anchor.diff:    each matching element, injecting an Angular `ElementRef` 
     into the constructor.
     
  +  Angular会为每个被指令匹配上的元素创建一个控制器类的实例，并把Angular的`ElementRef`注入进它的构造函数。
  +  
     `ElementRef` is a service that grants us direct access to the DOM element
     through its `nativeElement` property.
     That's all we need to set the element's background color using the browser DOM API.
  +  
  +  `ElementRef`是一个服务，通过它的`nativeElement`属性，我们可以直接访问到这个DOM元素。
  +  我们所要做的，就是使用浏览器的DOM API来设置这个元素的背景色。
   
   .l-main-section
   :marked
     ## Apply the attribute directive
  +  ## 应用Attribute型指令
     The `AppComponent` in this sample is a test harness for our `HighlightDirective`.
     Let's give it a new template that 
     applies the directive as an attribute to a `span` element.
     In Angular terms, the `<span>` element will be the attribute **host**.
     
  +  这个例子中的`AppComponent`只是我们用来测试`HighlightDirective`的一个壳儿。
  +  我们来给它一个新的模板，把这个指令作为Attribute应用到一个`span`元素上。
  +  用Angular的话说，`<span>`元素就是这个Attribute型指令的**宿主**。
  +  
     We'll put the template in its own `app.component.html` file that looks like this:
  +  
  +  我们把这个模板放到它自己的`app.component.html`文件中，就像这样：
   +makeExample('attribute-directives/ts/app/app.component.1.html',null,'app/app.component.html')(format=".")
   :marked
     A separate template file is clearly overkill for a 2-line template. 
     Hang in there; we're going to expand it later.
     Meanwhile, we'll revise the `AppComponent` to reference this template.
  +  
  +  对于一个只有两行的模板来说，使用一个独立的模板文件确实有点过分了。
  +  先别管它，我们后面很快就会扩展它。
  +  同时，我们要修改`AppComponent`，使其引用这个模板。
   +makeExample('attribute-directives/ts/app/app.component.ts',null,'app/app.component.ts')
   :marked
     We've added an `import` statement to fetch the 'Highlight' directive and 
     added that class to a `directives` array in the component metadata so that Angular 
     will recognize our directive when it encounters `myHighlight` in the template. 
  +  
  +  我们添加了一个`import`语句来获得'Highlight'指令类，并把这个类添加到`AppComponent`组件的`directives`数组中。
  +  这样，当Angular在模板中遇到`myHighlight`时，就能认出这是我们的指令了。
   
     We run the app and see that our directive highlights the span text.
  +  
  +  运行应用，就会看到我们的指令确实高亮了span中的文本。
     
   figure.image-display
     img(src="/resources/images/devguide/attribute-directives/first-highlight.png" alt="First Highlight")
   .l-sub-section
     :marked
       #### Why isn't my directive working? 
  +    #### 为什么我的指令不能工作？
   
       Did you remember to set the `directives` array? It is easy to forget!
       
  +    你记着设置`directives`数组了吗？它很容易被忘掉。
  +    
       Open the console in the browser tools and look for an error like this:
  +    
  +    打开浏览器调试工具的控制台，会看到像这样的错误信息：
     code-example.format("").
       EXCEPTION: Template parse errors:
         Can't bind to 'myHighlight' since it isn't a known native property 
     :marked
       Angular detects that we're trying to bind to *something* but it doesn't know what.
       We have to tell it by listing `HighlightDirective` in the `directives` metadata array.
  +    
  +    这是因为Angular检测到我们正在尝试绑定到*某些东西*，但它不认识。
  +    我们得通过把`HighlightDirective`列在元数据的`directives`数组中来告诉它有这样一个指令。
   :marked
     Let's recap what happened.
  +  
  +  我们来概括一下发生了什么。
     
     Angular found the `myHighlight` attribute on the `<span>` element. It created
     an instance of the `HighlightDirective` class, 
     injecting a reference to the element into the constructor 
     where we set the `<span>` element's background style to yellow.
  +  
  +  Angular在`<span>`元素上发现了一个`myHighlight`属性。
  +  然后它创建了一个`HighlightDirective`类的实例，并把所在元素的引用注入到了指令的构造函数中。
  +  在构造函数中，我们把`<span>`元素的背景设置为了黄色。
   
   .l-main-section
   :marked
     ## Respond to user action
  +  ## 响应用户的操作
   
     We are not satisfied to simply set an element color.
     Our directive should set the color in response to a user action.
     Specifically, we want to set the color when the user mouses over the element.
     
  +  我们不能满足于设置元素的颜色。
  +  我们的指令设置颜色是要用来响应用户的操作。
  +  特别是，希望在用户的鼠标滑过当前元素时设置颜色。
  +  
     We'll need to
  +  
  +  我们需要：
     1. detect when the user mouses into and out of the element
  +  1. 检测用户的鼠标啥时候进入和离开这个元素。
     1. respond to those actions by setting and clearing the highlight color.
  +  1. 通过设置和清除背景颜色来响应这些操作。
     
     Start with event detection. 
     We add a `host` property to the directive metadata and give it a configuration object
     that specifies two mouse events and the directive methods to call when they are raised.
  +  
  +  从事件检测开始。
  +  我们把`host`属性加入指令的元数据中，并给它一个配置对象，用来指定两个鼠标事件，并在它们被触发时，调用指令中的方法。
   +makeExample('attribute-directives/ts/app/highlight.directive.2.ts','host')(format=".")
   :marked
   .l-sub-section
     :marked
       The `host` property refers to the DOM element that hosts our attribute directive, the `<span>` in our case.
  +    
  +    `host`属性引用的是我们这个Attribute指令的宿主元素，在这个例子中就是`<span>`。
        
       We could have attached an event listener to the native element (`el.nativeElement`) with 
       plain old JavaScript. 
       There are at least three problems with that approach:
       
  +    我们可以通过老旧的JavaScript方式来给这个原生元素(`el.nativeElement`)挂上一个事件监听器。
  +    但这种方法至少有三个问题：
       1. We have to write the listeners correctly.
  +    1. 我们必须正确的书写事件监听器。
       1. We must *detach* our listener when the directive is destroyed to avoid memory leaks.
  +    1. 当指令被销毁的时候，我们必须*摘掉*我们的事件监听器，否则就会导致内存泄露。
       1. We'd be talking to DOM API directly which, we learned, is something to avoid.
  +    1. 我们必须直接和DOM API打交道，但正如我们学过的那样，应该避免这样做。
   
       Let's roll with the `host` property. 
  +    
  +    我们还是围绕`host`属性来吧。
   :marked
     Now we implement those two mouse event handlers:
  +  
  +  现在，我们实现那两个鼠标事件处理器：
   +makeExample('attribute-directives/ts/app/highlight.directive.2.ts','mouse-methods')(format=".")
   :marked
     Notice that they delegate to a helper method that sets the color via a private local variable, `_el`.
     We revise the constructor to capture the `ElementRef.nativeElement` in `_el`. 
  +  
  +  注意，它们把处理逻辑委托给了一个辅助方法，这个方法会通过一个私有变量`_el`来设置颜色。
  +  我们要修改构造函数，来把`ElementRef.nativeElement`存进私有变量`_el`。
   
   +makeExample('attribute-directives/ts/app/highlight.directive.2.ts','ctor')(format=".")
   :marked
     Here's the updated directive:
  +  
  +  这里是更新过的指令：
   +makeExample('attribute-directives/ts/app/highlight.directive.2.ts',null, 'app/highlight.directive.ts')
   :marked
     We run the app and confirm that the background color appears as we move the mouse over the `span` and
     disappears as we move out.
  +  
  +  运行本应用，我们就可以确认：当把鼠标移到`span`上的时候，背景色就出现了，而移开的时候，它消失了。
   figure.image-display
     img(src="/resources/images/devguide/attribute-directives/highlight-directive-anim.gif" alt="Second Highlight")
   :marked
   .l-main-section
   :marked
     ## Configure the directive with binding
  +  ## 通过绑定来配置指令
   
     Currently the highlight color is hard-coded within the directive. That's inflexible.
     We should set the highlight color externally with a binding like this:
  +  
  +  现在的高亮颜色是在指令中硬编码进去的。这样没有弹性。
  +  我们应该通过绑定从外部设置这个高亮颜色。就像这样：
   +makeExample('attribute-directives/ts/app/app.component.html','span')
   :marked
     We'll extend our directive class with a bindable **input** `highlightColor` property and use it when we highlight text.
  +  
  +  我们将给指令类增加一个可绑定**输入**属性`highlightColor`，当我们需要高亮文本的时候，就用它。
   
     Here is the final version of the class:
  +  
  +  这里是该类的最终版：
     
   +makeExample('attribute-directives/ts/app/highlight.directive.ts', 'class-1', 'app/highlight.directive.ts (class only)')
   <a id="input"></a>
   :marked
     The new `highlightColor` property is called an "input" property because data flows from the binding expression into our directive.
     Notice that we call the `@Input()` decorator function while defining the property.
  +  
  +  新的`highlightColor`属性被称为“输入”属性，这是因为数据流是从绑定表达式到这个指令的。
  +  注意，我们在定义这个属性的时候，调用了`@Input()`装饰器函数。
   +makeExample('attribute-directives/ts/app/highlight.directive.ts', 'color')
   :marked
     This `@Input` decorator adds metadata to the class that makes the `highlightColor` property  available for property binding
     under the `myHighlight` alias. 
     We must add this input metadata or Angular will reject the binding.
     See the [appendix](#why-input) below to learn why.
  +  
  +  这个`@Input`装饰器把元数据添加到了类上，这让`highlightColor`能被以`myHighlight`为别名进行绑定。
  +  我们必须添加这个input元数据，否则Angular会拒绝绑定。
  +  参见下面的[附录](#why-input)来了解为何如此。
   .l-sub-section
     :marked
       ### @Input(alias) 
  +    ### @Input(别名)
       The developer who uses our directive expects to bind to the attribute name, `myHighlight`.
       The directive property name is `highlightColor`. That's a disconnect.
       
  +    使用我们这个指令的开发人员会期望绑定到Attribute名`myHighlight`上，
  +    而指令中的属性名是`highlightColor`。两者联系不起来。
  +    
       We can resolve the discrepancy by renaming the property to `myHighlight` and define it as follows:
  +    
  +    我们固然可以通过把属性名改为`myHighlight`来解决这个矛盾，就像这样：
   
     +makeExample('attribute-directives/ts/app/highlight.directive.ts', 'highlight')
     <br>
  
positive.code: .l-main-section
  :marked
    ### Appendix: Input properties
    
    Earlier we declared the `highlightColor` property to be an ***input*** property of our
    `HighlightDirective`
   
    We've seen properties in bindings before. 
    We never had to declare them as anything. Why now?
    
    Angular makes a subtle but important distinction between binding **sources** and **targets**.
    
    In all previous bindings, the directive or component property was a binding ***source***.
    A property is a *source* if it appears in the template expression to the ***right*** of the equals (=).
    
    A property is a *target* when it appears in **square brackets** ([ ]) to the **left** of the equals (=) ...
    as it is does when we bind to the `myHighlight` property of the `HighlightDirective`, 
  +makeExample('attribute-directives/ts/app/app.component.html','span')(format=".")
  :marked
    The 'color' in `[myHighlight]="color"` is a binding ***source***.
    A source property doesn't require a declaration.
    
    The 'myHighlight' in `[myHighlight]="color"` *is* a binding ***target***.
    We must declare it as an *input* property.
    Angular rejects the binding with a clear error if we don't.
    
    Angular treats a *target* property differently for a good reason.
    A component or directive in target position needs protection.
    
    Imagine that our `HighlightDirective` did truly wonderous things.
    We graciously made a gift of it to the world. 
    
    To our surprise, some people &mdash; perhaps naively &mdash;
    started binding to *every* property of our directive. 
    Not just the one or two properties we expected them to target. *Every* property.
    That could really mess up our directive in ways we didn't anticipate and have no desire to support.
    
    The *input* declaration ensures that consumers of our directive can only bind to
    the properties of our public API ... nothing else.
  
negative.code:     doesn't express our intention well. 
      We can **alias** the `highlightColor` property with the attribute name by
      passing `myHighlight` into the `@Input` decorator:
      
      但我们可能在指令中不想要那样一个属性名，因为它不能很好的表示我们的意图。
      我们可以通过把`myHighlight`传给`@Input`装饰器来把这个Attribute名字作为`highlightColor`属性的别名。
    +makeExample('attribute-directives/ts/app/highlight.directive.ts', 'color')
  :marked
    Now that we're getting the highlight color as an input, we modify the `onMouseEnter()` method to use
    it instead of the hard-coded color name.
    We also define a red default color as a fallback in case
    the user neglects to bind with a color.
    
    现在，我们通过输入型属性得到了高亮的颜色，然后修改`onMouseEnter()`来使用它代替我们硬编码的那个颜色名。
    我们还把红色定义为默认颜色，以便在用户忘了绑定颜色时作为备用。
  +makeExample('attribute-directives/ts/app/highlight.directive.ts', 'mouse-enter')
  :marked
    Now we'll update our `AppComponent` template to let 
    users pick the highlight color and bind their choice to our directive.
    
    我们这就更新`AppComponent`的模板，来让用户选择一个高亮颜色，并把选择结果绑定到我们的指令上。
    
    Here is the updated template:
    
    这里是更新后的模板：
  
  +makeExample('attribute-directives/ts/app/app.component.html', 'v2')
  
  .l-sub-section
    :marked
      ### Where is the templated *color* property?
      ### 模板的*color*属性在哪里？
      
      The eagle-eyed may notice that the radio button click handlers in the template set a `color` property
      and we are binding that `color` to the directive. 
      We should expect to find a `color` on the host `AppComponent`.
      
      眼光锐利的你可能发现了，模板中的单选按钮的点击事件处理器设置了一个`color`属性，而且我们把`color`绑定到了我们的指令上。
      我们会期望在这个宿主`AppComponent`上发现一个`color`属性。
      
      **We never defined a color property for the host *AppComponent***!
      And yet this code works. Where is the template `color` value going?
      
      **但我们从来没有在宿主`AppComponent`上定义过color属性**！
      不过这段代码却能正常工作。模板中的`color`值哪儿去了？
      
      Browser debugging reveals that Angular dynamically added a `color` property 
      to the runtime instance of the `AppComponent`.
      
      在浏览器中调试就会发现，Angular在`AppComponent`的运行期实例上添加了一个`color`属性。
      
      This is *convenient* behavior but it is also *implicit* behavior that could be confusing. 
      While it's cool that this technique works, we recommend adding the `color` property to the `AppComponent`.
      
      这是一个*很便利的*行为，但它也是*隐式的*行为，这容易让人困惑。
      虽然用这种技术可能显得比较酷，但我们建议你还是要把`color`属性加到`AppComponent`中。
  
  :marked
    Here is our second version of the directive in action.
    
    下面是指令操作演示的第二版。
  figure.image-display
    img(src="/resources/images/devguide/attribute-directives/highlight-directive-v2-anim.gif" alt="Highlight v.2")
  
  .l-main-section
  :marked
    ## Bind to a second property
    ## 绑定到第二个属性
    Our directive only has a single, customizable property. What if we had ***two properties***?
    
    我们的指令只有一个可定制属性，如果有***两个***呢？
    
    Let's let the template developer set the default color, the color that prevails until the user picks a highlight color.
    We'll add a second **input** property to `HighlightDirective` called `defaultColor`:
    
    我们要让模板开发者设置一个默认颜色，直到用户选择了一个高亮颜色才失效。
    让我们给`HighlightDirective`添加第二个**输入型**属性`defaultColor`：
  +makeExample('attribute-directives/ts/app/highlight.directive.ts', 'defaultColor')(format=".")
  :marked
    The `defaultColor` property has a setter that overrides the hard-coded default color, "red".
    We don't need a getter.
    
    `defaultColor`属性是一个setter函数，它代替了硬编码的默认颜色“red”。我们不需要getter函数。
    
    How do we bind to it? We already "burned" the `myHighlight` attribute name as a binding target.
    
    我们该如何绑定到它？别忘了我们已经把`myHighlight`属性名用作绑定目标了。
    
    Remember that a *component is a directive too*. 
    We can add as many component property bindings as we need by stringing them along in the template
    as in this example that sets the `a`, `b`, `c` properties to the string literals 'a', 'b', and 'c'. 
    
    记住，*组件也是指令*。
    只要需要，我们就可以通过把它们依次串在模板中来为组件添加多个属性绑定。
    下面这个例子中就把`a`、`b`、`c`属性设置为了字符串字面量'a', 'b', 'c'。
  code-example(format="." ).
    &lt;my-component [a]="'a'" [b]="'b'" [c]="'c'">&lt;my-component>
  :marked
    We do the same thing with an attribute directive.
    
    在Attribute型指令中也可以这样做。
  +makeExample('attribute-directives/ts/app/app.component.html', 'defaultColor')(format=".")
  :marked
    Here we're binding the user's color choice to the `myHighlight` attribute as we did before.
    We're *also* binding the literal string, 'violet', to the `defaultColor`.
    
    这里，我们像以前一样把用户选择的颜色绑定到了`myHighlight`上。
    我们*还*把字符串字面量'violet'绑定到了`defaultColor`上。
      
    Here is the final version of the directive in action.
    
    下面就是该指令最终版的操作演示。
  figure.image-display
    img(src="/resources/images/devguide/attribute-directives/highlight-directive-final-anim.gif" alt="Final Highlight")
  
  .l-main-section
  :marked
    ## Summary
    ## 总结
    Now we know how to
    
    现在，我们知道了该如何：
    - build a simple **attribute directive** to attach behavior to an HTML element,
    - 构建一个简单的**Attribute型指令**来为一个HTML元素添加行为，
    - use that directive in a template,
    - 在模板中使用那个指令，
    - respond to **events** to change behavior based on an event,
    - 响应**事件**，以便基于事件改变行为。
    - and use **binding** to pass values to the attribute directive.
    - 以及使用**绑定**来把值传给Attribute型指令。
  
    The final source:
  
    最终的源码如下：
  +makeTabs(
    `attribute-directives/ts/app/app.component.ts,
     attribute-directives/ts/app/app.component.html, 
  