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# Introduction to components
<img src="generated/images/guide/architecture/hero-component.png" alt="Component" class="left">
A _component_ controls a patch of screen called a *view*. For example, individual components define and control each of the following views from the Tutorial:
* The app root with the navigation links.
* The list of heroes.
* The hero editor.
You define a component's application logic&mdash;what it does to support the view&mdash;inside a class. The class interacts with the view through an API of properties and methods.
For example, the `HeroListComponent` has a `heroes` property that returns an array of heroes that it acquires from a service. `HeroListComponent` also has a `selectHero()` method that sets a `selectedHero` property when the user clicks to choose a hero from that list.
<code-example path="architecture/src/app/hero-list.component.ts" linenums="false" title="src/app/hero-list.component.ts (class)" region="class"></code-example>
Angular creates, updates, and destroys components as the user moves through the application. Your app can take action at each moment in this lifecycle through optional [lifecycle hooks](guide/lifecycle-hooks), like `ngOnInit()`.
<hr/>
## Component metadata
<img src="generated/images/guide/architecture/metadata.png" alt="Metadata" class="left">
The `@Component` decorator identifies the class immediately below it as a component class, and specifies its metadata. In the example code below, you can see that `HeroListComponent` is just a class, with no special Angular notation or syntax at all. It's not a component until mark it as one with the `@Component` decorator.
The metadata for a component tells Angular where to get the major building blocks it needs to create and present the component and its view. In particular, it associates a _template_ with the component, either directly with inline code, or by reference. Together, the component and its template describe a _view_.
In addition to containing or pointing to the template, the `@Component` metadata configures, for example, how the component can be referenced in HTML and what services it requires.
Here's an example of basic metadata for `HeroListComponent`:
<code-example path="architecture/src/app/hero-list.component.ts" linenums="false" title="src/app/hero-list.component.ts (metadata)" region="metadata"></code-example>
This example shows some of the most useful `@Component` configuration options:
* `selector`: A CSS selector that tells Angular to create and insert an instance of this component wherever it finds the corresponding tag in template HTML. For example, if an app's HTML contains `<app-hero-list></app-hero-list>`, then
Angular inserts an instance of the `HeroListComponent` view between those tags.
* `templateUrl`: The module-relative address of this component's HTML template. Alternatively, you can provide the HTML template inline, as the value of the `template` property. This template defines the component's _host view_.
* `providers`: An array of **dependency injection providers** for services that the component requires. In the example, this tells Angular that the component's constructor requires a `HeroService` instance
in order to get the list of heroes to display.
<hr/>
## Templates and views
<img src="generated/images/guide/architecture/template.png" alt="Template" class="left">
You define a component's view with its companion template. A template is a form of HTML that tells Angular how to render the component.
Components and their associated views are typically arranged hierarchically, allowing you to modify or show and hide entire UI sections or pages as a unit. The template immediately associated with a component defines that component's _host view_. The template can also define a _view hierarchy_, which can contain _embedded views_, as well as views hosted by other components.
<figure>
<img src="generated/images/guide/architecture/component-tree.png" alt="Component tree" class="left">
</figure>
A view hierarchy can include views from the component's own child components, but it also can (and often does) include views from components that are defined in different NgModules.
## Template syntax
A template looks like regular HTML, except that it also contains Angular [template syntax](guide/template-syntax), which alters the HTML before the view is rendered, based on your app's logic and the state of app and DOM data. Your template can use _data binding_ to coordinate the app and DOM data, _pipes_ to transform data before it is displayed, and _directives_ to apply app logic to what gets displayed.
For example, here is a template for the Tutorial's `HeroListComponent`:
<code-example path="architecture/src/app/hero-list.component.html" title="src/app/hero-list.component.html"></code-example>
This template uses typical HTML elements like `<h2>` and `<p>`, and also includes Angular template-syntax elements, `*ngFor`, `{{hero.name}}`, `(click)`, `[hero]`, and `<app-hero-detail>`. The template-syntax elements tell Angular how to render the HTML to the screen, using program logic and data.
* The `*ngFor` directive tells Angular to iterate over a list.
* The `{{hero.name}}`, `(click)`, and `[hero]` bind program data to and from the DOM, responding to user input. See more about [data binding](#data-binding) below.
* The `<app-hero-detail>` tag in the example is a custom element that represents a new component, `HeroDetailComponent`. The `HeroDetailComponent` (code not shown) is a child component of the `HeroListComponent` that defines the Hero-detail view. Notice how custom components like this mix seamlessly with native HTML in the same layouts.
### Data binding
Without a framework, you 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.
Angular supports *two-way data binding*, a mechanism for coordinating parts of a template with parts of a component. Add binding markup to the template HTML to tell Angular how to connect both sides.
The following diagram shows the four forms of data binding markup. Each form has a direction&mdash;to the DOM, from the DOM, or in both directions.
<figure>
<img src="generated/images/guide/architecture/databinding.png" alt="Data Binding" class="left">
</figure>
This example from the `HeroListComponent` template uses three of these forms:
<code-example path="architecture/src/app/hero-list.component.1.html" linenums="false" title="src/app/hero-list.component.html (binding)" region="binding"></code-example>
* The `{{hero.name}}` [*interpolation*](guide/displaying-data#interpolation)
displays the component's `hero.name` property value within the `<li>` element.
* The `[hero]` [*property binding*](guide/template-syntax#property-binding) passes the value of `selectedHero` from
the parent `HeroListComponent` to the `hero` property of the child `HeroDetailComponent`.
* The `(click)` [*event binding*](guide/user-input#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. Here's an example from the `HeroDetailComponent` template:
<code-example path="architecture/src/app/hero-detail.component.html" linenums="false" title="src/app/hero-detail.component.html (ngModel)" region="ngModel"></code-example>
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,
from the root of the application component tree through all child components.
<figure>
<img src="generated/images/guide/architecture/component-databinding.png" alt="Data Binding" class="left">
</figure>
Data binding plays an important role in communication between a template and its component, and is also important for communication between parent and child components.
<figure>
<img src="generated/images/guide/architecture/parent-child-binding.png" alt="Parent/Child binding" class="left">
</figure>
### Pipes
Angular pipes let you declare display-value transformations in your template HTML. A class with the `@Pipe` decorator defines a function that transforms input values to output values for display in a view.
Angular defines various pipes, such as the [Date](https://angular.io/api/common/DatePipe) pipe and [Currency](https://angular.io/api/common/CurrencyPipe) pipe; for a complete list, see the [Pipes API list](https://angular.io/api?type=pipe). You can also define new pipes.
To specify a value transformation in an HTML template, use the [pipe operator (|)](https://angular.io/guide/template-syntax#pipe):
`{{interpolated_value | pipe_name}}`
You can chain pipes, sending the output of one pipe function to be transformed by another pipe function.
<hr/>
### Directives
<img src="generated/images/guide/architecture/directive.png" alt="Directives" class="left">
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 a `@Directive` decorator.
A component is technically a directive - but components are so distinctive and central to Angular applications that Angular defines the `@Component` decorator, which extends the `@Directive` decorator with template-oriented features.
There are two kinds of directives besides components: _structural_ and _attribute_ directives. Just as for components, the metadata for a directive associates the class with a `selector` that you use to insert it into HTML. In templates, directives typically appear within an element tag as attributes, either by name or as the target of an assignment or a binding.
#### Structural directives
Structural directives alter layout by adding, removing, and replacing elements in DOM. The example template uses two built-in structural directives to add application logic to how the view is rendered:
<code-example path="architecture/src/app/hero-list.component.1.html" linenums="false" title="src/app/hero-list.component.html (structural)" region="structural"></code-example>
* [`*ngFor`](guide/displaying-data#ngFor) is an iterative; it tells Angular to stamp out one `<li>` per hero in the `heroes` list.
* [`*ngIf`](guide/displaying-data#ngIf) is a conditional; it includes the `HeroDetail` component only if a selected hero exists.
#### Attribute directives
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.
<code-example path="architecture/src/app/hero-detail.component.html" linenums="false" title="src/app/hero-detail.component.html (ngModel)" region="ngModel"></code-example>
Angular has more pre-defined directives that either alter the layout structure
(for example, [ngSwitch](guide/template-syntax#ngSwitch))
or modify aspects of DOM elements and components
(for example, [ngStyle](guide/template-syntax#ngStyle) and [ngClass](guide/template-syntax#ngClass)).
You can also write your own directives. Components such as `HeroListComponent` are one kind of custom directive. You can also create custom structural and attribute directives.
<!-- PENDING: link to where to learn more about other kinds! -->

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# Introduction to modules
<img src="generated/images/guide/architecture/module.png" alt="Module" class="left">
Angular apps are modular and Angular has its own modularity system called _NgModules_. An NgModule is a container for a cohesive block of code dedicated to an application domain, a workflow, or a closely related set of capabilities. It can contain components, service providers, and other code files whose scope is defined by the containing NgModule. It can import functionality that is exported from other NgModules, and export selected functionality for use by other NgModules.
Every Angular app has at least one NgModule class, [the _root module_](guide/bootstrapping), which is conventionally named `AppModule` and resides in a file named `app.module.ts`. You launch your app by *bootstrapping* the root NgModule.
While a small application might have only one NgModule, most apps have many more _feature modules_. The _root_ NgModule for an app is so named because it can include child NgModules in a hierarchy of any depth.
## NgModule metadata
An NgModule is defined as a class decorated with `@NgModule`. The `@NgModule` decorator is a function that takes a single metadata object, whose properties describe the module. The most important properties are as follows.
* `declarations`&mdash;The [components](guide/architecture-components), _directives_, and _pipes_ that belong to this NgModule.
* `exports`&mdash;The subset of declarations that should be visible and usable in the _component templates_ of other NgModules.
* `imports`&mdash;Other modules whose exported classes are needed by component templates declared in _this_ NgModule.
* `providers`&mdash;Creators of [services](guide/architecture-services) that this NgModule contributes to the global collection of services; they become accessible in all parts of the app.
* `bootstrap`&mdash;The main application view, called the _root component_, which hosts all other app views. Only the _root NgModule_ should set this `bootstrap` property.
Here's a simple root NgModule definition:
<code-example path="architecture/src/app/mini-app.ts" region="module" title="src/app/app.module.ts" linenums="false"></code-example>
<div class="l-sub-section">
The `export` of `AppComponent` is just to show how to export; it isn't actually necessary in this example. A root NgModule has no reason to _export_ anything because other components don't need to _import_ the root NgModule.
</div>
## NgModules and components
NgModules provide a _compilation context_ for their components. An NgModule has a root component, created during bootstrap, and any number of additional components that can be loaded (through the router) or created (through the template). The components that belong to an NgModule share a compilation context.
<figure>
<img src="generated/images/guide/architecture/compilation-context.png" alt="Component compilation context" class="left">
</figure>
<br class="clear">
A component and its template together define a _view_. A component can contain a _view hierarchy_, which allows you to define arbitrarily complex areas of the screen that can be created, modified, and destroyed as a unit. A view hierarchy can mix views defined in components that belong to different NgModules. This is often the case, especially for UI libraries.
<figure>
<img src="generated/images/guide/architecture/view-hierarchy.png" alt="View hierarchy" class="left">
</figure>
<br class="clear">
When you create a component, it is associated directly with a single view, called the _host view_. The host view can be the root of a view hierarchy, which can contain _embedded views_, which are in turn the host views of other components. Those components can be in the same NgModule, or can be imported from other NgModules. Views in the tree can be nested to any depth.
<div class="l-sub-section">
The hierarchical structure of views is a key factor in the way Angular detects and responds to changes in the DOM and app data.
</div>
## NgModules and JavaScript modules
The NgModule system is different from and unrelated to the JavaScript (ES2015) module system for managing collections of JavaScript objects. These are two different and _complementary_ module systems. You can use them both to write your apps.
In JavaScript each _file_ is a module and all objects defined in the file belong to that module.
The module declares some objects to be public by marking them with the `export` key word.
Other JavaScript modules use *import statements* to access public objects from other modules.
<code-example path="architecture/src/app/app.module.ts" region="imports" linenums="false"></code-example>
<code-example path="architecture/src/app/app.module.ts" region="export" linenums="false"></code-example>
<div class="l-sub-section">
<a href="http://exploringjs.com/es6/ch_modules.html">Learn more about the JavaScript module system on the web.</a>
</div>
## Angular libraries
<img src="generated/images/guide/architecture/library-module.png" alt="Component" class="left">
Angular ships as a collection of JavaScript modules. You can think of them as library modules. Each Angular library name begins with the `@angular` prefix. Install them with the `npm` package manager and import parts of them with JavaScript `import` statements.
<br class="clear">
For example, import Angular's `Component` decorator from the `@angular/core` library like this:
<code-example path="architecture/src/app/app.component.ts" region="import" linenums="false"></code-example>
You also import NgModules from Angular _libraries_ using JavaScript import statements:
<code-example path="architecture/src/app/mini-app.ts" region="import-browser-module" linenums="false"></code-example>
In the example of the simple root module above, the application module needs material from within the `BrowserModule`. To access that material, add it to the `@NgModule` metadata `imports` like this.
<code-example path="architecture/src/app/mini-app.ts" region="ngmodule-imports" linenums="false"></code-example>
In this way you're using both the Angular and JavaScript module systems _together_. Although it's easy to confuse the two systems, which share the common vocabulary of "imports" and "exports", you will become familiar with the different contexts in which they are used.
<div class="l-sub-section">
Learn more from the [NgModules](guide/ngmodules) page.
</div>
<hr/>

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# Next steps: tools and techniques
Once you have understood the basic building blocks, you can begin to learn more about the features and tools that are available to help you develop and deliver Angular applications. Angular provides a lot more features and services that are covered in this documentation.
#### Responsive programming tools
* [Lifecycle hooks](guide/lifecycle-hooks): Tap into key moments in the lifetime of a component, from its creation to its destruction, by implementing the lifecycle hook interfaces.
* [Observables and event processing](guide/observables): How to use observables with components and services to publish and subscribe to messages of any type, such as user-interaction events and asynchronous operation results.
#### Client-server interaction tools
* [HTTP](guide/http): Communicate with a server to get data, save data, and invoke server-side actions with an HTTP client.
* [Server-side Rendering](guide/universal): Angular Universal generates static application pages on the server through server-side rendering (SSR). This allows you to run your Angular app on the server in order to improve performance and show the first page quickly on mobile and low-powered devices, and also facilitate web crawlers.
* [Service Workers](guide/service-worker-intro): A service worker is a script that runs in the web browser and manages caching for an application. Service workers function as a network proxy. They intercept outgoing HTTP requests and can, for example, deliver a cached response if one is available. You can significantly improve the user experience by using a service worker to reduce dependency on the network.
#### Domain-specific libraries
* [Animations](guide/animations): Animate component behavior
without deep knowledge of animation techniques or CSS with Angular's animation library.
* [Forms](guide/forms): Support complex data entry scenarios with HTML-based validation and dirty checking.
#### Support for the development cycle
* [Testing Platform](guide/testing): Run unit tests on your application parts as they interact with the Angular framework.
* [Internationalization](guide/i18n): Angular's internationalization (i18n) tools can help you make your app available in multiple languages.
* [Compilation](guide/aot-compiler): Angular provides just-in-time (JIT) compilation for the development environment, and ahead-of-time (AOT) compilation for the production environment.
* [Security guidelines](guide/security): Learn about Angular's built-in protections against common web-app vulnerabilities and attacks such as cross-site scripting attacks.
#### Setup and deployment tools
* [Setup for local development](guide/setup): Learn how to set up a new project for development with QuickStart.
* [Installation](guide/npm-packages): The [Angular CLI](https://cli.angular.io/), Angular applications, and Angular itself depend on features and functionality provided by libraries that are available as [npm](https://docs.npmjs.com/) packages.
* [Typescript Configuration](guide/typescript-configuration): TypeScript is the primary language for Angular application development.
* [Browser support](guide/browser-support): Learn how to make your apps compatible across a wide range of browsers.
* [Deployment](guide/deployment): Learn techniques for deploying your Angular application to a remote server.
<hr/>

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# Introduction to services and dependency injection
<img src="generated/images/guide/architecture/service.png" alt="Service" class="left">
_Service_ is a broad category encompassing any value, function, or feature that an app needs. A service is typically a class with a narrow, well-defined purpose. It should do something specific and do it well.
<br class="clear">
Angular distinguishes components from services in order to increase modularity and reusability.
* By separating a component's view-related functionality from other kinds of processing, you can make your component classes lean and efficient. Ideally, a component's job is to enable the user experience and nothing more. It should present properties and methods for data binding, in order to mediate between the view (rendered by the template) and the application logic (which often includes some notion of a _model_).
* A component should not need to define things like how to fetch data from the server, validate user input, or log directly to the console. Instead, it can delegate such tasks to services. By defining that kind of processing task in an injectable service class, you make it available to any component. You can also make your app more adaptable by injecting different providers of the same kind of service, as appropriate in different circumstances.
Angular doesn't *enforce* these principles. Angular does help you *follow* these principles by making it easy to factor your
application logic into services and make those services available to components through *dependency injection*.
## Service examples
Here's an example of a service class that logs to the browser console:
<code-example path="architecture/src/app/logger.service.ts" linenums="false" title="src/app/logger.service.ts (class)" region="class"></code-example>
Here's a `HeroService` that uses a [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) to fetch heroes.
The `HeroService` depends on the `Logger` service and another `BackendService` that handles the server communication grunt work.
<code-example path="architecture/src/app/hero.service.ts" linenums="false" title="src/app/hero.service.ts (class)" region="class"></code-example>
<hr/>
## Dependency injection
<img src="generated/images/guide/architecture/dependency-injection.png" alt="Service" class="left">
Components consume services; that is, you can *inject* a service into a component, giving the component access to that service class. To define a class as a service in Angular, use the `@Injectable` decorator to provide the metadata that allows Angular to inject it into a component as a *dependency*. Most dependencies are services.
*Dependency injection* (often called DI) is wired into the Angular framework and used everywhere to provide new components with the services they need.
* The *injector* is the main mechanism. You don't have to create an Angular injector. Angular creates an application-wide injector for you during the bootstrap process.
* The injector maintains a *container* of service instances that it has already created, and reuses them if possible.
* A *provider* is a recipe for creating a service -- typically the service class itself. For any service you need in your app, you must register a provider with the app's injector, so that the injector can use it to create new service instances.
When Angular creates a new instance of a component class, it determines which services that component needs by looking at the types of its constructor parameters. For example, the constructor of `HeroListComponent` needs a `HeroService`:
<code-example path="architecture/src/app/hero-list.component.ts" linenums="false" title="src/app/hero-list.component.ts (constructor)" region="ctor"></code-example>
When Angular discovers that a component depends on a service, it first checks if the injector already has any existing instances of that service. If a requested service instance does not yet exist, the injector makes one using the registered provider, and adds it to the injector 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.
The process of `HeroService` injection looks something like this:
<figure>
<img src="generated/images/guide/architecture/injector-injects.png" alt="Service" class="left">
</figure>
### Providing services
You must register at least one *provider* of any service you are going to use. You can register providers in modules or in components.
* When you add providers to the [root module](guide/architecture-modules), the same instance of a service is available to all components in your app.
<code-example path="architecture/src/app/app.module.ts" linenums="false" title="src/app/app.module.ts (module providers)" region="providers"></code-example>
* When you register a provider at the component level, you get a new instance of the
service with each new instance of that component. At the component level, register a service provider in the `providers` property of the `@Component` metadata:
<code-example path="architecture/src/app/hero-list.component.ts" linenums="false" title="src/app/hero-list.component.ts (component providers)" region="providers"></code-example>
For more detailed information, see the [Dependency Injection](guide/dependency-injection) section.
<hr/>

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# Architecture Overview # Architecture overview
Angular is a framework for building client applications in HTML and Angular is a platform and framework for building client applications in HTML and TypeScript.
either JavaScript or a language like TypeScript that compiles to JavaScript. Angular is itself written in TypeScript. It implements core and optional functionality as a set of TypeScript libraries that you import into your apps.
The framework consists of several libraries, some of them core and some optional. The basic building blocks of an Angular application are _NgModules_, which provide a compilation context for _components_. NgModules collect related code into functional sets; an Angular app is defined by a set of NgModules. An app always has at least a _root module_ that enables bootstrapping, and typically has many more _feature modules_.
You write Angular applications by composing HTML *templates* with Angularized markup, * Components define *views*, which are sets of screen elements that Angular can choose among and modify according to your program logic and data. Every app has at least a root component.
writing *component* classes to manage those templates, adding application logic in *services*,
and boxing components and services in *modules*.
Then you launch the app by *bootstrapping* the _root module_. * Components use *services*, which provide specific functionality not directly related to views. Service providers can be *injected* into components as *dependencies*, making your code modular, reusable, and efficient.
Angular takes over, presenting your application content in a browser and
responding to user interactions according to the instructions you've provided.
Of course, there is more to it than this. Both components and services are simply classes, with *decorators* that mark their type and provide metadata that tells Angular how to use them.
You'll learn the details in the pages that follow. For now, focus on the big picture.
* The metadata for a component class associates it with a *template* that defines a view. A template combines ordinary HTML with Angular *directives* and *binding markup* that allow Angular to modify the HTML before rendering it for display.
* The metadata for a service class provides the information Angular needs to make it available to components through *Dependency Injection (DI)*.
An app's components typically define many views, arranged hierarchically. Angular provides the `Router` service to help you define navigation paths among views. The router provides sophisticated in-browser navigational capabilities.
## Modules
Angular defines the `NgModule`, which differs from and complements the JavaScript (ES2015) module. An NgModule declares a compilation context for a set of components that is dedicated to an application domain, a workflow, or a closely related set of capabilities. An NgModule can associate its components with related code, such as services, to form functional units.
Every Angular app has a _root module_, conventionally named `AppModule`, which provides the bootstrap mechanism that launches the application. An app typically contains many functional modules.
Like JavaScript modules, NgModules can import functionality from other NgModules, and allow their own functionality to be exported and used by other NgModules. For example, to use the router service in your app, you import the `Router` NgModule.
Organizing your code into distinct functional modules helps in managing development of complex applications, and in designing for reusability. In addition, this technique lets you take advantage of _lazy-loading_&mdash;that is, loading modules on demand&mdash;in order to minimize the amount of code that needs to be loaded at startup.
<div class="l-sub-section">
For a more detailed discussion, see [Introduction to modules](guide/architecture-modules).
</div>
## Components
Every Angular application has at least one component, the *root component* that connects a component hierarchy with the page DOM. Each component defines a class that contains application data and logic, and is associated with an HTML *template* that defines a view to be displayed in a target environment.
The `@Component` decorator identifies the class immediately below it as a component, and provides the template and related component-specific metadata.
<div class="l-sub-section">
Decorators are functions that modify JavaScript classes. Angular defines a number of such decorators that attach specific kinds of metadata to classes, so that it knows what those classes mean and how they should work.
<a href="https://medium.com/google-developers/exploring-es7-decorators-76ecb65fb841#.x5c2ndtx0">Learn more about decorators on the web.</a>
</div>
### Templates, directives, and data binding
A template combines HTML with Angular markup that can modify the HTML elements before they are displayed.
Template *directives* provide program logic, and *binding markup* connects your application data and the document object model (DOM).
* *Event binding* lets your app respond to user input in the target environment by updating your application data.
* *Property binding* lets you interpolate values that are computed from your application data into the HTML.
Before a view is displayed, Angular evaluates the directives and resolves the binding syntax in the template to modify the HTML elements and the DOM, according to your program data and logic. Angular supports *two-way data binding*, meaning that changes in the DOM, such as user choices, can also be reflected back into your program data.
Your templates can also use *pipes* to improve the user experience by transforming values for display. Use pipes to display, for example, dates and currency values in a way appropriate to the user's locale. Angular provides predefined pipes for common transformations, and you can also define your own.
<div class="l-sub-section">
For a more detailed discussion of these concepts, see [Introduction to components](guide/architecture-components).
</div>
{@a dependency-injection}
## Services and dependency injection
For data or logic that is not associated with a specific view, and that you want to share across components, you create a *service* class. A service class definition is immediately preceded by the `@Injectable` decorator. The decorator provides the metadata that allows your service to be *injected* into client components as a dependency.
*Dependency injection* (or DI) lets you keep your component classes lean and efficient. They don't fetch data from the server, validate user input, or log directly to the console; they delegate such tasks to services.
<div class="l-sub-section">
For a more detailed discusssion, see [Introduction to services and DI](guide/architecture-services).
</div>
### Routing
The Angular `Router` NgModule provides a service that lets you define a navigation path among the different application states and view hierarchies in your app. It is modeled on the familiar browser navigation conventions:
* Enter a URL in the address bar and the browser navigates to a corresponding page.
* Click links on the page and the browser navigates to a new page.
* Click the browser's back and forward buttons and the browser navigates backward and forward through the history of pages you've seen.
The router maps URL-like paths to views instead of pages. When a user performs an action, such as clicking a link, that would load a new page in the browser, the router intercepts the browser's behavior, and shows or hides view hierarchies.
If the router determines that the current application state requires particular functionality, and the module that defines it has not been loaded, the router can _lazy-load_ the module on demand.
The router interprets a link URL according to your app's view navigation rules and data state. You can navigate to new views when the user clicks a button, selects from a drop box, or in response to some other stimulus from any source. The Router logs activity in the browser's history journal, so the back and forward buttons work as well.
To define navigation rules, you associate *navigation paths* with your components. A path uses a URL-like syntax that integrates your program data, in much the same way that template syntax integrates your views with your program data. You can then apply program logic to choose which views to show or to hide, in response to user input and your own access rules.
<div class="l-sub-section">
For a more detailed discussion, see [Routing and navigation](guide/router).
</div>
<hr/>
## What's next
You've learned the basics about the main building blocks of an Angular application. The following diagram shows how these basic pieces are related.
<figure> <figure>
<img src="generated/images/guide/architecture/overview2.png" alt="overview"> <img src="generated/images/guide/architecture/overview2.png" alt="overview">
</figure> </figure>
<div class="l-sub-section"> * Together, a component and template define an Angular view.
* A decorator on a component class adds the metadata, including a pointer to the associated template.
* Directives and binding markup in a component's template modify views based on program data and logic.
* The dependency injector provides services to a component, such as the router service that lets you define navigation among views.
The code referenced on this page is available as a <live-example></live-example>. Each of these subjects is introduced in more detail in the following pages.
</div> * [Modules](guide/architecture-modules)
* [Components](guide/architecture-components)
## Modules * [Templates](guide/architecture-components#templates-and-views)
* [Metadata](guide/architecture-components#component-metadata)
<img src="generated/images/guide/architecture/module.png" alt="Component" class="left"> * [Data binding](guide/architecture-components#data-binding)
* [Directives](guide/architecture-components#directives)
* [Pipes](guide/architecture-components#pipes)
Angular apps are modular and Angular has its own modularity system called _NgModules_. * [Services and dependency injection](guide/architecture-services)
NgModules are a big deal.
This page introduces modules; the [NgModules](guide/ngmodules) pages
relating to NgModules covers them in detail.
<br class="clear">
Every Angular app has at least one NgModule class, [the _root module_](guide/bootstrapping "Bootstrapping"),
conventionally named `AppModule`.
While the _root module_ may be the only module in a small application, most apps have many more
_feature modules_, each a cohesive block of code dedicated to an application domain,
a workflow, or a closely related set of capabilities.
An NgModule, whether a _root_ or _feature_, is a class with an `@NgModule` decorator.
<div class="l-sub-section"> <div class="l-sub-section">
Decorators are functions that modify JavaScript classes. Note that the code referenced on these pages is available as a <live-example></live-example>.
Angular has many decorators that attach metadata to classes so that it knows
what those classes mean and how they should work.
<a href="https://medium.com/google-developers/exploring-es7-decorators-76ecb65fb841#.x5c2ndtx0">
Learn more</a> about decorators on the web.
</div> </div>
`NgModule` is a decorator function that takes a single metadata object whose properties describe the module. When you are familiar with these fundamental building blocks, you can explore them in more detail in the documentation. To learn about more tools and techniques that are available to help you build and deploy Angular applications, see [Next steps](guide/architecture-next-steps).
The most important properties are:
* `declarations` - the _view classes_ that belong to this module.
Angular has three kinds of view classes: [components](guide/architecture#components), [directives](guide/architecture#directives), and [pipes](guide/pipes).
* `exports` - the subset of declarations that should be visible and usable in the component [templates](guide/architecture#templates) of other modules.
* `imports` - other modules whose exported classes are needed by component templates declared in _this_ module.
* `providers` - creators of [services](guide/architecture#services) that this module contributes to
the global collection of services; they become accessible in all parts of the app.
* `bootstrap` - the main application view, called the _root component_,
that hosts all other app views. Only the _root module_ should set this `bootstrap` property.
Here's a simple root module:
<code-example path="architecture/src/app/mini-app.ts" region="module" title="src/app/app.module.ts" linenums="false"></code-example>
<div class="l-sub-section">
The `export` of `AppComponent` is just to show how to use the `exports` array to export a component; it isn't actually necessary in this example. A root module has no reason to _export_ anything because other components don't need to _import_ the root module.
</div> </div>
Launch an application by _bootstrapping_ its root module.
During development you're likely to bootstrap the `AppModule` in a `main.ts` file like this one.
<code-example path="architecture/src/main.ts" title="src/main.ts" linenums="false"></code-example>
### NgModules vs. JavaScript modules
The NgModule &mdash; a class decorated with `@NgModule` &mdash; is a fundamental feature of Angular.
JavaScript also has its own module system for managing collections of JavaScript objects.
It's completely different and unrelated to the NgModule system.
In JavaScript each _file_ is a module and all objects defined in the file belong to that module.
The module declares some objects to be public by marking them with the `export` key word.
Other JavaScript modules use *import statements* to access public objects from other modules.
<code-example path="architecture/src/app/app.module.ts" region="imports" linenums="false"></code-example>
<code-example path="architecture/src/app/app.module.ts" region="export" linenums="false"></code-example>
<div class="l-sub-section">
<a href="http://exploringjs.com/es6/ch_modules.html">Learn more about the JavaScript module system on the web.</a>
</div>
These are two different and _complementary_ module systems. Use them both to write your apps.
### Angular libraries
<img src="generated/images/guide/architecture/library-module.png" alt="Component" class="left">
Angular ships as a collection of JavaScript modules. You can think of them as library modules.
Each Angular library name begins with the `@angular` prefix.
You install them with the **npm** package manager and import parts of them with JavaScript `import` statements.
<br class="clear">
For example, import Angular's `Component` decorator from the `@angular/core` library like this:
<code-example path="architecture/src/app/app.component.ts" region="import" linenums="false"></code-example>
You also import NgModules from Angular _libraries_ using JavaScript import statements:
<code-example path="architecture/src/app/mini-app.ts" region="import-browser-module" linenums="false"></code-example>
In the example of the simple root module above, the application module needs material from within that `BrowserModule`. To access that material, add it to the `@NgModule` metadata `imports` like this.
<code-example path="architecture/src/app/mini-app.ts" region="ngmodule-imports" linenums="false"></code-example>
In this way you're using both the Angular and JavaScript module systems _together_.
It's easy to confuse the two systems because they share the common vocabulary of "imports" and "exports".
Hang in there. The confusion yields to clarity with time and experience.
<div class="l-sub-section">
Learn more from the [NgModules](guide/ngmodules) page.
</div>
<hr/>
## Components
<img src="generated/images/guide/architecture/hero-component.png" alt="Component" class="left">
A _component_ controls a patch of screen called a *view*.
For example, the following views are controlled by components:
* The app root with the navigation links.
* The list of heroes.
* The hero editor.
You define a component's application logic&mdash;what it does to support the view&mdash;inside a class.
The class interacts with the view through an API of properties and methods.
{@a component-code}
For example, this `HeroListComponent` has a `heroes` property that returns an array of heroes
that it acquires from a service.
`HeroListComponent` also has a `selectHero()` method that sets a `selectedHero` property when the user clicks to choose a hero from that list.
<code-example path="architecture/src/app/hero-list.component.ts" linenums="false" title="src/app/hero-list.component.ts (class)" region="class"></code-example>
Angular creates, updates, and destroys components as the user moves through the application.
Your app can take action at each moment in this lifecycle through optional [lifecycle hooks](guide/lifecycle-hooks), like `ngOnInit()` declared above.
<hr/>
## Templates
<img src="generated/images/guide/architecture/template.png" alt="Template" class="left">
You 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, except for a few differences. Here is a
template for our `HeroListComponent`:
<code-example path="architecture/src/app/hero-list.component.html" title="src/app/hero-list.component.html"></code-example>
Although this template uses typical HTML elements like `<h2>` and `<p>`, it also has some differences. Code like `*ngFor`, `{{hero.name}}`, `(click)`, `[hero]`, and `<app-hero-detail>` uses Angular's [template syntax](guide/template-syntax).
In the last line of the template, the `<app-hero-detail>` tag is a custom element that represents a new component, `HeroDetailComponent`.
The `HeroDetailComponent` is a *different* component than the `HeroListComponent` you've been reviewing.
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`.
<img src="generated/images/guide/architecture/component-tree.png" alt="Metadata" class="left">
Notice how `<app-hero-detail>` rests comfortably among native HTML elements. Custom components mix seamlessly with native HTML in the same layouts.
<hr class="clear"/>
## Metadata
<img src="generated/images/guide/architecture/metadata.png" alt="Metadata" class="left">
Metadata tells Angular how to process a class.
<br class="clear">
[Looking back at the code](guide/architecture#component-code) for `HeroListComponent`, you can see that it's just a class.
There is no evidence of a framework, no "Angular" in it at all.
In fact, `HeroListComponent` really is *just a class*. It's not a component until you *tell Angular about it*.
To tell Angular that `HeroListComponent` is a component, attach **metadata** to the class.
In TypeScript, you attach metadata by using a **decorator**.
Here's some metadata for `HeroListComponent`:
<code-example path="architecture/src/app/hero-list.component.ts" linenums="false" title="src/app/hero-list.component.ts (metadata)" region="metadata"></code-example>
Here is the `@Component` decorator, which identifies the class
immediately below it as a component class.
The `@Component` decorator takes a required configuration object with the
information Angular needs to create and present the component and its view.
Here are a few of the most useful `@Component` configuration options:
* `selector`: CSS selector that tells Angular to create and insert an instance of this component
where it finds a `<app-hero-list>` tag in *parent* HTML.
For example, if an app's HTML contains `<app-hero-list></app-hero-list>`, then
Angular inserts an instance of the `HeroListComponent` view between those tags.
* `templateUrl`: module-relative address of this component's HTML template, shown [above](guide/architecture#templates).
* `providers`: array of **dependency injection providers** for services that the component requires.
This is one way to tell Angular that the component's constructor requires a `HeroService`
so it can get the list of heroes to display.
<img src="generated/images/guide/architecture/template-metadata-component.png" alt="Metadata" class="left">
The metadata in the `@Component` tells Angular where to get the major building blocks you specify for the component.
The template, metadata, and component together describe a view.
Apply other metadata decorators in a similar fashion to guide Angular behavior.
`@Injectable`, `@Input`, and `@Output` are a few of the more popular decorators.
<br class="clear">
The architectural takeaway is that you must add metadata to your code
so that Angular knows what to do.
<hr/>
## Data binding
Without a framework, you 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.
<img src="generated/images/guide/architecture/databinding.png" alt="Data Binding" class="left">
Angular supports **data binding**,
a mechanism for coordinating parts of a template with parts of a component.
Add binding markup to the template HTML to tell Angular how to connect both sides.
As the diagram shows, there are four forms of data binding syntax. Each form has a direction &mdash; to the DOM, from the DOM, or in both directions.
<br class="clear">
The `HeroListComponent` [example](guide/architecture#templates) template has three forms:
<code-example path="architecture/src/app/hero-list.component.1.html" linenums="false" title="src/app/hero-list.component.html (binding)" region="binding"></code-example>
* The `{{hero.name}}` [*interpolation*](guide/displaying-data#interpolation)
displays the component's `hero.name` property value within the `<li>` element.
* The `[hero]` [*property binding*](guide/template-syntax#property-binding) passes the value of `selectedHero` from
the parent `HeroListComponent` to the `hero` property of the child `HeroDetailComponent`.
* The `(click)` [*event binding*](guide/user-input#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.
Here's an example from the `HeroDetailComponent` template:
<code-example path="architecture/src/app/hero-detail.component.html" linenums="false" title="src/app/hero-detail.component.html (ngModel)" region="ngModel"></code-example>
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,
from the root of the application component tree through all child components.
<figure>
<img src="generated/images/guide/architecture/component-databinding.png" alt="Data Binding">
</figure>
Data binding plays an important role in communication between a template and its component.
<figure>
<img src="generated/images/guide/architecture/parent-child-binding.png" alt="Parent/Child binding">
</figure>
Data binding is also important for communication between parent and child components.
<hr/>
## Directives
<img src="generated/images/guide/architecture/directive.png" alt="Parent child" class="left">
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 a `@Directive` decorator.
A component is a *directive-with-a-template*;
a `@Component` decorator is actually a `@Directive` decorator extended with template-oriented features.
<div class="l-sub-section">
While **a component is technically a directive**,
components are so distinctive and central to Angular applications that this architectural overview separates components from directives.
</div>
Two *other* kinds of directives exist: _structural_ and _attribute_ directives.
They tend to appear within an element tag as attributes do,
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.
The [example template](guide/architecture#templates) uses two built-in structural directives:
<code-example path="architecture/src/app/hero-list.component.1.html" linenums="false" title="src/app/hero-list.component.html (structural)" region="structural"></code-example>
* [`*ngFor`](guide/displaying-data#ngFor) tells Angular to stamp out one `<li>` per hero in the `heroes` list.
* [`*ngIf`](guide/displaying-data#ngIf) includes the `HeroDetail` component only if a selected hero exists.
**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.
<code-example path="architecture/src/app/hero-detail.component.html" linenums="false" title="src/app/hero-detail.component.html (ngModel)" region="ngModel"></code-example>
Angular has a few more directives that either alter the layout structure
(for example, [ngSwitch](guide/template-syntax#ngSwitch))
or modify aspects of DOM elements and components
(for example, [ngStyle](guide/template-syntax#ngStyle) and [ngClass](guide/template-syntax#ngClass)).
Of course, you can also write your own directives. Components such as
`HeroListComponent` are one kind of custom directive.
<!-- PENDING: link to where to learn more about other kinds! -->
<hr/>
## Services
<img src="generated/images/guide/architecture/service.png" alt="Service" class="left">
_Service_ is a broad category encompassing any value, function, or feature that your 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 class="clear">
Examples include:
* logging service
* data service
* message bus
* tax calculator
* application configuration
There is nothing specifically _Angular_ about services. Angular 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. Components are big consumers of services.
Here's an example of a service class that logs to the browser console:
<code-example path="architecture/src/app/logger.service.ts" linenums="false" title="src/app/logger.service.ts (class)" region="class"></code-example>
Here's a `HeroService` that uses a [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) to fetch heroes.
The `HeroService` depends on the `Logger` service and another `BackendService` that handles the server communication grunt work.
<code-example path="architecture/src/app/hero.service.ts" linenums="false" title="src/app/hero.service.ts (class)" region="class"></code-example>
Services are everywhere.
Component classes should be lean. They don't fetch data from the server,
validate user input, or log directly to the 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 you write a "kitchen sink" component with 3000 lines.
Angular does help you *follow* these principles by making it easy to factor your
application logic into services and make those services available to components through *dependency injection*.
<hr/>
## Dependency injection
<img src="generated/images/guide/architecture/dependency-injection.png" alt="Service" class="left">
_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 class="clear">
Angular can tell which services a component needs by looking at the types of its constructor parameters.
For example, the constructor of your `HeroListComponent` needs a `HeroService`:
<code-example path="architecture/src/app/hero-list.component.ts" linenums="false" title="src/app/hero-list.component.ts (constructor)" region="ctor"></code-example>
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 *dependency injection*.
The process of `HeroService` injection looks a bit like this:
<figure>
<img src="generated/images/guide/architecture/injector-injects.png" alt="Service">
</figure>
If the injector doesn't have a `HeroService`, how does it know how to make one?
In brief, you 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.
You can register providers in modules or in components.
In general, add providers to the [root module](guide/architecture#modules) so that
the same instance of a service is available everywhere.
<code-example path="architecture/src/app/app.module.ts" linenums="false" title="src/app/app.module.ts (module providers)" region="providers"></code-example>
Alternatively, register at a component level in the `providers` property of the `@Component` metadata:
<code-example path="architecture/src/app/hero-list.component.ts" linenums="false" title="src/app/hero-list.component.ts (component providers)" region="providers"></code-example>
Registering at a component level means you 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](guide/dependency-injection) page. -->
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.
* Register *providers* with injectors.
<hr/>
## Wrap up
You've learned the basics about the eight main building blocks of an Angular application:
* [Modules](guide/architecture#modules)
* [Components](guide/architecture#components)
* [Templates](guide/architecture#templates)
* [Metadata](guide/architecture#metadata)
* [Data binding](guide/architecture#data-binding)
* [Directives](guide/architecture#directives)
* [Services](guide/architecture#services)
* [Dependency injection](guide/architecture#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 you need to know.
Here is a brief, alphabetical list of other important Angular features and services.
Most of them are covered in this documentation (or soon will be).
> [**Animations**](guide/animations): Animate component behavior
without deep knowledge of animation techniques or CSS with Angular's animation library.
> **Change detection**: The change detection documentation will cover how Angular decides that a component property value has changed,
when to update the screen, and how it uses **zones** to intercept asynchronous activity and run its change detection strategies.
> **Events**: The events documentation will cover how to use components and services to raise events with mechanisms for
publishing and subscribing to events.
> [**Forms**](guide/forms): Support complex data entry scenarios with HTML-based validation and dirty checking.
> [**HTTP**](guide/http): Communicate with a server to get data, save data, and invoke server-side actions with an HTTP client.
> [**Lifecycle hooks**](guide/lifecycle-hooks): Tap into key moments in the lifetime of a component, from its creation to its destruction,
by implementing the lifecycle hook interfaces.
> [**Pipes**](guide/pipes): Use pipes in your templates to improve the user experience by transforming values for display. Consider this `currency` pipe expression:
>
> > `price | currency:'USD':true`
>
> It displays a price of 42.33 as `$42.33`.
> [**Router**](guide/router): Navigate from page to page within the client
application and never leave the browser.
> [**Testing**](guide/testing): Run unit tests on your application parts as they interact with the Angular framework
using the _Angular Testing Platform_.

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