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 <live-example></live-example>.

## 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](tutorial/toh-pt1) page.
If your structure doesn't match, go back to that page to figure out what you missed.


<aio-filetree>

  <aio-folder>
    angular-tour-of-heroes
    <aio-folder>
      src
      <aio-folder>
        app
        <aio-file>
          app.component.ts
        </aio-file>

        <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>

  </aio-folder>

</aio-filetree>

## Keep the app transpiling and running
Enter the following command in the terminal window:


<code-example language="sh" class="code-shell">
  npm start  
    
</code-example>

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.


<code-example path="toh-2/src/app/app.component.ts" region="hero-array">

</code-example>

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.



<code-example path="toh-2/app/app.component.1.html" region="hero-array-1">

</code-example>

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


~~~ {.l-sub-section}

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.



<code-example path="toh-2/app/app.component.1.html" region="heroes-template-1" linenums="false">

</code-example>

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`.


<code-example path="toh-2/app/app.component.1.html" region="heroes-ngfor-1">

</code-example>



~~~ {.l-sub-section}

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) section of the
[Displaying Data](guide/displaying-data) page and the
[ngFor](guide/template-syntax) 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.



<code-example path="toh-2/app/app.component.1.html" region="ng-for" linenums="false">

</code-example>

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:


<code-example path="toh-2/src/app/app.component.ts" region="styles" linenums="false">

</code-example>

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:



<code-example path="toh-2/app/app.component.1.html" region="heroes-styled" linenums="false">

</code-example>


## 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:



<code-example path="toh-2/app/app.component.1.html" region="selectedHero-click" linenums="false">

</code-example>

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.

~~~ {.l-sub-section}

Learn more about event binding at the
[User Input](guide/user-input) page and the
[Event binding](guide/template-syntax) 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:


<code-example path="toh-2/src/app/app.component.ts" region="selected-hero">

</code-example>

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.

<code-example path="toh-2/src/app/app.component.ts" region="on-select" linenums="false">

</code-example>

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



<code-example path="toh-2/app/app.component.1.html" region="selectedHero-details" linenums="false">

</code-example>

### 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:


<code-example format="nocode">
  EXCEPTION: TypeError: Cannot read property 'name' of undefined in [null]  
    
</code-example>

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.


<code-example path="toh-2/app/app.component.1.html" region="ng-if" linenums="false">

</code-example>



~~~ {.alert.is-critical}

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.


~~~ {.l-sub-section}

Read more about `ngIf` and `ngFor` in the
[Structural Directives](guide/structural-directives) page and the
[Built-in directives](guide/template-syntax) 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:


<figure class='image-display'>
  <img src='assets/images/devguide/toh/heroes-list-selected.png' alt="Selected hero">  </img>
</figure>

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

<code-example path="toh-2/app/app.component.1.html" region="class-selected-1" linenums="false">

</code-example>

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



~~~ {.l-sub-section}

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


~~~

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

<code-example path="toh-2/app/app.component.1.html" region="class-selected-2" linenums="false">

</code-example>

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


<figure class='image-display'>
  <img src='assets/images/devguide/toh/heroes-list-1.png' alt="Output of heroes list app">  </img>
</figure>

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


<code-example path="toh-2/src/app/app.component.ts">

</code-example>


## 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 <live-example></live-example>.

## 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](tutorial/toh-pt3), you'll split the app into sub-components and make them work together.