Although this template uses typical HTML elements like `<h2>` and `<p>`, it also has some differences. Code like `*ngFor`, `{{hero.name}}`, `(click)`, `[hero]`, and `<hero-detail>` uses Angular's [template syntax](template-syntax.html).
In the last line of the template, the `<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`.
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 !{_decorator}s in a similar fashion to guide Angular behavior.
`@Injectable`, `@Input`, and `@Output` are a few of the more popular !{_decorator}s.<brclass="l-clear-both">The architectural takeaway is that you must add metadata to your code
so that Angular knows what to do.
<divclass='l-hr'>
</div>
## 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.
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 — to the DOM, from the DOM, or in both directions.<brclass="l-clear-both">The `HeroListComponent` [example](#templates) template has three forms:
* The `{{hero.name}}` [*interpolation*](displaying-data.html#interpolation)
displays the component's `hero.name` property value within the `<li>` element.
* The `[hero]` [*property binding*](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.
Here's an example from the `HeroDetailComponent` template:
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.
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.
<brclass="l-clear-both">
While **a component is technically a directive**,
components are so distinctive and central to Angular applications that this architectural overview separates components from directives.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](#templates) uses two built-in structural directives:
* [`*ngFor`](displaying-data.html#ngFor) tells Angular to stamp out one `<li>` per hero in the `heroes` list.
* [`*ngIf`](displaying-data.html#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.
Angular has a few more directives that either alter the layout structure
(for example, [ngSwitch](template-syntax.html#ngSwitch))
or modify aspects of DOM elements and components
(for example, [ngStyle](template-syntax.html#ngStyle) and [ngClass](template-syntax.html#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! -->
_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.<brclass="l-clear-both">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:
Here's a `HeroService` that uses a !{_PromiseLinked} to fetch heroes.
The `HeroService` depends on the `Logger` service and another `BackendService` that handles the server communication grunt work.
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*.
_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.<brclass="l-clear-both">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`:
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:
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**](forms.html): Support complex data entry scenarios with HTML-based validation and dirty checking.
> [**HTTP**](server-communication.html): Communicate with a server to get data, save data, and invoke server-side actions with an HTTP client.
> [**Lifecycle hooks**](lifecycle-hooks.html): Tap into key moments in the lifetime of a component, from its creation to its destruction,
by implementing the lifecycle hook interfaces.
> [**Pipes**](pipes.html): 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**](router.html): Navigate from page to page within the client
