angular-cn/aio/content/tutorial/toh-pt3.md

@title Multiple Components

@intro We refactor the master/detail view into separate components.

@description Our app is growing. Use cases are flowing in for reusing components, passing data to components, and creating more reusable assets. Let's separate the heroes list from the hero details and make the details component reusable.

Run the for this part.

Where We Left Off

Before we continue with our Tour of Heroes, let’s verify we have the following structure. If not, we’ll need to go back and follow the previous chapters.

angular-tour-of-heroes src app app.component.ts

    <aio-file>
      app.module.ts
    </aio-file>


  </aio-folder>


  <aio-file>
    main.ts
  </aio-file>


  <aio-file>
    index.html
  </aio-file>


  <aio-file>
    styles.css
  </aio-file>


  <aio-file>
    systemjs.config.js
  </aio-file>


  <aio-file>
    tsconfig.json
  </aio-file>


</aio-folder>


<aio-file>
  node_modules ...   
</aio-file>


<aio-file>
  package.json
</aio-file>

Keep the app transpiling and running

We want to start the TypeScript compiler, have it watch for changes, and start our server. We'll do this by typing

npm start

This will keep the application running while we continue to build the Tour of Heroes.

Making a Hero Detail Component

Our heroes list and our hero details are in the same component in the same file. They're small now but each could grow. We are sure to receive new requirements for one and not the other. Yet every change puts both components at risk and doubles the testing burden without benefit. If we had to reuse the hero details elsewhere in our app, the heroes list would tag along for the ride.

Our current component violates the Single Responsibility Principle. It's only a tutorial but we can still do things right — especially if doing them right is easy and we learn how to build Angular apps in the process.

Let’s break the hero details out into its own component.

Separating the Hero Detail Component

Add a new file named hero-detail.component.ts to the app folder and create HeroDetailComponent as follows.

{@example 'toh-3/ts/src/app/hero-detail.component.ts' region='v1'}

Naming conventions

We like to identify at a glance which classes are components and which files contain components.

Notice that we have an AppComponent in a file named app.component.ts and our new HeroDetailComponent is in a file named hero-detail.component.ts.

All of our component names end in "Component". All of our component file names end in ".component".

We spell our file names in lower dash case (AKA kebab-case) so we don't worry about case sensitivity on the server or in source control.

We begin by importing the `Component` and `Input` decorators from Angular because we're going to need them soon.

We create metadata with the @Component decorator where we specify the selector name that identifies this component's element. Then we export the class to make it available to other components.

When we finish here, we'll import it into AppComponent and create a corresponding <my-hero-detail> element.#### Hero Detail Template At the moment, the Heroes and Hero Detail views are combined in one template in AppComponent. Let’s cut the Hero Detail content from AppComponent and paste it into the new template property of HeroDetailComponent.

We previously bound to the selectedHero.name property of the AppComponent. Our HeroDetailComponent will have a hero property, not a selectedHero property. So we replace selectedHero with hero everywhere in our new template. That's our only change. The result looks like this:

{@example 'toh-3/ts/src/app/hero-detail.component.ts' region='template'}

Now our hero detail layout exists only in the HeroDetailComponent.

Add the hero property

Let’s add that hero property we were talking about to the component class.

{@example 'toh-3/ts/src/app/hero-detail.component.ts' region='hero'}

Uh oh. We declared the hero property as type Hero but our Hero class is over in the app.component.ts file. We have two components, each in their own file, that need to reference the Hero class.

We solve the problem by relocating the Hero class from app.component.ts to its own hero.ts file.

{@example 'toh-3/ts/src/app/hero.ts'}

We export the Hero class from hero.ts because we'll need to reference it in both component files. Add the following import statement near the top of both app.component.ts and hero-detail.component.ts.

{@example 'toh-3/ts/src/app/hero-detail.component.ts' region='hero-import'}

The hero property is an input

The HeroDetailComponent must be told what hero to display. Who will tell it? The parent AppComponent!

The AppComponent knows which hero to show: the hero that the user selected from the list. The user's selection is in its selectedHero property.

We will soon update the AppComponent template so that it binds its selectedHero property to the hero property of our HeroDetailComponent. The binding might look like this: <my-hero-detail [hero]="selectedHero"></my-hero-detail>

Notice that the hero property is the target of a property binding — it's in square brackets to the left of the (=).

Angular insists that we declare a target property to be an input property. If we don't, Angular rejects the binding and throws an error. We explain input properties in more detail here where we also explain why target properties require this special treatment and source properties do not.There are a couple of ways we can declare that hero is an input. We'll do it the way we prefer, by annotating the hero property with the @Input decorator that we imported earlier.

{@example 'toh-3/ts/src/app/hero-detail.component.ts' region='hero-input'}

Learn more about the @Input() decorator in the Attribute Directives chapter.

Refresh the AppModule

We return to the AppModule, the application's root module, and teach it to use the HeroDetailComponent.

We begin by importing the HeroDetailComponent so we can refer to it.

{@example 'toh-3/ts/src/app/app.module.ts' region='hero-detail-import'}

Then we add HeroDetailComponent to the NgModule decorator's declarations array. This array contains the list of all components, pipes, and directives that we created and that belong in our application's module.

{@example 'toh-3/ts/src/app/app.module.ts' region='declarations'}

Refresh the AppComponentNow that the application knows about our HeroDetailComponent,

find the location in the AppComponent template where we removed the Hero Detail content and add an element tag that represents the HeroDetailComponent. <my-hero-detail></my-hero-detail>

my-hero-detail is the name we set as the selector in the HeroDetailComponent metadata.The two components won't coordinate until we bind the selectedHero property of the AppComponent to the HeroDetailComponent element's hero property like this: <my-hero-detail [hero]="selectedHero"></my-hero-detail>

The AppComponent’s template should now look like this

{@example 'toh-3/ts/src/app/app.component.ts' region='hero-detail-template'}

Thanks to the binding, the HeroDetailComponent should receive the hero from the AppComponent and display that hero's detail beneath the list. The detail should update every time the user picks a new hero.

It works!

When we view our app in the browser we see the list of heroes. When we select a hero we can see the selected hero’s details.

What's fundamentally new is that we can use this HeroDetailComponent to show hero details anywhere in the app.

We’ve created our first reusable component!

Reviewing the App Structure

Let’s verify that we have the following structure after all of our good refactoring in this chapter:

angular-tour-of-heroes src app app.component.ts

    <aio-file>
      app.module.ts
    </aio-file>


    <aio-file>
      hero.ts
    </aio-file>


    <aio-file>
      hero-detail.component.ts
    </aio-file>


  </aio-folder>


  <aio-file>
    main.ts
  </aio-file>


  <aio-file>
    index.html
  </aio-file>


  <aio-file>
    styles.css
  </aio-file>


  <aio-file>
    systemjs.config.js
  </aio-file>


  <aio-file>
    tsconfig.json
  </aio-file>


</aio-folder>


<aio-file>
  node_modules ...
</aio-file>


<aio-file>
  package.json
</aio-file>

Here are the code files we discussed in this chapter.

{@example 'toh-3/ts/src/app/hero-detail.component.ts'} {@example 'toh-3/ts/src/app/app.component.ts'} {@example 'toh-3/ts/src/app/hero.ts'} {@example 'toh-3/ts/src/app/app.module.ts'}

The Road We’ve Travelled

Let’s take stock of what we’ve built.

• We created a reusable component
• We learned how to make a component accept input
• We learned to declare the application directives we need in an Angular module. We list the directives in the NgModule decorator's declarations array.
• We learned to bind a parent component to a child component.

Run the for this part.

The Road Ahead

Our Tour of Heroes has become more reusable with shared components.

We're still getting our (mock) data within the AppComponent. That's not sustainable. We should refactor data access to a separate service and share it among the components that need data.

We’ll learn to create services in the next tutorial chapter.