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

Master/Detail Components

At the moment, the HeroesComponent displays both the list of heroes and the selected hero's details.

Keeping all features in one component as the application grows will not be maintainable. You'll want to split up large components into smaller sub-components, each focused on a specific task or workflow.

In this page, you'll take the first step in that direction by moving the hero details into a separate, reusable HeroDetailComponent.

The HeroesComponent will only present the list of heroes. The HeroDetailComponent will present details of a selected hero.

Make the HeroDetailComponent

Use the Angular CLI to generate a new component named hero-detail.

ng generate component hero-detail

The command scaffolds the HeroDetailComponent files and declares the component in AppModule.

Write the template

Cut the HTML for the hero detail from the bottom of the HeroesComponent template and paste it over the generated boilerplate in the HeroDetailComponent template.

The pasted HTML refers to a selectedHero. The new HeroDetailComponent can present any hero, not just a selected hero. So replace "selectedHero" with "hero" everywhere in the template.

When you're done, the HeroDetailComponent template should look like this:

Add the @Input() hero property

The HeroDetailComponent template binds to the component's hero property which is of type Hero.

Open the HeroDetailComponent class file and import the Hero symbol.

The hero property must be an Input property, annotated with the @Input() decorator, because the external HeroesComponent will bind to it like this.

Amend the @angular/core import statement to include the Input symbol.

Add a hero property, preceded by the @Input() decorator.

That's the only change you should make to the HeroDetailComponent class. There are no more properties. There's no presentation logic. This component simply receives a hero object through its hero property and displays it.

Show the HeroDetailComponent

The HeroesComponent is still a master/detail view.

It used to display the hero details on its own, before you cut that portion of the template. Now it will delegate to the HeroDetailComponent.

The two components will have a parent/child relationship. The parent HeroesComponent will control the child HeroDetailComponent by sending it a new hero to display whenever the user selects a hero from the list.

You won't change the HeroesComponent class but you will change its template.

{@a heroes-component-template}

Update the HeroesComponent template

The HeroDetailComponent selector is 'app-hero-detail'. Add an <app-hero-detail> element near the bottom of the HeroesComponent template, where the hero detail view used to be.

Bind the HeroesComponent.selectedHero to the element's hero property like this.

[hero]="selectedHero" is an Angular property binding.

It's a one way data binding from the selectedHero property of the HeroesComponent to the hero property of the target element, which maps to the hero property of the HeroDetailComponent.

Now when the user clicks a hero in the list, the selectedHero changes. When the selectedHero changes, the property binding updates hero and the HeroDetailComponent displays the new hero.

The revised HeroesComponent template should look like this:

The browser refreshes and the app starts working again as it did before.

What changed?

As before, whenever a user clicks on a hero name, the hero detail appears below the hero list. Now the HeroDetailComponent is presenting those details instead of the HeroesComponent.

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

1. You simplified the HeroesComponent by reducing its responsibilities.

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

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

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

Final code review

Here are the code files discussed on this page and your app should look like this .

Summary

• You created a separate, reusable HeroDetailComponent.

• You used a property binding to give the parent HeroesComponent control over the child HeroDetailComponent.

• You used the @Input decorator to make the hero property available for binding by the external HeroesComponent.