include ../_util-fns

<!-- http://plnkr.co/edit/x9JYbC -->

:marked
  We typically display data in Angular by binding controls in an HTML template
  to properties of an Angular component.

  Angular中典型的显示数据的方式就是把HTML模板中的控件绑定到Angular组件上的一个属性。

  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 selected hero in a detail area below the list.
  
  本章中，我们将创建一个英雄列表组件。每个英雄都有一个名字。我们将显示英雄名字的列表，并在列表下方的详情区显示所选英雄的详情。

  The final UI looks like this:
  
  最终的UI类似于这样：

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

:marked
  [Run the live example](/resources/live-examples/displaying-data/ts/plnkr.html)
  
  [运行鲜活范例](/resources/live-examples/displaying-data/ts/plnkr.html)

<a id="interpolation"></a>
.l-main-section
:marked
  ## Showing component properties with interpolation
  ## 使用插值表达式显示组件属性
  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}}`.
  
  显示组件属性的最简单方式就是通过插值表达式来绑定属性名。
  通过插值表达式，我们把属性名放进视图模板中，包裹在双重花括号中。

  Let's build a small illustrative example together.
  
  接下来就我们一起构建一个简明的范例。

  Create a new project folder (`displaying-data`) and follow the steps in the [QuickStart](../quickstart.html).
  
  创建一个新的项目文件夹(`displaying-data`)，并且遵循[QuickStart](../quickstart.html)中的下列步骤。

include ../_quickstart_repo
:marked
  Then modify the `app.component.ts` file by changing the template and the body of the component.
  When we're done, it should look like this:
  
  然后，修改`app.component.ts`文件中的模板和组件体。
  修改完之后，它看起来应该是这样：

+makeExample('displaying-data/ts/app/app.component.1.ts', null, 'app/app.component.ts')

:marked
  We added two properties to the formerly empty component: `title` and `myHero`.
  
  再添加两个属性`title`和`myHero`到以前的空白组件中。

  Our revised template displays the two component properties using double curly brace
  interpolation:
  
  修改过的模板使用双花括号插值表达式来显示这两个模板属性：

+makeExample('displaying-data/ts/app/app.component.1.ts', 'template')(format=".")
.l-sub-section
  :marked
    The template is a multi-line string within ECMAScript 2015 backticks (\`).
    The backtick (\`) &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.
    
    模板是包括在反引号(\`)中的一个多行字符串。
    反引号(\`) —— 不是单引号(') —— 有很多好用的特性。我们在这里用到的是它把一个字符串写成多行的能力，以便写出更具可读性的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.
  
  Angular自动从组件中拉取`title`和`myHero`属性的值，并且把这些值插入浏览器中。一旦这些属性发生变化，Angular就会刷新显示。
.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.
    
    更准确的说，重新显示发生在某些关联到视图中的异步事件之后，比如：按键、定时器或收到异步`XHR`响应。
    本例子中没有展示这些。但显然，属性肯定不会无缘无故的变化。现在，我们只要相信这一点就行了。
:marked
  Notice that we haven't called **new** to create an instance of the `AppComponent` class.
  Angular is creating an instance for us. How?
  
  注意，我们从没调用 **new** 来创建`AppComponent`类的实例。
  Angular为我们创建了一个实例。如何创建？

  Notice the CSS `selector` in the `@Component` decorator that specifies an element named "my-app".
  Remember back in QuickStart that we added the `<my-app>` element to the body of our `index.html`
  
  注意`@Component`装饰器中指定的CSS选择器`selector`，它指定了一个叫`my-app`的元素。
  回忆下，在QuickStart中，我们曾把一个`<my-app>`元素添加到`index.html`的`body`里。
+makeExample('displaying-data/ts/index.html', 'my-app')(format=".")

:marked
  When we bootstrap with the `AppComponent` class (see `main.ts`), 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.
  
  当我们通过`main.ts`中的`AppComponent`类启动时，Angular在`index.html`中查找一个`<my-app>`元素，
  找到它，实例化一个`AppComponent`，然后把这个实例渲染进`<my-app>`标签中。

  We're ready to see changes in a running app by firing up the npm script that both compiles and serves our applications
  while watching for changes.
  
  通过运行npm脚本（它能编译并启动一个能监听变化的服务器），我们能看到运行中的应用发生的变化。
code-example(format="").
  npm start
:marked
  We should see the title and hero name:
  
  我们应该看到标题和英雄名变了：
figure.image-display
  img(src="/resources/images/devguide/displaying-data/title-and-hero.png" alt="标题和英雄")
:marked
  Let's review some of the choices we made and consider alternatives.
  
  我们来回顾一下以前所做的决定，看看还有哪些其它选择。

  ## Template inline or template file?
  ## 行内模板还是模板文件？

  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.
  
  我们可以把组件模板放在两种地方之一。
  我们可以使用`template`属性把它定义为 *行内Inline* 的，就像这里所做的一样。
  或者，可以把模板定义在一个独立的HTML文件中，并且在组件元数据中使用`@Component`装饰器的`templateUrl`属性链接到它。

  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 HTML file.
  
  选择行内HTML还是独立HTML，取决于：个人喜好、具体状况和组织级策略。
  这里我们使用行内HTML，是因为模板很小，并且这个演示很简单，没必要用HTML文件。

  In either style, the template data bindings have the same access to the component's properties.
  
  无论哪种风格，模板中的数据绑定在访问组件属性方面都是完全一样的。

  ## Constructor or variable initialization?
  ## 用构造函数进行初始化还是用变量？

  We initialized our component properties using variable assignment.
  This is a wonderfully concise and compact technique.
  
  这里我们使用了变量赋值的方式初始化组件的属性。
  这在技术上简洁明了。

  Some folks prefer to declare the properties and initialize them within a constructor like this:
  
  也有些人喜欢单独声明属性，并且在构造函数中初始化它们，就像这样：
+makeExample('displaying-data/ts/app/app-ctor.component.ts', 'app-ctor')(format=".")

: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.

  这也挺好。这个选择取决于个人喜好和组织级策略。
  本章中，我们简单的选用了更简短的“变量赋值”风格，因为它们代码更少，更容易阅读。
<a id="ngFor"></a>
.l-main-section
:marked
  ## Showing an array property with ***ngFor***
  ## 使用 ***ngFor*** 显示数组属性

  We want to display a list of heroes. We begin by adding a mock heroes name array to the component,
  just above `myHero`, and redefine `myHero` to be the first name in the array.
  
  我们想显示一个英雄列表。我们先在组件的`myHero`属性上方添加一个模拟的英雄名字数组，并且把`myHero`重定义为数组中的第一个名字。
+makeExample('displaying-data/ts/app/app.component.2.ts', 'mock-heroes', 'app/app.component.ts (类)')(format=".")

:marked
  Now we use the Angular `ngFor` "repeater" directive in the template to display
  each item in the `heroes` list.
  
  现在我们在模板中使用Angular的`NgFor`“重复器”指令来显示`heroes`列表中的每一个条目。

+makeExample('displaying-data/ts/app/app.component.2.ts', 'template','app/app.component.ts (模板)')(format=".")

:marked
  Our presentation is the familiar HTML unordered list with `<ul>` and `<li>` tags. Let's focus on the `<li>` tag.
  
  我们的表现层是熟悉的HTML —— 由`<ul>`和`<li>`标签组成的无序列表。我们重点来看`<li>`标签。
+makeExample('displaying-data/ts/app/app.component.2.ts', 'li-repeater')(format=".")

: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".

  我们把看起来颇有神秘感的`*ngFor`属性添加到`<li>`元素上。
  这就是Angular的“重复器”指令。
  它出现在`<li>`标签上就表示把`<li>`元素（及其子元素）作为“重复器的模板”。
.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.
    
    不要忘记`*ngFor`中的前导星号(\*)。它是语法中不可或缺的一部分。
    要了解关于此语法和`NgFor`的更多知识，请参见[模板语法](./template-syntax.html#ngFor)一章。
:marked
  Notice the `hero` in the `NgFor` double-quoted instruction;
  it is an example of a [template input variable](./template-syntax.html#ngForMicrosyntax).

  注意`ngFor`的双引号表达式中的`hero`部分。
  它是一个[模板输入变量](./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.
  
  Angular为列表中的每一个条目复制`<li>`元素。在每个迭代中，都会把`hero`变量设置为当前条目（此英雄）。
  Angular把`hero`变量作为双花括号中插值表达式的上下文。
.l-sub-section
  :marked
    We happened to give `ngFor` an array to display.
    In fact, `ngFor` can repeat items for any [iterable](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols)
    object.
    
    我们这里所做的是给`NgFor`一个数组供它显示。
    实际上，`NgFor`可以为任何[可迭代Iterable](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols)对象重复渲染条目。
:marked
  Assuming we're still running under the `npm start` command,
  we should see heroes appearing in an unordered list.
  
  如果我们仍在运行`npm start`命令，我们将看到英雄们的数据展现在了一个无序列表中。
figure.image-display
  img(src="/resources/images/devguide/displaying-data/hero-names-list.png" alt="ngfor之后")

.l-main-section
:marked
  ## Creating a class for the data
  ## 为数据创建一个类

  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.
  
  我们在组件内部直接定义了我们的对象。
  这对于演示来说挺不错，但它当然不是最佳实践。它甚至不是一个好的实践。
  虽然我们在本章中不会改进它，但是我们先记下来，等将来再修复这个问题。

  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 displaying objects &mdash; potentially instances of classes.
  
  现在，我们绑定到了一个字符串数组。在真实的应用中，这是特例。大多数时候，我们显示的是对象 —— 由类创建的实例 —— 的数组。

  Let's turn our array of hero names into an array of `Hero` objects. For that we'll need a `Hero` class.
  
  我们来把英雄名字的数组转换成`Hero`对象的数组。我们得先有一个`Hero`类。

  Create a new file in the `app/` folder called `hero.ts` with the following short bit of code.
  
  在`app/`目录下创建一个名叫`hero.ts`的新文件，内容如下：
+makeExample('displaying-data/ts/app/hero.ts', null, 'app/hero.ts')(format = ".")

:marked
  We've defined a class with a constructor and two properties: `id` and `name`.
  
  我们已经定义了一个带有构造函数和两个属性：`id`和`name`的类。

  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.
  
  它可能看上去不像是有属性的，但确实有。我们正在使用TypeScript的高级简写形式：用构造函数的参数定义属性。
  
  Consider the first parameter:
  
  来看第一个参数：
+makeExample('displaying-data/ts/app/hero.ts', 'id-parameter')

:marked
  That brief syntax does a lot:
  这个简写语法做了很多：
  * declares a constructor parameter and its type
  * 定义了一个构造函数参数及其类型
  * declares a public property of the same name
  * 定义了一个同名的公开属性
  * initializes that property with the corresponding argument when we "new" an instance of the class
  * 当我们`new`出该类的一个实例时，把该属性初始化为相应的参数值

.l-main-section
:marked
  ## Using the Hero class
  ## 使用Hero类
  Let's redefine the `heroes` property in our component to return an array of these Hero objects
  and also set the `myHero` property with the first of these mock heroes.
  
  我们要把组件的`heroes`属性重定义为这些Hero对象构成的数组，并且把这个数组中的第一项赋值给`myHero`属性。
+makeExample('displaying-data/ts/app/app.component.3.ts', 'heroes', 'app.component.ts (节选)')(format=".")

:marked
  We'll have to update the template.
  At the moment it displays the entire `hero` object, which used to be a string value.
  Let's fix that so we interpolate the `hero.name` property.
  
  我们还得更新下模板。
  现在它显示的是整个`hero`对象的值，它是被当做字符串值使用了。
  我们要修复它，所以，我们的插值表达式应该使用`hero.name`属性。
+makeExample('displaying-data/ts/app/app.component.3.ts', 'template','app.component.ts (模板)')(format=".")

:marked
  Our display looks the same, but now we know much better what a hero really is.
  
  从显示上看还是那样，但现在除了名字之外，我们对该英雄有了更多了解。

<a id="ngIf"></a>
.l-main-section
:marked
  ## Conditional display with NgIf
  ## 通过NgIf进行条件显示

  Sometimes the app should display a view or a portion of a view only under specific circumstances.
  
  有时候，本应用希望只在特定情况下才显示视图或视图的一部分。

  In our example, we'd like to display a message if we have a large number of heroes &mdash; say, more than 3.
  
  在我们的例子中，如果有大量的英雄 —— 比如大于3，我们会希望显示一条消息。

  The Angular `ngIf` directive inserts or removes an element based on a truthy/falsey condition.
  We can see it in action by adding the following paragraph at the bottom of the template:
  
  Angular的`NgIf`指令会基于条件的真假来显示或移除一个元素。
  我们来亲自动手试一下，把下列语句加到模板的底部：
+makeExample('displaying-data/ts/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.

    不要忘了`*ngIf`中的前导星号(\*)。它是本语法中不可或缺的一部分。
    要学习此语法和`NgIf`，参见[模板语法](./template-syntax.html#ngIf)一章。
:marked
  The [template expression](./template-syntax.html#template-expressions) inside the double quotes
  looks much like JavaScript and it _is_ much like JavaScript.
  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.

  双引号中的[模板表达式](./template-syntax.html#template-expressions)，看起来很像JavaScript，并且它 _只是_ 像JavaScript。
  当组件中的英雄列表有三个以上的条目时，Angular把这些语句添加到DOM中，于是消息显示了出来。
  如果少于或等于三个条目，Angular会移除这些语句，于是不显示任何消息。
.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.
    
    Angular并不是在显示和隐藏这条消息，它是在从DOM中添加和移除这段元素。
    在这个范例中，他们几乎等价。但是如果我们要根据条件包含或排除一大段具有很多数据绑定的HTML，性能上的区别就会很明显。
:marked
  Try it out. Because the array has four items, the message should appear.
  Go back into `app.component.ts` and delete or comment out one of the elements from the hero array.
  The browser should refresh automatically and the message should disappear.

  试一下。因为数组中有四个条目，所以消息应该显示出来。
  回到`app.component.ts`，并从英雄数组中删除或注释掉一个元素。
  浏览器应该自动刷新，而且消息应该消失了。
.l-main-section
:marked
  ## Summary
  ## 小结
  Now we know how to use:
  现在我们知道了如何使用：
  - **interpolation** with double curly braces to display a component property
  - 带有双花括号的 **插值表达式interpolation** 用来显示组件的一个属性
  - **`ngFor`** to display a list of items
  - **`ngFor`** 用来显示条目列表
  - a TypeScript class to shape the **model data** for our component and display properties of that model
  - 一个TypeScript类，用来为我们的组件描述 **模型数据** 并且显示模型的那些属性。
  - **`ngIf`** to conditionally display a chunk of HTML based on a boolean expression
  - **`ngIf`** 用来基于一个boolean表达式有条件的显示一段HTML

  Here's our final code:
  
  下面是我们的最终代码：

+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')
