't tell the router which child route to use.
+makeExample('router/ts/app/app.component.3.ts', 'cc-anchor-fail')(format=".")
:marked
  We'd need to write our anchor with a link array like this:
+makeExample('router/ts/app/app.component.3.ts', 'cc-anchor-no-default')(format=".")
:marked
  Let's parse it out.
  * The first item in the array identifies the parent route ('CrisisCenter').
  * 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 name is 'CrisisList'
  * So we add that 'CrisisList' as the second item in the array.
  * Voila! `['CrisisCenter', 'CrisisList']`.

  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 ('CrisisCenter').
  * 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 ('CrisisDetail').
  * The details child route requires an `id` route parameter
  * We add `id` of the *Dragon Crisis* as the third item in the array (`{id: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
  ### Link Parameters Arrays in Redirects
  What if we weren't constructing anchor tags with `RouterLink` directives?
  What if we wanted to add a disaster route as part of the top-level router's configuration?

  We can do that!

  We compose a 3-item link parameters array following the recipe we just created.
  This time we set the id to the *Asteroid Crisis* (`{id:3}`).

  We can't define a normal route because that requires setting a target component.
  We're not defining a *route to a component*. We're defining a *route to a route*. A *route to a route* is a **redirect**.
  Here's the redirect route we'll add to our configuration.
+makeExample('router/ts/app/app.component.ts', 'asteroid-route')(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 names 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](https://angular.io/docs/ts/latest/api/core/OnInit-interface.html) and
  [ngOnDestroy](https://angular.io/docs/ts/latest/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 `RouteParams`
  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>
.l-main-section
:marked
  ## Appendix: 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 `ROUTER_PROVIDERS` array 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.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](https://angular.io/docs/ts/latest/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 `ROUTE_PROVIDERS` by
  providing the `HashLocationStrategy` later in the `bootstrap` providers array argument:
+makeExample('router/ts/app/main.2.ts','', 'main.ts (hash URL strategy)')