include ../_util-fns

:marked
  Angular 2 is a framework to help us build client applications in HTML and
  either JavaScript or a language (like Dart or TypeScript) that compiles to JavaScript.
  Angular 2 for Dart is published as the `angular2` package, which
  (like many other Dart packages) is available via the Pub tool.

  With Angular, we write applications by composing HTML *templates* with Angularized markup,
  writing *component* classes to manage those templates, adding application logic in *services*,
  and handing the top root component to Angular's *bootstrapper*.

  Angular takes over, presenting our application content in a browser and responding to user interactions
  according to the instructions we provided.

<!-- figure img(src="/resources/images/devguide/architecture/airplane.png" alt="Us" align="left" style="width:200px; margin-left:-40px;margin-right:10px" ) -->
:marked
  Of course there is more to it than this.
  We're cruising at high altitude in this overview.
  We're looking for landmarks. We should expect the object below to be fuzzy and obscured by occasional clouds.
  Details become more clear and precise when we land in the chapters themselves.
<br clear="all">

:marked
  An Angular 2 for Dart application rests on seven main building blocks:
  1. [Components](#component)
  1. [Templates](#template)
  1. [Metadata](#metadata)
  1. [Data binding](#data-binding)
  1. [Directives](#directive)
  1. [Services](#service)
  1. [Dependency injection](#dependency-injection)

figure
  img(src="/resources/images/devguide/architecture/overview.png" alt="overview" style="margin-left:-40px;" width="700")
:marked
  Learn these seven and we're on our way.

.l-main-section
<a id="component"></a>
:marked
  ## Components
figure
  img(src="/resources/images/devguide/architecture/hero-component.png" alt="Component" align="left" style="width:200px; margin-left:-40px;margin-right:10px" )
:marked
  A **component** controls a patch of screen real estate that we could call a *view*.
  A set of navigation links, a list of heroes, a hero editor ...
  they're all views controlled by components.

  We define a component's application logic — what it does to support the view — inside a class.
  The class interacts with the view through an API of properties and methods.

  <a id="component-code"></a>
  A `HeroListComponent`, for example, might have a `heroes` property that returns an array of heroes
  that it acquired from a service.
  It might have a `selectHero()` method that sets a `selectedHero` property when the user clicks to choose a hero from that list.
  The component might be a class like this:

+makeExample('architecture/dart/lib/hero_list_component.dart', 'class', 'lib/hero_list_component.dart')
:marked
  Angular creates, updates, and destroys components as the user moves through the application.
  The developer can take action at each moment in this lifecycle through optional lifecycle hooks.
  <!-- PENDING: What was that supposed to link to? -->

.l-sub-section
  :marked
    We may wonder who is calling the component's constructor? Who provides the service parameter?
    For the moment, have faith that Angular will call the constructor and deliver an
    appropriate `HeroService` when we need it.

.l-main-section
<a id="template"></a>
:marked
  ## Templates
figure
  img(src="/resources/images/devguide/architecture/template.png" alt="Template" align="left" style="width:200px; margin-left:-40px;margin-right:10px" )
:marked
  We define a component's view with its companion **template**. A template is a form of HTML
  that tells Angular how to render the component.

  A template looks like regular HTML much of the time ... and then it gets a bit strange. Here is a
  template for our `HeroListComponent`:
+makeExample('architecture/dart/lib/hero_list_component.html', null, 'lib/hero_list_component.html')
:marked
  This template features typical HTML elements like `<h2>` and  `<div>`.
  But what are `*ngFor`, <code>{&zwnj;{hero.name}}</code>, `(click)`, `[hero]`, and `<hero-detail>`?
  They're examples of Angular's template syntax. <!-- TODO: link to template-syntax.html -->
  We'll grow accustomed to that syntax and may even learn to love it.

  Take a look at the last line,
  which has the `<hero-detail>` tag.
  That tag adds a custom element representing a component we haven't seen yet,
  a `HeroDetailComponent`.

  The `HeroDetailComponent` is a *different* component than the `HeroListComponent` we've seen.
  The `HeroDetailComponent` (code not shown) presents facts about a particular hero, the
  hero that the user selects from the list presented by the `HeroListComponent`.
  The `HeroDetailComponent` is a **child** of the `HeroListComponent`.

figure
  img(src="/resources/images/devguide/architecture/component-tree.png" alt="Metadata" align="left" style="width:300px; margin-left:-40px;margin-right:10px" )
:marked
  Notice how `<hero-detail>` rests comfortably among native HTML elements.
  We can and _will_ mix our custom components with native HTML in the same layouts.

  In this manner we'll compose complex component trees to build out our richly featured application.
<br clear="all">

.l-main-section
<a id="metadata"></a>
:marked
  ## Metadata
figure
  img(src="/resources/images/devguide/architecture/metadata.png" alt="Metadata" align="left" style="width:150px; margin-left:-40px;margin-right:10px" )
:marked
<p style="padding-top:10px">Metadata tells Angular how to process a class.</p>
<br clear="all">
:marked
  [Looking back at the code](#component-code) for `HeroListComponent`, we see that it's just a class.
  There is no evidence of a framework, no "Angular" in it at all.

  In fact, it really is *just a class*. It's not a component until we *tell Angular about it*.

  We tell Angular that `HeroListComponent` is a component by attaching **metadata** to the class.

  In Dart, we attach metadata by using an **annotation**.
  Here's some metadata for `HeroListComponent`:

+makeExample('architecture/dart/lib/hero_list_component.dart', 'metadata', 'lib/hero_list_component.dart')
:marked
  Here we see the `@Component` annotation, which (no surprise) identifies the class
  immediately below it as a component class.

  Annotations often have configuration parameters.
  The `@Component` annotation takes parameters to provide the
  information Angular needs to create and present the component and its view.

  Here we see a few of the possible `@Component` parameters:

  * `selector`: A CSS selector that tells Angular to create and insert an instance of this component
  where it finds a `<hero-list>` tag in *parent* HTML.
  For example, if an app's  HTML contains `<hero-list></hero-list>`, then
  Angular inserts an instance of the `HeroListComponent` view between those tags.

  * `templateUrl`: The address of this component's template, which we showed [above](#the-template).

  * `directives`: An array of the components or directives that *this* template requires.
  We saw in the last line of our template that we expect Angular to insert a `HeroDetailComponent`
  in the space indicated by `<hero-detail>` tags.
  Angular will do so only if we mention the `HeroDetailComponent` in this `directives` array.

  * `providers`: An array of **dependency injection providers** for services that the component requires.
  This is one way to tell Angular that our component's constructor requires a `HeroService`
  so it can get the list of heroes to display. We'll get to dependency injection later.
figure
  img(src="/resources/images/devguide/architecture/template-metadata-component.png" alt="Metadata" align="left" style="height:200px; margin-left:-40px;margin-right:10px" )

:marked
  At runtime, Angular discovers the metadata specified by the `@Component`
  annotation. That's how Angular learns how to do "the right thing".

  The template, metadata, and component together describe the view.

  We apply other metadata annotations in a similar fashion to guide Angular behavior.
  `@Injectable`, `@Input`, `@Output`, and `@RouterConfig` are a few of the more popular annotations
  we'll master as our Angular knowledge grows.
<br clear="all">
:marked
  The architectural takeaway is that we must add metadata to our code
  so that Angular knows what to do.

.l-main-section
<a id="data-binding"></a>
:marked
  ## Data binding
  Without a framework, we would be responsible for pushing data values into the HTML controls and turning user responses
  into actions and value updates. Writing such push/pull logic by hand is tedious, error-prone, and a nightmare to
  read as any experienced jQuery programmer can attest.
figure
  img(src="/resources/images/devguide/architecture/databinding.png" alt="Data Binding" style="width:220px; float:left; margin-left:-40px;margin-right:20px" )
:marked
  Angular supports **data binding**,
  a mechanism for coordinating parts of a template with parts of a component.
  We add binding markup to the template HTML to tell Angular how to connect both sides.

  There are four forms of data binding syntax. Each form has a direction — to the DOM, from the DOM, or in both directions —
  as indicated by the arrows in the diagram.
<br clear="all">
:marked
  We saw three forms of data binding in our [example](#template) template:
+makeExample('architecture/dart/lib/hero_list_component_1.html', 'binding')(format=".")
:marked
  * The <code>{&zwnj;{hero.name}}</code> [interpolation](displaying-data.html#interpolation)
  displays the component's `hero.name` property value within the `<div>` tags.

  * The `[hero]` property binding <!-- TODO: link to template-syntax.html#property-binding-->
  passes the value of `selectedHero` from
  the parent `HeroListComponent` to the `hero` property of the child `HeroDetailComponent`.

  * The `(click)` [event binding](user-input.html#click) calls the component's `selectHero` method when the user clicks a hero's name.

  **Two-way data binding** is an important fourth form
  that combines property and event binding in a single notation, using the `ngModel` directive.
  We didn't have a two-way binding in the `HeroListComponent` template;
  here's an example from the `HeroDetailComponent` template:

+makeExample('architecture/dart/lib/hero_detail_component.html', 'ng-model', 'lib/hero_detail_component.html (excerpt)')(format=".")
:marked
  In two-way binding, a data property value flows to the input box from the component as with property binding.
  The user's changes also flow back to the component, resetting the property to the latest value,
  as with event binding.

  Angular processes *all* data bindings once per JavaScript event cycle,
  depth-first from the root of the application component tree.
  <!-- PENDING: clarify what "depth-first from the root" really means,
  or reassure that they'll learn it soon. -->
figure
  img(src="/resources/images/devguide/architecture/component-databinding.png" alt="Data Binding" style="float:left; width:300px; margin-left:-40px;margin-right:10px" )
:marked
  We don't know all the details yet,
  but it's clear from these examples that data binding plays an important role in communication
  between a template and its component.
<br clear="all">
figure
  img(src="/resources/images/devguide/architecture/parent-child-binding.png" alt="Parent/Child binding" style="float:left; width:300px; margin-left:-40px;margin-right:10px" )
:marked
  Data binding is also important for communication between parent and child components.
<br clear="all">

.l-main-section
<a id="directive"></a>
:marked
  ## Directives
figure
  img(src="/resources/images/devguide/architecture/directive.png" alt="Parent child" style="float:left; width:150px; margin-left:-40px;margin-right:10px" )
:marked
  Angular templates are *dynamic*. When Angular renders them, it transforms the DOM
  according to the instructions given by **directives**.

  A directive is a class with directive metadata. In Dart we apply the `@Directive` annotation
  to attach metadata to the class.
<br clear="all">
:marked
  We already met one form of directive: the component. A component is a *directive-with-a-template*;
  a `@Component` annotation is actually a `@Directive` annotation extended with template-oriented features.

.l-sub-section
  :marked
    While **a component is technically a directive**,
    components are so distinctive and central to Angular applications that we chose
    to separate components from directives in this architectural overview.
:marked
  Two *other* kinds of directives exist: _structural_ and _attribute_ directives.

  They tend to appear within an element tag like attributes,
  sometimes by name but more often as the target of an assignment or a binding.

  **Structural** directives alter layout by adding, removing, and replacing elements in DOM.

  Our [example](#template) template uses two built-in structural directives:
+makeExample('architecture/dart/lib/hero_list_component_1.html', 'structural')(format=".")
:marked
  * [`*ngFor`](displaying-data.html#ng-for) tells Angular to stamp out one `<div>` per hero in the `heroes` list.
  * [`*ngIf`](displaying-data.html#ng-if) includes the `HeroDetail` component only if a selected hero exists.

.l-sub-section
  :marked
    In Dart, **the only value that is true is the boolean value `true`**; all
    other values are false. JavaScript and TypeScript, in contrast, treat values
    such as 1 and most non-null objects as true. For this reason, the JavaScript
    and TypeScript versions of this app can use just `selectedHero` as the value
    of the `*ngIf` expression. The Dart version must use a boolean operator such
    as `!=` instead.

:marked
  **Attribute** directives alter the appearance or behavior of an existing element.
  In templates they look like regular HTML attributes, hence the name.

  The `ngModel` directive, which implements two-way data binding, is
  an example of an attribute directive. `ngModel` modifies the behavior of
  an existing element (typically an `<input>`)
  by setting its display value property and responding to change events.

+makeExample('architecture/dart/lib/hero_detail_component.html', 'ng-model')(format=".")
:marked
  Angular ships with a small number of other directives that either alter the layout structure
  (for example, `ngSwitch`) <!-- TODO: link to template-syntax.html#ng-switch -->
  or modify aspects of DOM elements and components
  (for example, `ngStyle` and `ngClass`).
  <!-- PENDING: link to template-syntax.html#ng-style template-syntax.html#ng-class-->

  Of course, we can also write our own directives. Components such as
  `HeroListComponent` are one kind of custom directive.
  <!-- PENDING: link to where to learn more about other kinds! -->

.l-main-section
<a id="service"></a>
:marked
  ## Services
figure
  img(src="/resources/images/devguide/architecture/service.png" alt="Service" style="float:left; margin-left:-40px;margin-right:10px" )
:marked
  _Services_ is a broad category encompassing any value, function, or feature that our application needs.

  Almost anything can be a service.
  A service is typically a class with a narrow, well-defined purpose. It should do something specific and do it well.
<br clear="all">
:marked
  Examples include:
  * logging service
  * data service
  * message bus
  * tax calculator
  * application configuration

  There is nothing specifically _Angular_ about services. Angular itself has no definition of a service.
  There is no service base class, and no place to register a service.

  Yet services are fundamental to any Angular application. Our components are big consumers of services.

  We prefer our component classes lean. Our components don't fetch data from the server,
  they don't validate user input, and they don't log directly to console. They delegate such tasks to services.

  A component's job is to enable the user experience and nothing more. It mediates between the view (rendered by the template)
  and the application logic (which often includes some notion of a _model_).
  A good component presents properties and methods for data binding.
  It delegates everything nontrivial to services.

  Angular doesn't *enforce* these principles.
  It won't complain if we write a "kitchen sink" component with 3000 lines.

  Angular does help us *follow* these principles by making it easy to factor our
  application logic into services and make those services available to components through *dependency injection*.

.l-main-section
<a id="dependency-injection"></a>
:marked
  ## Dependency injection
figure
  img(src="/resources/images/devguide/architecture/dependency-injection.png" alt="Service" style="float:left; width:200px; margin-left:-40px;margin-right:10px" )
:marked
  Dependency injection is a way to supply a new instance of a class
  with the fully-formed dependencies it requires. Most dependencies are services.
  Angular uses dependency injection to provide new components with the services they need.
<br clear="all">
:marked
  Angular can tell which services a component needs by looking at the types of its constructor parameters.
  For example, the constructor of our `HeroListComponent` needs a `HeroService`:
+makeExample('architecture/dart/lib/hero_list_component.dart', 'ctor', 'lib/hero_list_component.dart (excerpt)')(format='.')
:marked
  When Angular creates a component, it first asks an **injector** for
  the services that the component requires.

  An injector maintains a container of service instances that it has previously created.
  If a requested service instance is not in the container, the injector makes one and adds it to the container
  before returning the service to Angular.
  When all requested services have been resolved and returned,
  Angular can call the component's constructor with those services as arguments.
  This is what we mean by *dependency injection*.

  The process of `HeroService` injection looks a bit like this:
figure
  img(src="/resources/images/devguide/architecture/injector-injects.png" alt="Service" )
:marked
  If the injector doesn't have a `HeroService`, how does it know how to make one?

  In brief, we must have previously registered a **provider** of the `HeroService` with the injector.
  A provider is something that can create or return a service, typically  the service class itself.

  We can register providers at any level of the application component tree.
  We often do so at the root when we bootstrap the application so that
  the same instance of a service is available everywhere.
+makeExample('architecture/dart/web/main.dart', 'bootstrap', 'web/main.dart (excerpt)')(format='.')
:marked
  Alternatively, we might register at a component level:
+makeExample('architecture/dart/lib/hero_list_component.dart', 'providers', 'lib/hero_list_component.dart (excerpt)')(format='.')
:marked
  Registering at a component level means we get a new instance of the
  service with each new instance of that component.

  <!-- We've vastly oversimplified dependency injection for this overview.
  The full story is in the [Dependency Injection](dependency-injection.html) chapter. -->

  Points to remember about dependency injection:

  * Dependency injection is wired into the Angular framework and used everywhere.

  * The *injector* is the main mechanism.
    * An injector maintains a *container* of service instances that it created.
    * An injector can create a new service instance from a *provider*.

  * A *provider* is a recipe for creating a service.

  * We register *providers* with injectors.

<a id="other-stuff"></a>
.l-main-section
:marked
  ## Other stuff

  We've learned just a bit about the seven main building blocks of an Angular application:
  1. [Components](#component)
  1. [Templates](#template)
  1. [Metadata](#metadata)
  1. [Data binding](#data-binding)
  1. [Directives](#directive)
  1. [Services](#service)
  1. [Dependency injection](#dependency-injection)

  That's a foundation for everything else in an Angular application,
  and it's more than enough to get going.
  But it doesn't include everything we'll need or want to know.

  Here is a brief, alphabetical list of other important Angular features and services.
  Most of them are covered in this Developers Guide (or soon will be).

  >**Animations:** A forthcoming animation library makes it easy for developers to animate component behavior
  without deep knowledge of animation techniques or CSS.

  >**Bootstrap:** A method to configure and launch the root application component.

  >**Change detection:** Learn how Angular decides that a component property value has changed and
  when to update the screen.
  Learn how it uses **zones** to intercept asynchronous activity and run its change detection strategies.

  >**Component router:** With the component Router service, users can navigate a multi-screen application
  in a familiar web browsing style using URLs.

  >**Events:** The DOM raises events. So can components and services. Angular offers mechanisms for
  publishing and subscribing to events including an implementation of the [RxJS Observable](https://github.com/zenparsing/es-observable) proposal.

  >**[Forms](forms.html):** Support complex data entry scenarios with HTML-based validation and dirty checking.

  >**HTTP:** Communicate with a server to get data, save data, and invoke server-side actions with this Angular HTTP client.

  >**Lifecycle hooks:** We can tap into key moments in the lifetime of a component, from its creation to its destruction,
  by implementing the lifecycle hook interfaces.

  >**Pipes:** Services that transform values for display.
  We can put pipes in our templates to improve the user experience. Consider
  this `currency` pipe expression:
<div style="margin-left:40px">
code-example(language="javascript" linenumbers=".").
  price | currency:'USD':true'
</div>
:marked
  >It displays a price of "42.33" as `$42.33`.

  >**Testing:** Angular provides a
  [testing library](https://pub.dartlang.org/packages/angular2_testing)
  to run unit tests on our application parts as they interact with the Angular framework.
