angular-docs-cn/public/docs/ts/latest/guide/displaying-data.jade

block includes
  include ../_util-fns
  - var _iterableUrl = 'https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols';
  - var _boolean = 'truthy/falsey';
  
:marked
  We typically display data in Angular by binding controls in an HTML template
  to properties of an Angular component.

  In this chapter, we'll create a component with a list of heroes. Each hero has a name.
  We'll display the list of hero names and
  conditionally show a message below the list.

  The final UI looks like this:

figure.image-display
  img(src="/resources/images/devguide/displaying-data/final.png" alt="Final UI")

:marked
  # Table Of Contents

  * [Showing component properties with interpolation](#interpolation)
  * [Showing !{_an} !{_array} property with NgFor](#ngFor)
  * [Conditional display with NgIf](#ngIf)

.l-sub-section
  :marked
    The <live-example></live-example> demonstrates all of the syntax and code
    snippets described in this chapter.

.l-main-section#interpolation
:marked
  ## Showing component properties with interpolation
  The easiest way to display a component property
  is to bind the property name through interpolation.
  With interpolation, we put the property name in the view template, enclosed in double curly braces: `{{myHero}}`.

  Let's build a small illustrative example together.

  Create a new project folder (<ngio-ex path="displaying-data"></ngio-ex>) and follow the steps in the [QuickStart](../quickstart.html).

block quickstart-repo
  include ../_quickstart_repo

:marked
  Then modify the <ngio-ex path="app.component.ts"></ngio-ex> file by 
  changing the template and the body of the component.
  When we're done, it should look like this:

+makeExample('app/app.component.1.ts')

:marked
  We added two properties to the formerly empty component: `title` and `myHero`.

  Our revised template displays the two component properties using double curly brace
  interpolation:

+makeExcerpt('app/app.component.1.ts', 'template', '')

+ifDocsFor('ts')
  .l-sub-section
    :marked
      The template is a multi-line string within ECMAScript 2015 backticks (<code>\`</code>).
      The backtick (<code>\`</code>) &mdash; which is *not* the same character as a single
      quote (`'`) &mdash; has many nice features. The feature we're exploiting here
      is the ability to compose the string over several lines, which makes for
      much more readable HTML.

:marked
  Angular automatically pulls the value of the `title` and `myHero` properties from the component and
  inserts those values into the browser. Angular updates the display
  when these properties change.

.l-sub-section
  :marked
    More precisely, the redisplay occurs after some kind of asynchronous event related to
    the view such as a keystroke, a timer completion, or an async `XHR` response.
    We don't have those in this sample.
    But then the properties aren't changing on their own either. For the moment we must operate on faith.

:marked
  Notice that we haven't called **new** to create an instance of the `AppComponent` class.
  Angular is creating an instance for us. How?

  Notice the CSS `selector` in the `@Component` !{_decorator} that specifies an element named `my-app`.
  Remember back in [QuickStart](../quickstart.html) that we added the `<my-app>` element to the body of our `index.html` file:

+makeExcerpt('index.html', 'body')

:marked
  When we bootstrap with the `AppComponent` class (in <ngio-ex path="main.ts"></ngio-ex>), Angular looks for a `<my-app>`
  in the `index.html`, finds it, instantiates an instance of `AppComponent`, and renders it
  inside the `<my-app>` tag.

  Try running the app. It should display the title and hero name:
figure.image-display
  img(src="/resources/images/devguide/displaying-data/title-and-hero.png" alt="Title and Hero")

+ifDocsFor('ts')
  :marked
    Let's review some of the choices we made and consider alternatives.

:marked
  ## Template inline or template file?

  We can store our component's template in one of two places.
  We can define it *inline* using the `template` property, as we do here.
  Or we can define the template in a separate HTML file and link to it in
  the component metadata using the `@Component` !{_decorator}'s `templateUrl` property.

  The choice between inline and separate HTML is a matter of taste,
  circumstances, and organization policy.
  Here we're using inline HTML because the template is small, and the demo
  is simpler without the additional HTML file.

  In either style, the template data bindings have the same access to the component's properties.

+ifDocsFor('ts')
  :marked
    ## Constructor or variable initialization?

    We initialized our component properties using variable assignment.
    This is a wonderfully concise and compact technique.

    Some folks prefer to declare the properties and initialize them within a constructor like this:
  +makeExcerpt('app/app-ctor.component.ts', 'class')

  :marked
    That's fine too. The choice is a matter of taste and organization policy.
    We'll adopt the more terse "variable assignment" style in this chapter simply because
    there will be less code to read.

.l-main-section#ngFor
:marked
  ## Showing !{_an} !{_array} property with ***ngFor**

  We want to display a list of heroes. We begin by adding !{_an} !{_array} of hero names to the component and redefine `myHero` to be the first name in the !{_array}.

+makeExcerpt('app/app.component.2.ts', 'class')

:marked
  Now we use the Angular `ngFor` directive in the template to display
  each item in the `heroes` list.

+makeExcerpt('app/app.component.2.ts', 'template')

:marked
  Our presentation is the familiar HTML unordered list with `<ul>` and `<li>` tags. Let's focus on the `<li>` tag.

+makeExcerpt('app/app.component.2.ts ()', 'li', '')

:marked
  We added a somewhat mysterious `*ngFor` to the `<li>` element.
  That's the Angular "repeater" directive.
  Its presence on the `<li>` tag marks that `<li>` element (and its children) as the "repeater template".

.alert.is-important
  :marked
    Don't forget the leading asterisk (\*) in `*ngFor`. It is an essential part of the syntax.
    Learn more about this and `ngFor` in the [Template Syntax](./template-syntax.html#ngFor) chapter.

:marked
  Notice the `hero` in the `ngFor` double-quoted instruction;
  it is an example of a [template input variable](./template-syntax.html#ngForMicrosyntax).

  Angular duplicates the `<li>` for each item in the list, setting the `hero` variable
  to the item (the hero) in the current iteration. Angular uses that variable as the
  context for the interpolation in the double curly braces.

.l-sub-section
  :marked
    We happened to give `ngFor` !{_an} !{_array} to display.
    In fact, `ngFor` can repeat items for any [iterable](!{_iterableUrl})
    object.
:marked
  Now the heroes appear in an unordered list.

figure.image-display
  img(src="/resources/images/devguide/displaying-data/hero-names-list.png" alt="After ngfor")

.l-main-section
:marked
  ## Creating a class for the data

  We are defining our data directly inside our component.
  That's fine for a demo but certainly isn't a best practice. It's not even a good practice.
  Although we won't do anything about that in this chapter, we'll make a mental note to fix this down the road.

  At the moment, we're binding to !{_an} !{_array} of strings. We do that occasionally in real applications, but
  most of the time we're binding to more specialized objects.

  Let's turn our !{_array} of hero names into !{_an} !{_array} of `Hero` objects. For that we'll need a `Hero` class.

  Create a new file in the `!{_appDir}` folder called  <ngio-ex path="hero.ts"></ngio-ex> with the following code:
  
+makeExcerpt('app/hero.ts')

block hero-class
  :marked
    We've defined a class with a constructor and two properties: `id` and `name`.

    It might not look like we have properties, but we do. We're taking
    advantage of a TypeScript shortcut in our declaration of the constructor parameters.

    Consider the first parameter:

  +makeExcerpt('app/hero.ts ()', 'id')

  :marked
    That brief syntax does a lot:
    * Declares a constructor parameter and its type
    * Declares a public property of the same name
    * Initializes that property with the corresponding argument when we "new" an instance of the class

.l-main-section
:marked
  ## Using the Hero class
  Let's make the `heroes` property in our component return !{_an} !{_array} of these `Hero` objects.

+makeExcerpt('app/app.component.3.ts', 'heroes')

:marked
  We'll have to update the template.
  At the moment it displays the hero's `id` and `name`.
  Let's fix that so we display only the hero's `name` property.

+makeExcerpt('app/app.component.3.ts', 'template')

:marked
  Our display looks the same, but now we know much better what a hero really is.

.l-main-section#ngIf
:marked
  ## Conditional display with NgIf

  Sometimes an app needs to display a view or a portion of a view only under specific circumstances.

  In our example, we'd like to display a message if we have a large number of heroes, say, more than 3.

  The Angular `ngIf` directive inserts or removes an element based on a !{_boolean} condition.
  We can see it in action by adding the following paragraph at the bottom of the template:

+makeExcerpt('app/app.component.ts', 'message')

.alert.is-important
  :marked
    Don't forget the leading asterisk (\*) in `*ngIf`. It is an essential part of the syntax.
    Learn more about this and `ngIf` in the [Template Syntax](./template-syntax.html#ngIf) chapter.

:marked
  The [template expression](./template-syntax.html#template-expressions) inside the double quotes
  looks much like !{_Lang}, and it _is_ much like !{_Lang}.
  When the component's list of heroes has more than 3 items, Angular adds the paragraph to the DOM and the message appears.
  If there are 3 or fewer items, Angular omits the paragraph, so no message appears.

.alert.is-helpful
  :marked
    Angular isn't showing and hiding the message. It is adding and removing the paragraph element from the DOM.
    That hardly matters here. But it would matter a great deal, from a performance perspective, if
    we were conditionally including or excluding a big chunk of HTML with many data bindings.

:marked
  Try it out. Because the !{_array} has four items, the message should appear.
  Go back into <ngio-ex path="app.component.ts"></ngio-ex> and delete or comment out one of the elements from the hero !{_array}.
  The browser should refresh automatically and the message should disappear.

.l-main-section
:marked
  ## Summary
  Now we know how to use:
  - **Interpolation** with double curly braces to display a component property
  - **ngFor** to display !{_an} !{_array} of items
  - A !{_Lang} class to shape the **model data** for our component and display properties of that model
  - **ngIf** to conditionally display a chunk of HTML based on a boolean expression

  Here's our final code:

block final-code
  +makeTabs(`displaying-data/ts/app/app.component.ts,
             displaying-data/ts/app/hero.ts,
             displaying-data/ts/app/main.ts`,
             'final,,',
             'app/app.component.ts, app/hero.ts, main.ts')