angular-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
  You can display data by binding controls in an HTML template to properties of an Angular component.

  In this page, you'll create a component with a list of heroes.
  You'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
  # 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 page.

.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, you put the property name in the view template, enclosed in double curly braces: `{{myHero}}`.

  To build an illustrative example, start by creating a new project folder called <ngio-ex path="displaying-data"></ngio-ex>
  and following the steps in [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 you're done, it should look like this:

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

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

  The 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;allows you to compose a string over several lines, which makes the
      HTML more readable.

: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 a response to an HTTP request.

:marked
  Notice that you don't call **new** to create an instance of the `AppComponent` class.
  Angular is creating an instance for you. How?

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

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

:marked
  When you 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.

  Now run 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
    The next few sections review some of the coding choices in the app.

:marked
  ## Template inline or template file?

  You can store your component's template in one of two places.
  You can define it *inline* using the `template` property, or you 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 the app uses 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?

    Although this example uses variable assignment to initialize the components, you can instead declare and initialize the properties using a constructor:

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

  :marked
    This app uses more terse "variable assignment" style simply for brevity.

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

  To display a list of heroes, 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 use the Angular `ngFor` directive in the template to display
  each item in the `heroes` list.

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

:marked
  This UI uses the HTML unordered list with `<ul>` and `<li>` tags. The `*ngFor`
  in the `<li>` element is the Angular "repeater" directive.
  It marks that `<li>` element (and its children) as the "repeater template":

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

.alert.is-important
  :marked
    Don't forget the leading asterisk (\*) in `*ngFor`. It is an essential part of the syntax.
    For more information, see the [Template Syntax](./template-syntax.html#ngFor) page.

:marked
  Notice the `hero` in the `ngFor` double-quoted instruction;
  it is an example of a template input variable. Read
  more about template input variables in the [microsyntax](./template-syntax.html#ngForMicrosyntax) section of
  the [Template Syntax](./template-syntax.html) page.

  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
    In this case, `ngFor` is displaying !{_an} !{_array}, but `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

  The app's code defines the data directly inside the component, which isn't best practice.
  In a simple demo, however, it's fine.

  At the moment, the binding is to !{_an} !{_array} of strings.
  In real applications, most bindings are to more specialized objects.

  To convert this binding to use specialized objects, turn the !{_array}
  of hero names into !{_an} !{_array} of `Hero` objects. For that you'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
    You've defined a class with a constructor and two properties: `id` and `name`.

    It might not look like the class has properties, but it does.
    The declaration of the constructor parameters takes advantage of a TypeScript shortcut.

    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 creating an instance of the class.

.l-main-section
:marked
  ## Using the Hero class

  The `heroes` property in the component can now use the `Hero` class to return !{_an} !{_array}
  of `Hero` objects:

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

:marked
  Next, update the template.
  At the moment it displays the hero's `id` and `name`.
  Fix that to display only the hero's `name` property.

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

:marked
  The display looks the same, but the code is clearer.

.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.

  Let's change the example to display a message if there are more than three heroes.

  The Angular `ngIf` directive inserts or removes an element based on a !{_boolean} condition.
  To see it in action, add 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.
    Read more about `ngIf` and `*` in the [ngIf section](./template-syntax.html#ngIf) of the [Template Syntax](./template-syntax.html) page.

:marked
  The template expression inside the double quotes,
  `*ngIf="heros.length > 3"`, looks and behaves much like !{_Lang}.
  When the component's list of heroes has more than three items, Angular adds the paragraph
  to the DOM and the message appears. If there are three or fewer items, Angular omits the
  paragraph, so no message appears. For more information,
  see the [template expressions](./template-syntax.html#template-expressions) section of the
  [Template Syntax](./template-syntax.html) page.

.alert.is-helpful
  :marked
    Angular isn't showing and hiding the message. It is adding and removing the paragraph element from the DOM. That improves performance, especially in larger projects when conditionally including or excluding
    big chunks 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 you 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 your component and display properties of that model.
  - **ngIf** to conditionally display a chunk of HTML based on a boolean expression.

  Here's the final code:

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