angular-cn/aio/content/tutorial/toh-pt2.md

@title Master/Detail

@intro Build a master/detail page with a list of heroes.

@description

In this page, you'll expand the Tour of Heroes app to display a list of heroes, and allow users to select a hero and display the hero's details.

When you're done with this page, the app should look like this .

Where you left off

Before you continue with this page of the Tour of Heroes, verify that you have the following structure after The Hero Editor page. If your structure doesn't match, go back to that page to figure out what you missed.

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

Enter the following command in the terminal window:

npm start

This command runs the TypeScript compiler in "watch mode", recompiling automatically when the code changes. The command simultaneously launches the app in a browser and refreshes the browser when the code changes.

You can keep building the Tour of Heroes without pausing to recompile or refresh the browser.

Displaying heroes

To display a list of heroes, you'll add heroes to the view's template.

Create heroes

Create an array of ten heroes.

The HEROES array is of type Hero, the class defined in the previous page. Eventually this app will fetch the list of heroes from a web service, but for now you can display mock heroes.

Expose heroes

Create a public property in AppComponent that exposes the heroes for binding.

The heroes type isn't defined because TypeScript infers it from the HEROES array.

The hero data is separated from the class implementation
because ultimately the hero names will come from a data service.

Display hero names in a template

To display the hero names in an unordered list, insert the following chunk of HTML below the title and above the hero details.

Now you can fill the template with hero names.

List heroes with ngFor

The goal is to bind the array of heroes in the component to the template, iterate over them, and display them individually.

Modify the <li> tag by adding the built-in directive *ngFor.

The (`*`) prefix to `ngFor` is a critical part of this syntax.
It indicates that the `<li>` element and its children
constitute a master template.

The `ngFor` directive iterates over the component's `heroes` array
and renders an instance of this template for each hero in that array.

The `let hero` part of the expression identifies `hero` as the  template input variable,
which holds the current hero item for each iteration.
You can reference this variable within the template to access the current hero's properties.

Read more about `ngFor` and template input variables in the
[Showing an array property with *ngFor](guide/displaying-data#ngFor) section of the
[Displaying Data](guide/displaying-data) page and the
[ngFor](guide/template-syntax#ngFor) section of the
[Template Syntax](guide/template-syntax) page.

Within the <li> tags, add content that uses the hero template variable to display the hero's properties.

When the browser refreshes, a list of heroes appears.

Style the heroes

Users should get a visual cue of which hero they are hovering over and which hero is selected.

To add styles to your component, set the styles property on the @Component decorator to the following CSS classes:

Remember to use the backtick notation for multi-line strings.

Adding these styles makes the file much longer. In a later page you'll move the styles to a separate file.

When you assign styles to a component, they are scoped to that specific component. These styles apply only to the AppComponent and don't affect the outer HTML.

The template for displaying heroes should look like this:

Selecting a hero

The app now displays a list of heroes as well as a single hero in the details view. But the list and the details view are not connected. When users select a hero from the list, the selected hero should appear in the details view. This UI pattern is known as "master/detail." In this case, the master is the heroes list and the detail is the selected hero.

Next you'll connect the master to the detail through a selectedHero component property, which is bound to a click event.

Add a click event

Add a click event binding to the <li> like this:

The parentheses identify the <li> element's click event as the target. The onSelect(hero) expression calls the AppComponent method, onSelect(), passing the template input variable hero, as an argument. That's the same hero variable you defined previously in the ngFor directive.

Learn more about event binding at the
[User Input](guide/user-input) page and the
[Event binding](guide/template-syntax#event-binding) section of the
[Template Syntax](guide/template-syntax) page.

Add a click handler to expose the selected hero

You no longer need the hero property because you're no longer displaying a single hero; you're displaying a list of heroes. But the user will be able to select one of the heroes by clicking on it. So replace the hero property with this simple selectedHero property:

The hero names should all be unselected before the user picks a hero, so you won't initialize the selectedHero as you did with hero.

Add an onSelect() method that sets the selectedHero property to the hero that the user clicks.

The template still refers to the old hero property. Bind to the new selectedHero property instead as follows:

Hide the empty detail with ngIf

When the app loads, the selectedHero is undefined and won't be defined until you click a hero's name. Angular can't display properties of the undefined selectedHero and throws the following error, visible in the browser's console:

EXCEPTION: TypeError: Cannot read property 'name' of undefined in [null]

Although selectedHero.name is displayed in the template, you must keep the hero detail out of the DOM until there is a selected hero.

Wrap the HTML hero detail content of the template with a <div>. Then add the ngIf built-in directive and set it to the selectedHero property of the component.

Don't forget the asterisk (`*`) in front of `ngIf`.

The app no longer fails and the list of names displays again in the browser.

When there is no selectedHero, the ngIf directive removes the hero detail HTML from the DOM. There are no hero detail elements or bindings to worry about.

When the user picks a hero, selectedHero becomes defined and ngIf puts the hero detail content into the DOM and evaluates the nested bindings.

Read more about `ngIf` and `ngFor` in the
[Structural Directives](guide/structural-directives) page and the
[Built-in directives](guide/template-syntax#directives) section of the
[Template Syntax](guide/template-syntax) page.

Style the selected hero

While the selected hero details appear below the list, it's difficult to identify the selected hero within the list itself.

In the styles metadata that you added above, there is a custom CSS class named selected. To make the selected hero more visible, you'll apply this selected class to the <li> when the user clicks on a hero name. For example, when the user clicks "Magneta", it should render with a distinctive but subtle background color like this:

In the template, add the following [class.selected] binding to the <li>:

When the expression (hero === selectedHero) is true, Angular adds the selected CSS class. When the expression is false, Angular removes the selected class.

Read more about the `[class]` binding in the [Template Syntax](guide/template-syntax#ngClass "Template syntax: NgClass") guide.

The final version of the <li> looks like this:

After clicking "Magneta", the list should look like this:

Here's the complete app.component.ts as of now:

The road you've travelled

Here's what you achieved in this page:

• The Tour of Heroes app displays a list of selectable heroes.
• You added the ability to select a hero and show the hero's details.
• You learned how to use the built-in directives ngIf and ngFor in a component's template.

Your app should look like this .

The road ahead

You've expanded the Tour of Heroes app, but it's far from complete. You can't put the entire app into a single component. In the next page, you'll split the app into sub-components and make them work together.