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

@title Multiple Components

@intro Refactor the master/detail view into separate components.

@description

The AppComponent is doing everything at the moment. In the beginning, it showed details of a single hero. Then it became a master/detail form with both a list of heroes and the hero detail. Soon there will be new requirements and capabilities. You can't keep piling features on top of features in one component; that's not maintainable.

You'll need to break it up into sub-components, each focused on a specific task or workflow. Eventually, the AppComponent could become a simple shell that hosts those sub-components.

In this page, you'll take the first step in that direction by carving out the hero details into a separate, reusable component. When you're done, the app should look like this .

Where you left off

Before getting started on this page, verify that you have the following structure from earlier in the Tour of Heroes. If not, go back to the previous pages.

angular-tour-of-heroes

<div class='file'>
  src
</div>

<div class='children'>

  <div class='file'>
    app
  </div>

  <div class='children'>

    <div class='file'>
      app.component.ts
    </div>

    <div class='file'>
      app.module.ts
    </div>

  </div>

  <div class='file'>
    main.ts
  </div>

  <div class='file'>
    index.html
  </div>

  <div class='file'>
    styles.css
  </div>

  <div class='file'>
    systemjs.config.js
  </div>

  <div class='file'>
    tsconfig.json
  </div>

</div>

<div class='file'>
  node_modules ...
</div>

<div class='file'>
  package.json
</div>

Keep the app transpiling and running while you build the Tour of Heroes by entering the npm start command in a terminal window as you did before.

Make a hero detail component

Add a file named hero-detail.component.ts to the app/ folder. This file will hold the new HeroDetailComponent.

The file and component names follow the standard described in the Angular style guide.

• The component class name should be written in upper camel case and end in the word "Component". The hero detail component class is HeroDetailComponent.

• The component file name should be spelled in lower dash case, each word separated by dashes, and end in .component.ts. The HeroDetailComponent class goes in the hero-detail.component.ts file.

Start writing the HeroDetailComponent as follows:

{@a selector}

To define a component, you always import the Component symbol.

The @Component decorator provides the Angular metadata for the component. The CSS selector name, hero-detail, will match the element tag that identifies this component within a parent component's template. Near the end of this tutorial page, you'll add a <hero-detail> element to the AppComponent template.

Always export the component class because you'll always import it elsewhere.

Hero detail template

To move the hero detail view to the HeroDetailComponent, cut the hero detail content from the bottom of the AppComponent template and paste it into a new template property in the @Component metadata.

The HeroDetailComponent has a hero, not a selected hero. Replace the word, "selectedHero", with the word, "hero", everywhere in the template. When you're done, the new template should look like this:

Add the hero property

The HeroDetailComponent template binds to the component's hero property. Add that property to the HeroDetailComponent class like this:

The hero property is typed as an instance of Hero. The Hero class is still in the app.component.ts file. Now there are two components that need to reference the Hero class. The Angular style guide recommends one class per file anyway.

Move the Hero class from app.component.ts to its own hero.ts file.

Now that the Hero class is in its own file, the AppComponent and the HeroDetailComponent have to import it. Add the following import statement near the top of both the app.component.ts and the hero-detail.component.ts files.

The hero property is an input property

Later in this page, the parent AppComponent will tell the child HeroDetailComponent which hero to display by binding its selectedHero to the hero property of the HeroDetailComponent. The binding will look like this:

Putting square brackets around the hero property, to the left of the equal sign (=), makes it the target of a property binding expression. You must declare a target binding property to be an input property. Otherwise, Angular rejects the binding and throws an error.

First, amend the @angular/core import statement to include the Input symbol.

Then declare that hero is an input property by preceding it with the @Input decorator that you imported earlier.

Read more about input properties in the Attribute Directives page.

That's it. The hero property is the only thing in the HeroDetailComponent class.

All it does is receive a hero object through its hero input property and then bind to that property with its template.

Here's the complete HeroDetailComponent.

Declare HeroDetailComponent in the AppModule

Every component must be declared in one—and only one—NgModule.

Open app.module.ts in your editor and import the HeroDetailComponent so you can refer to it.

Add HeroDetailComponent to the module's declarations array.

In general, the declarations array contains a list of application components, pipes, and directives that belong to the module. A component must be declared in a module before other components can reference it. This module declares only the two application components, AppComponent and HeroDetailComponent.

Read more about NgModules in the NgModules guide.

{@a add-hero-detail}

Add the HeroDetailComponent to the AppComponent

The AppComponent is still a master/detail view. It used to display the hero details on its own, before you cut out that portion of the template. Now it will delegate to the HeroDetailComponent.

Recall that hero-detail is the CSS selector in the HeroDetailComponent metadata. That's the tag name of the element that represents the HeroDetailComponent.

Add a <hero-detail> element near the bottom of the AppComponent template, where the hero detail view used to be.

Coordinate the master AppComponent with the HeroDetailComponent by binding the selectedHero property of the AppComponent to the hero property of the HeroDetailComponent.

Now every time the selectedHero changes, the HeroDetailComponent gets a new hero to display.

The revised AppComponent template should look like this:

What changed?

As before, whenever a user clicks on a hero name, the hero detail appears below the hero list. But now the HeroDetailView is presenting those details.

Refactoring the original AppComponent into two components yields benefits, both now and in the future:

1. You simplified the AppComponent by reducing its responsibilities.

2. You can evolve the HeroDetailComponent into a rich hero editor without touching the parent AppComponent.

3. You can evolve the AppComponent without touching the hero detail view.

4. You can re-use the HeroDetailComponent in the template of some future parent component.

Review the app structure

Verify that you have the following structure:

angular-tour-of-heroes

<div class='file'>
  src
</div>

<div class='children'>

  <div class='file'>
    app
  </div>

  <div class='children'>

    <div class='file'>
      app.component.ts
    </div>

    <div class='file'>
      app.module.ts
    </div>

    <div class='file'>
      hero.ts
    </div>

    <div class='file'>
      hero-detail.component.ts
    </div>

  </div>

  <div class='file'>
    main.ts
  </div>

  <div class='file'>
    index.html
  </div>

  <div class='file'>
    styles.css
  </div>

  <div class='file'>
    systemjs.config.js
  </div>

  <div class='file'>
    tsconfig.json
  </div>

</div>

<div class='file'>
  node_modules ...
</div>

<div class='file'>
  package.json
</div>

Here are the code files discussed in this page.

The road you’ve travelled

Here's what you achieved in this page:

• You created a reusable component.
• You learned how to make a component accept input.
• You learned to declare the required application directives in an NgModule. You listed the directives in the @NgModule decorator's declarations array.
• You learned to bind a parent component to a child component.

Your app should look like this .

The road ahead

The Tour of Heroes app is more reusable with shared components, but its (mock) data is still hard coded within the AppComponent. That's not sustainable. Data access should be refactored to a separate service and shared among the components that need data.

You’ll learn to create services in the next tutorial page.