308 lines
13 KiB
Plaintext
308 lines
13 KiB
Plaintext
include ../_util-fns
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:marked
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# It Takes Many Heroes
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Our story needs more heroes.
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We’ll expand our Tour of Heroes app to display a list of heroes,
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allow the user to select a hero, and display the hero’s details.
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[Run the live example for part 2](/resources/live-examples/toh-2/ts/plnkr.html)
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Let’s take stock of what we’ll need to display a list of heroes.
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First, we need a list of heroes. We want to display those heroes in the view’s template,
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so we’ll need a way to do that.
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.l-main-section
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:marked
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## Where We Left Off
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Before we continue with Part 2 of the Tour of Heroes,
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let’s verify we have the following structure after [Part 1](./toh-pt1.html).
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If not, we’ll need to go back to Part 1 and figure out what we missed.
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.filetree
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.file angular2-tour-of-heroes
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.children
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.file app
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.children
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.file app.component.ts
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.file main.ts
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.file node_modules ...
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.file typings ...
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.file index.html
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.file package.json
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.file tsconfig.json
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.file typings.json
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:marked
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### Keep the app transpiling and running
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We want to start the TypeScript compiler, have it watch for changes, and start our server. We'll do this by typing
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code-example(format="." language="bash").
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npm start
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:marked
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This will keep the application running while we continue to build the Tour of Heroes.
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.l-main-section
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:marked
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## Displaying Our Heroes
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### Creating heroes
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Let’s create an array of ten heroes at the bottom of `app.component.ts`.
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+makeExample('toh-2/ts/app/app.component.ts', 'hero-array', 'app.component.ts (Hero array)')
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:marked
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The `HEROES` array is of type `Hero`, the class defined in part one,
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to create an array of heroes.
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We aspire to fetch this list of heroes from a web service, but let’s take small steps
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first and display mock heroes.
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### Exposing heroes
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Let’s create a property in `AppComponent` that exposes the heroes for binding.
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'hero-array-1', 'app.component.ts (Hero array property)')
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:marked
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We did not have to define the `heroes` type. TypeScript can infer it from the `HEROES` array.
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.l-sub-section
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:marked
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We could have defined the heroes list here in this component class.
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But we know that ultimately we’ll get the heroes from a data service.
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Because we know where we are heading, it makes sense to separate the hero data
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from the class implementation from the start.
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:marked
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### Displaying heroes in a template
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Our component has `heroes`. Let’s create an unordered list in our template to display them.
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We’ll insert the following chunk of HTML below the title and above the hero details.
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'heroes-template-1', 'app.component.ts (Heroes template)')
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:marked
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Now we have a template that we can fill with our heroes.
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### Listing heroes with ngFor
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We want to bind the array of `heroes` in our component to our template, iterate over them,
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and display them individually.
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We’ll need some help from Angular to do this. Let’s do this step by step.
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First modify the `<li>` tag by adding the built-in directive `*ngFor`.
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'heroes-ngfor-1', 'app.component.ts (ngFor)')
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.alert.is-critical
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:marked
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The leading asterisk (`*`) in front of `ngFor` is a critical part of this syntax.
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.l-sub-section
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:marked
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The (`*`) prefix to `ngFor` indicates that the `<li>` element and its children
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constitute a master template.
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The `ngFor` directive iterates over the `heroes` array returned by the `AppComponent.heroes` property
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and stamps out instances of this template.
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The quoted text assigned to `ngFor` means
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“*take each hero in the `heroes` array, store it in the local `hero` variable,
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and make it available to the corresponding template instance*”.
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The `#` prefix before "hero" identifies the `hero` as a local template variable.
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We can reference this variable within the template to access a hero’s properties.
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Learn more about `ngFor` and local template variables in the
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[Displaying Data](../guide/displaying-data.html#ngFor) and
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[Template Syntax](../guide/template-syntax.html#ngFor) chapters.
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:marked
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Now we insert some content between the `<li>` tags
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that uses the `hero` template variable to display the hero’s properties.
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'ng-for', 'app.component.ts (ngFor template)')(format=".")
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:marked
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When the browser refreshes, we see a list of heroes!
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### Styling our heroes
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Our list of heroes looks pretty bland.
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We want to make it visually obvious to a user which hero we are hovering over and which hero is selected.
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Let’s add some styles to our component by setting the `styles` property on the `@Component` decorator
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to the following CSS classes:
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+makeExample('toh-2/ts/app/app.component.ts', 'styles-1', 'app.component.ts (Styling)')
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:marked
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Notice that we again use the back-tick notation for multi-line strings.
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When we assign styles to a component they are scoped to that specific component.
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Our styles will only apply to our `AppComponent` and won't "leak" to the outer HTML.
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Our template for displaying the heroes should now look like this:
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'heroes-styled', 'app.component.ts (Styled heroes)')
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:marked
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That's a lot of styles! We can put them inline as shown here, or we can move them out to their own file which will make it easier to code our component.
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We'll do this in a later chapter. For now let's keep rolling.
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.l-main-section
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:marked
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## Selecting a Hero
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We have a list of heroes and we have a single hero displayed in our app.
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The list and the single hero are not connected in any way.
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We want the user to select a hero from our list, and have the selected hero appear in the details view.
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This UI pattern is widely known as "master-detail".
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In our case, the master is the heroes list and the detail is the selected hero.
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Let’s connect the master to the detail through a `selectedHero` component property bound to a click event.
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### Click event
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We modify the `<li>` by inserting an Angular event binding to its click event.
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'selectedHero-click', 'app.component.ts (Capturing the click event)')
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:marked
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Focus on the event binding
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code-example(format="." language="bash").
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(click)="onSelect(hero)"
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:marked
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The parenthesis identify the `<li>` element’s `click` event as the target.
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The expression to the right of the equal sign calls the `AppComponent` method, `onSelect()`,
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passing the local template variable `hero` as an argument.
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That’s the same `hero` variable we defined previously in the `ngFor`.
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.l-sub-section
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:marked
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Learn more about Event Binding in the
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[User Input](../guide/user-input.html) and
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[Templating Syntax](../guide/template-syntax.html#event-binding) chapters.
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:marked
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### Add the click handler
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Our event binding refers to an `onSelect` method that doesn’t exist yet.
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We’ll add that method to our component now.
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What should that method do? It should set the component’s selected hero to the hero that the user clicked.
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Our component doesn’t have a “selected hero” yet either. We’ll start there.
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### Expose the selected hero
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We no longer need the static `hero` property of the `AppComponent`.
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**Replace** it with this simple `selectedHero` property:
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+makeExample('toh-2/ts/app/app.component.ts', 'selected-hero-1', 'app.component.ts (selectedHero)')
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:marked
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We’ve decided that none of the heroes should be selected before the user picks a hero so
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we won’t initialize the `selectedHero` as we were doing with `hero`.
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Now **add an `onSelect` method** that sets the `selectedHero` property to the `hero` the user clicked.
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+makeExample('toh-2/ts/app/app.component.ts', 'on-select-1', 'app.component.ts (onSelect)')
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:marked
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We will be showing the selected hero's details in our template.
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At the moment, it is still referring to the old `hero` property.
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Let’s fix the template to bind to the new `selectedHero` property.
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'selectedHero-details', 'app.component.ts (Binding to the selectedHero\'s name)')
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:marked
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### Hide the empty detail with ngIf
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When our app loads we see a list of heroes, but a hero is not selected.
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The `selectedHero` is `undefined`.
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That’s why we'll see the following error in the browser’s console:
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code-example(language="html").
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EXCEPTION: TypeError: Cannot read property 'name' of undefined in [null]
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:marked
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Remember that we are displaying `selectedHero.name` in the template.
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This name property does not exist because `selectedHero` itself is undefined.
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We'll address this problem by keeping the hero detail out of the DOM until there is a selected hero.
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We wrap the HTML hero detail content of our template with a `<div>`.
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Then we add the `ngIf` built-in directive and set it to the `selectedHero` property of our component.
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'ng-if', 'app.component.ts (ngIf)')
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.alert.is-critical
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:marked
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Remember that the leading asterisk (`*`) in front of `ngIf` is
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a critical part of this syntax.
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:marked
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When there is no `selectedHero`, the `ngIf` directive removes the hero detail HTML from the DOM.
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There will be no hero detail elements and no bindings to worry about.
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When the user picks a hero, `selectedHero` becomes "truthy" and
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`ngIf` puts the hero detail content into the DOM and evaluates the nested bindings.
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.l-sub-section
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:marked
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`ngIf` and `ngFor` are called “structural directives” because they can change the
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structure of portions of the DOM.
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In other words, they give structure to the way Angular displays content in the DOM.
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Learn more about `ngIf`, `ngFor` and other structural directives in the
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[Structural Directives](../guide/structural-directives.html) and
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[Template Syntax](../guide/template-syntax.html#directives) chapters.
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:marked
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The browser refreshes and we see the list of heroes but not the selected hero detail.
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The `ngIf` keeps it out of the DOM as long as the `selectedHero` is undefined.
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When we click on a hero in the list, the selected hero displays in the hero details.
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Everything is working as we expect.
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### Styling the selection
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We see the selected hero in the details area below but we can’t quickly locate that hero in the list above.
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We can fix that by applying the `selected` CSS class to the appropriate `<li>` in the master list.
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For example, when we select Magneta from the heroes list,
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we can make it pop out visually by giving it a subtle background color as shown here.
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figure.image-display
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img(src='/resources/images/devguide/toh/heroes-list-selected.png' alt="Selected hero")
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:marked
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We’ll add a property binding on `class` for the `selected` class to the template. We'll set this to an expression that compares the current `selectedHero` to the `hero`.
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The key is the name of the CSS class (`selected`). The value is `true` if the two heroes match and `false` otherwise.
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We’re saying “*apply the `selected` class if the heroes match, remove it if they don’t*”.
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'class-selected-1', 'app.component.ts (Setting the CSS class)')(format=".")
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:marked
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Notice in the template that the `class.selected` is surrounded in square brackets (`[]`).
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This is the syntax for a Property Binding, a binding in which data flows one way
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from the data source (the expression `hero === selectedHero`) to a property of `class`.
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+makeExample('toh-2/ts-snippets/app.component.snippets.pt2.ts', 'class-selected-2', 'app.component.ts (Styling each hero)')(format=".")
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.l-sub-section
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:marked
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Learn more about [Property Binding](../guide/template-syntax.html#property-binding)
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in the Template Syntax chapter.
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:marked
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The browser reloads our app.
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We select the hero Magneta and the selection is clearly identified by the background color.
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figure.image-display
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img(src='/resources/images/devguide/toh/heroes-list-1.png' alt="Output of heroes list app")
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:marked
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We select a different hero and the tell-tale color switches to that hero.
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Here's the complete `app.component.ts` as it stands now:
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+makeExample('toh-2/ts/app/app.component.ts', 'pt2', 'app.component.ts')
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.l-main-section
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:marked
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## The Road We’ve Travelled
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Here’s what we achieved in this chapter:
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* Our Tour of Heroes now displays a list of selectable heroes
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* We added the ability to select a hero and show the hero’s details
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* We learned how to use the built-in directives `ngIf` and `ngFor` in a component’s template
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[Run the live example for part 2](/resources/live-examples/toh-2/ts/plnkr.html)
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### The Road Ahead
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Our Tour of Heroes has grown, but it’s far from complete.
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We can't put the entire app into a single component.
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We need to break it up into sub-components and teach them to work together
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as we learn in the [next chapter](toh-pt3.html).
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