# Attribute Directives

# 属性型指令

An **Attribute** directive changes the appearance or behavior of a DOM element.

**属性**型指令用于改变一个 DOM 元素的外观或行为。

Try the <live-example title="Attribute Directive example"></live-example>.

你可以到这里试试：<live-example title="属性型指令范例"></live-example>。

{@a directive-overview}

## Directives overview

## 指令概览

There are three kinds of directives in Angular:

在 Angular 中有三种类型的指令：

1. Components&mdash;directives with a template.

   组件 &mdash; 拥有模板的指令

1. Structural directives&mdash;change the DOM layout by adding and removing DOM elements.

   结构型指令 &mdash; 通过添加和移除 DOM 元素改变 DOM 布局的指令

1. Attribute directives&mdash;change the appearance or behavior of an element, component, or another directive.

   属性型指令 &mdash; 改变元素、组件或其它指令的外观和行为的指令。

*Components* are the most common of the three directives.
You saw a component for the first time in the [Getting Started](start "Getting Started with Angular") tutorial.

*组件*是这三种指令中最常用的。
你在[快速上手](start "Getting Started with Angular")例子中第一次见到组件。

*Structural Directives* change the structure of the view.
Two examples are [NgFor](guide/template-syntax#ngFor) and [NgIf](guide/template-syntax#ngIf).
Learn about them in the [Structural Directives](guide/structural-directives) guide.

*结构型*指令修改视图的结构。例如，[NgFor](guide/template-syntax#ngFor) 和 [NgIf](guide/template-syntax#ngIf)。
要了解更多，参见[结构型指令](guide/structural-directives) guide。

*Attribute directives* are used as attributes of elements.
The built-in [NgStyle](guide/template-syntax#ngStyle) directive in the
[Template Syntax](guide/template-syntax) guide, for example,
can change several element styles at the same time.

*属性型*指令改变一个元素的外观或行为。例如，内置的 [NgStyle](guide/template-syntax#ngStyle) 指令可以同时修改元素的多个样式。

## Build a simple attribute directive

## 创建一个简单的属性型指令

An attribute directive minimally requires building a controller class annotated with
`@Directive`, which specifies the selector that identifies
the attribute.
The controller class implements the desired directive behavior.

属性型指令至少需要一个带有 `@Directive` 装饰器的控制器类。该装饰器指定了一个用于标识属性的选择器。
控制器类实现了指令需要的指令行为。

This page demonstrates building a simple _appHighlight_ attribute
directive to set an element's background color
when the user hovers over that element. You can apply it like this:

本章展示了如何创建一个简单的属性型指令 _appHighlight_ ，当用户把鼠标悬停在一个元素上时，改变它的背景色。你可以这样用它：

<code-example path="attribute-directives/src/app/app.component.1.html" linenums="false" header="src/app/app.component.html (applied)" region="applied"></code-example>

{@a write-directive}

Please note that directives _do not_ support namespaces.

注意，指令**不支持**命名空间。

<code-example path="attribute-directives/src/app/app.component.avoid.html" linenums="false" header="src/app/app.component.avoid.html (unsupported)" region="unsupported"></code-example>

### Write the directive code

### 编写指令代码

Create the directive class file in a terminal window with the CLI command [`ng generate directive`](cli/generate).

在命令行窗口下用 CLI 命令 [`ng generate directive`](cli/generate) 创建指令类文件。

<code-example language="sh" class="code-shell">
ng generate directive highlight
</code-example>

The CLI creates `src/app/highlight.directive.ts`, a corresponding test file `src/app/highlight.directive.spec.ts`, and _declares_ the directive class in the root `AppModule`.

CLI 会创建 `src/app/highlight.directive.ts` 及相应的测试文件（`src/app/highlight.directive.spec.ts`），并且在根模块 `AppModule` 中声明这个指令类。

<div class="alert is-helpful">

_Directives_ must be declared in [Angular Modules](guide/ngmodules) in the same manner as _components_.

和**组件**一样，这些**指令**也必须在[Angular 模块](guide/ngmodules)中进行声明。

</div >

The generated `src/app/highlight.directive.ts` is as follows:

生成的 `src/app/highlight.directive.ts` 文件如下：

<code-example path="attribute-directives/src/app/highlight.directive.0.ts" header="src/app/highlight.directive.ts"></code-example>

The imported `Directive` symbol provides Angular the `@Directive` decorator.

这里导入的 `Directive` 符号提供了 Angular 的 `@Directive` 装饰器。

The `@Directive` decorator's lone configuration property specifies the directive's
[CSS attribute selector](https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors), `[appHighlight]`.

`@Directive` 装饰器的配置属性中指定了该指令的 [CSS 属性型选择器](https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors) `[appHighlight]`

It's the brackets (`[]`) that make it an attribute selector.
Angular locates each element in the template that has an attribute named `appHighlight` and applies the logic of this directive to that element.

这里的方括号(`[]`)表示它的属性型选择器。
Angular 会在模板中定位每个拥有名叫 `appHighlight` 属性的元素，并且为这些元素加上本指令的逻辑。

The _attribute selector_ pattern explains the name of this kind of directive.

正因如此，这类指令被称为 **属性选择器** 。

<div class="alert is-helpful">

#### Why not "highlight"?

#### 为什么不直接叫做 "highlight"？

Though *highlight* would be a more concise selector than *appHighlight* and it would work,
the best practice is to prefix selector names to ensure
they don't conflict with standard HTML attributes.
This also reduces the risk of colliding with third-party directive names.
The CLI added the `app` prefix for you.

尽管 *highlight* 是一个比 *appHighlight* 更简洁的名字，而且它确实也能工作。
但是最佳实践是在选择器名字前面添加前缀，以确保它们不会与标准 HTML 属性冲突。
它同时减少了与第三方指令名字发生冲突的危险。

Make sure you do **not** prefix the `highlight` directive name with **`ng`** because
that prefix is reserved for Angular and using it could cause bugs that are difficult to diagnose.

确认你**没有**给 `highlight` 指令添加**`ng`**前缀。
那个前缀属于 Angular，使用它可能导致难以诊断的 bug。例如，这个简短的前缀 `app` 可以帮助你区分自定义指令。

</div>

After the `@Directive` metadata comes the directive's controller class,
called `HighlightDirective`, which contains the (currently empty) logic for the directive.
Exporting `HighlightDirective` makes the directive accessible.

紧跟在 `@Directive` 元数据之后的就是该指令的控制器类，名叫 `HighlightDirective`，它包含了该指令的逻辑（目前为空逻辑）。然后导出 `HighlightDirective`，以便它能在别处访问到。

Now edit the generated `src/app/highlight.directive.ts` to look as follows:

现在，把刚才生成的 `src/app/highlight.directive.ts` 编辑成这样：

<code-example path="attribute-directives/src/app/highlight.directive.1.ts" header="src/app/highlight.directive.ts"></code-example>

The `import` statement specifies an additional `ElementRef` symbol from the Angular `core` library:

`import` 语句还从 Angular 的 `core` 库中导入了一个 `ElementRef` 符号。

You use the `ElementRef` in the directive's constructor
to [inject](guide/dependency-injection) a reference to the host DOM element, 
the element to which you applied `appHighlight`.

你可以在指令的构造函数中使用 `ElementRef` 来[注入](guide/dependency-injection)宿主 DOM 元素的引用，也就是你放置 `appHighlight` 的那个元素。

`ElementRef` grants direct access to the host DOM element
through its `nativeElement` property.

`ElementRef` 通过其 `nativeElement` 属性给你了直接访问宿主 DOM 元素的能力。

This first implementation sets the background color of the host element to yellow.

这里的第一个实现把宿主元素的背景色设置为了黄色。

{@a apply-directive}

## Apply the attribute directive

## 使用属性型指令

To use the new `HighlightDirective`, add a paragraph (`<p>`) element to the template of the root `AppComponent` and apply the directive as an attribute.

要想使用这个新的 `HighlightDirective`，就往根组件 `AppComponent` 的模板中添加一个 `<p>` 元素，并把该指令作为一个属性使用。

<code-example path="attribute-directives/src/app/app.component.1.html" header="src/app/app.component.html" region="applied"></code-example>

Now run the application to see the `HighlightDirective` in action.

运行这个应用以查看 `HighlightDirective` 的实际效果。

<code-example language="sh" class="code-shell">
ng serve
</code-example>

To summarize, Angular found the `appHighlight` attribute on the **host** `<p>` element.
It created an instance of the `HighlightDirective` class and
injected a reference to the `<p>` element into the directive's constructor
which sets the `<p>` element's background style to yellow.

总结：Angular 在**宿主**元素 `<p>` 上发现了一个 `appHighlight` 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，该指令把 `<p>` 元素的背景设置为了黄色。

{@a respond-to-user}

## Respond to user-initiated events

## 响应用户引发的事件

Currently, `appHighlight` simply sets an element color.
The directive could be more dynamic.
It could detect when the user mouses into or out of the element
and respond by setting or clearing the highlight color.

当前，`appHighlight` 只是简单的设置元素的颜色。
这个指令应该在用户鼠标悬浮一个元素时，设置它的颜色。

Begin by adding `HostListener` to the list of imported symbols.

先把 `HostListener` 加进导入列表中。

<code-example path="attribute-directives/src/app/highlight.directive.2.ts" linenums="false" header="src/app/highlight.directive.ts (imports)" region="imports"></code-example>

Then add two eventhandlers that respond when the mouse enters or leaves,
each adorned by the `HostListener` decorator.

然后使用 `HostListener` 装饰器添加两个事件处理器，它们会在鼠标进入或离开时进行响应。

<code-example path="attribute-directives/src/app/highlight.directive.2.ts" linenums="false" header="src/app/highlight.directive.ts (mouse-methods)" region="mouse-methods"></code-example>

The `@HostListener` decorator lets you subscribe to events of the DOM
element that hosts an attribute directive, the `<p>` in this case.

`@HostListener` 装饰器让你订阅某个属性型指令所在的宿主 DOM 元素的事件，在这个例子中就是 `<p>`。

<div class="alert is-helpful">

Of course you could reach into the DOM with standard JavaScript and attach event listeners manually.
There are at least three problems with _that_ approach:

当然，你可以通过标准的 JavaScript 方式手动给宿主 DOM 元素附加一个事件监听器。
但这种方法至少有三个问题：

1. You have to write the listeners correctly.

   必须正确的书写事件监听器。

1. The code must *detach* the listener when the directive is destroyed to avoid memory leaks.

   当指令被销毁的时候，必须*拆卸*事件监听器，否则会导致内存泄露。

1. Talking to DOM API directly isn't a best practice.

   必须直接和 DOM API 打交道，应该避免这样做。

</div>

The handlers delegate to a helper method that sets the color on the host DOM element, `el`.

这些处理器委托了一个辅助方法来为 DOM 元素（`el`）设置颜色。

The helper method, `highlight`, was extracted from the constructor.
The revised constructor simply declares the injected `el: ElementRef`.

这个辅助方法（`highlight`）被从构造函数中提取了出来。
修改后的构造函数只负责声明要注入的元素 `el: ElementRef`。

<code-example path="attribute-directives/src/app/highlight.directive.2.ts" linenums="false" header="src/app/highlight.directive.ts (constructor)" region="ctor"></code-example>

Here's the updated directive in full:

下面是修改后的指令代码：

<code-example path="attribute-directives/src/app/highlight.directive.2.ts" header="src/app/highlight.directive.ts"></code-example>

Run the app and confirm that the background color appears when
the mouse hovers over the `p` and disappears as it moves out.

运行本应用并确认：当把鼠标移到 `p` 上的时候，背景色就出现了，而移开时就消失了。

<figure>
  <img src="generated/images/guide/attribute-directives/highlight-directive-anim.gif" alt="Second Highlight">
</figure>

{@a bindings}

## Pass values into the directive with an _@Input_ data binding

## 使用 `@Input` 数据绑定向指令传递值

Currently the highlight color is hard-coded _within_ the directive. That's inflexible.
In this section, you give the developer the power to set the highlight color while applying the directive.

高亮的颜色目前是硬编码在指令中的，这不够灵活。
在这一节中，你应该让指令的使用者可以指定要用哪种颜色进行高亮。

Begin by adding `Input` to the list of symbols imported from `@angular/core`.

先从 `@angular/core` 中导入 `Input`。

<code-example path="attribute-directives/src/app/highlight.directive.3.ts" linenums="false" header="src/app/highlight.directive.ts (imports)" region="imports"></code-example>

Add a `highlightColor` property to the directive class like this:

然后把 `highlightColor` 属性添加到指令类中，就像这样：

<code-example path="attribute-directives/src/app/highlight.directive.2.ts" linenums="false" header="src/app/highlight.directive.ts (highlightColor)" region="color"></code-example>

{@a input}

### Binding to an _@Input_ property

### 绑定到 *@Input* 属性

Notice the `@Input` decorator. It adds metadata to the class that makes the directive's `highlightColor` property available for binding.

注意看 `@Input` 装饰器。它往类上添加了一些元数据，从而让该指令的 `highlightColor` 能用于绑定。

It's called an *input* property because data flows from the binding expression _into_ the directive.
Without that input metadata, Angular rejects the binding; see [below](guide/attribute-directives#why-input "Why add @Input?") for more about that.

它之所以称为*输入*属性，是因为数据流是从绑定表达式流向指令内部的。
如果没有这个元数据，Angular 就会拒绝绑定，参见[稍后](guide/attribute-directives#why-input "为什么要添加@Input?")了解更多。

Try it by adding the following directive binding variations to the `AppComponent` template:

试试把下列指令绑定变量添加到 `AppComponent` 的模板中：

<code-example path="attribute-directives/src/app/app.component.1.html" linenums="false" header="src/app/app.component.html (excerpt)" region="color-1"></code-example>

Add a `color` property to the `AppComponent`.

把 `color` 属性添加到 `AppComponent` 中：

<code-example path="attribute-directives/src/app/app.component.1.ts" linenums="false" header="src/app/app.component.ts (class)" region="class"></code-example>

Let it control the highlight color with a property binding.

让它通过属性绑定来控制高亮颜色。

<code-example path="attribute-directives/src/app/app.component.1.html" linenums="false" header="src/app/app.component.html (excerpt)" region="color-2"></code-example>

That's good, but it would be nice to _simultaneously_ apply the directive and set the color _in the same attribute_ like this.

很不错，但如果可以在应用该指令时在*同一个属性*中设置颜色就更好了，就像这样：

<code-example path="attribute-directives/src/app/app.component.html" linenums="false" header="src/app/app.component.html (color)" region="color"></code-example>

The `[appHighlight]` attribute binding both applies the highlighting directive to the `<p>` element
and sets the directive's highlight color with a property binding.
You're re-using the directive's attribute selector (`[appHighlight]`) to do both jobs.
That's a crisp, compact syntax.

`[appHighlight]` 属性同时做了两件事：把这个高亮指令应用到了 `<p>` 元素上，并且通过属性绑定设置了该指令的高亮颜色。
你复用了该指令的属性选择器 `[appHighlight]` 来同时完成它们。
这是清爽、简约的语法。

You'll have to rename the directive's `highlightColor` property to `appHighlight` because that's now the color property binding name.

你还要把该指令的 `highlightColor` 改名为 `appHighlight`，因为它是颜色属性目前的绑定名。

<code-example path="attribute-directives/src/app/highlight.directive.2.ts" linenums="false" header="src/app/highlight.directive.ts (renamed to match directive selector)" region="color-2"></code-example>

This is disagreeable. The word, `appHighlight`, is a terrible property name and it doesn't convey the property's intent.

这可不好。因为 `appHighlight` 是一个糟糕的属性名，而且不能反映该属性的意图。

{@a input-alias}

### Bind to an _@Input_ alias

### 绑定到 *@Input* 别名

Fortunately you can name the directive property whatever you want _and_ **_alias it_** for binding purposes.

幸运的是，你可以随意命名该指令的属性，并且**给它指定一个用于绑定的别名**。

Restore the original property name and specify the selector as the alias in the argument to `@Input`.

恢复原始属性名，并在 `@Input` 的参数中把该选择器指定为别名。

<code-example path="attribute-directives/src/app/highlight.directive.ts" linenums="false" header="src/app/highlight.directive.ts (color property with alias)" region="color"></code-example>

_Inside_ the directive the property is known as `highlightColor`.
_Outside_ the directive, where you bind to it, it's known as `appHighlight`.

在指令内部，该属性叫 `highlightColor`，在外部，你绑定到它地方，它叫 `appHighlight`。

You get the best of both worlds: the property name you want and the binding syntax you want:

这是最好的结果：理想的内部属性名，理想的绑定语法：

<code-example path="attribute-directives/src/app/app.component.html" linenums="false" header="src/app/app.component.html (color)" region="color"></code-example>

Now that you're binding via the alias to the `highlightColor`, modify the `onMouseEnter()` method to use that property.
If someone neglects to bind to `appHighlightColor`, highlight the host element in red:

现在，你通过别名绑定到了 `highlightColor` 属性，并修改 `onMouseEnter()` 方法来使用它。
如果有人忘了绑定到 `appHighlightColor`，那就用红色进行高亮。

<code-example path="attribute-directives/src/app/highlight.directive.3.ts" linenums="false" header="src/app/highlight.directive.ts (mouse enter)" region="mouse-enter"></code-example>

Here's the latest version of the directive class.

这是最终版本的指令类。

<code-example path="attribute-directives/src/app/highlight.directive.3.ts" linenums="false" header="src/app/highlight.directive.ts (excerpt)"></code-example>

## Write a harness to try it

## 写个测试程序试验下

It may be difficult to imagine how this directive actually works.
In this section, you'll turn `AppComponent` into a harness that
lets you pick the highlight color with a radio button and bind your color choice to the directive.

凭空想象该指令如何工作可不容易。
在本节，你将把 `AppComponent` 改成一个测试程序，它让你可以通过单选按钮来选取高亮颜色，并且把你选取的颜色绑定到指令中。

Update <code>app.component.html</code> as follows:

把 `app.component.html` 修改成这样：

<code-example path="attribute-directives/src/app/app.component.html" linenums="false" header="src/app/app.component.html (v2)" region="v2"></code-example>

Revise the `AppComponent.color` so that it has no initial value.

修改 `AppComponent.color`，让它不再有初始值。

<code-example path="attribute-directives/src/app/app.component.ts" linenums="false" header="src/app/app.component.ts (class)" region="class"></code-example>

Here are the harness and directive in action.

下面是测试程序和指令的动图。

<figure>
  <img src="generated/images/guide/attribute-directives/highlight-directive-v2-anim.gif" alt="Highlight v.2">
</figure>

{@a second-property}

## Bind to a second property

## 绑定到第二个属性

This highlight directive has a single customizable property. In a real app, it may need more.

本例的指令只有一个可定制属性，真实的应用通常需要更多。

At the moment, the default color&mdash;the color that prevails until
the user picks a highlight color&mdash;is hard-coded as "red".
Let the template developer set the default color.

目前，默认颜色（它在用户选取了高亮颜色之前一直有效）被硬编码为红色。应该允许模板的开发者设置默认颜色。

Add a second **input** property to `HighlightDirective` called `defaultColor`:

把第二个名叫 `defaultColor` 的**输入**属性添加到 `HighlightDirective` 中：

<code-example path="attribute-directives/src/app/highlight.directive.ts" linenums="false" header="src/app/highlight.directive.ts (defaultColor)" region="defaultColor"></code-example>

Revise the directive's `onMouseEnter` so that it first tries to highlight with the `highlightColor`,
then with the `defaultColor`, and falls back to "red" if both properties are undefined.

修改该指令的 `onMouseEnter`，让它首先尝试使用 `highlightColor` 进行高亮，然后用 `defaultColor`，如果它们都没有指定，那就用红色作为后备。

<code-example path="attribute-directives/src/app/highlight.directive.ts" linenums="false" header="src/app/highlight.directive.ts (mouse-enter)" region="mouse-enter"></code-example>

How do you bind to a second property when you're already binding to the `appHighlight` attribute name?

当已经绑定过 `appHighlight` 属性时，要如何绑定到第二个属性呢？

As with components, you can add as many directive property bindings as you need by stringing them along in the template.
The developer should be able to write the following template HTML to both bind to the `AppComponent.color`
and fall back to "violet" as the default color.

像组件一样，你也可以绑定到指令的很多属性，只要把它们依次写在模板中就行了。
开发者可以绑定到 `AppComponent.color`，并且用紫罗兰色作为默认颜色，代码如下：

<code-example path="attribute-directives/src/app/app.component.html" linenums="false" header="src/app/app.component.html (defaultColor)" region="defaultColor"></code-example>

Angular knows that the `defaultColor` binding belongs to the `HighlightDirective`
because you made it _public_ with the `@Input` decorator.

Angular 之所以知道 `defaultColor` 绑定属于 `HighlightDirective`，是因为你已经通过 `@Input` 装饰器把它设置成了*公共*属性。

Here's how the harness should work when you're done coding.

当这些代码完成时，测试程序工作时的动图如下：

<figure>
  <img src="generated/images/guide/attribute-directives/highlight-directive-final-anim.gif" alt="Final Highlight">
</figure>

## Summary

## 小结

This page covered how to:

本章介绍了如何：

* [Build an **attribute directive**](guide/attribute-directives#write-directive) that modifies the behavior of an element.

   [构建一个**属性型指令**](guide/attribute-directives#write-directive)，它用于修改一个元素的行为。

* [Apply the directive](guide/attribute-directives#apply-directive) to an element in a template.

   [把一个指令应用到](guide/attribute-directives#apply-directive)模板中的某个元素上。

* [Respond to **events**](guide/attribute-directives#respond-to-user) that change the directive's behavior.

   [响应**事件**](guide/attribute-directives#respond-to-user)以改变指令的行为。

* [**Bind** values to the directive](guide/attribute-directives#bindings).

   [把值**绑定**到指令中](guide/attribute-directives#bindings)。

The final source code follows:

最终的源码如下：

<code-tabs>
  <code-pane header="app/app.component.ts" path="attribute-directives/src/app/app.component.ts"></code-pane>
  <code-pane header="app/app.component.html" path="attribute-directives/src/app/app.component.html"></code-pane>
  <code-pane header="app/highlight.directive.ts" path="attribute-directives/src/app/highlight.directive.ts"></code-pane>
  <code-pane header="app/app.module.ts" path="attribute-directives/src/app/app.module.ts"></code-pane>
  <code-pane header="main.ts" path="attribute-directives/src/main.ts"></code-pane>
  <code-pane header="index.html" path="attribute-directives/src/index.html"></code-pane>
</code-tabs>

You can also experience and download the <live-example title="Attribute Directive example"></live-example>.

你还可以体验和下载<live-example title="属性型指令范例"></live-example>.

{@a why-input}

### Appendix: Why add _@Input_?

### 附录：为什么要加*@Input*？

In this demo, the `highlightColor` property is an ***input*** property of
the `HighlightDirective`. You've seen it applied without an alias:

在这个例子中 `hightlightColor` 是 `HighlightDirective` 的一个***输入型***属性。你见过它没有用别名时的代码：

<code-example path="attribute-directives/src/app/highlight.directive.2.ts" linenums="false" header="src/app/highlight.directive.ts (color)" region="color"></code-example>

You've seen it with an alias:

也见过用别名时的代码：

<code-example path="attribute-directives/src/app/highlight.directive.ts" linenums="false" header="src/app/highlight.directive.ts (color)" region="color"></code-example>

Either way, the `@Input` decorator tells Angular that this property is
_public_ and available for binding by a parent component.
Without  `@Input`, Angular refuses to bind to the property.

无论哪种方式，`@Input` 装饰器都告诉 Angular，该属性是*公共的*，并且能被父组件绑定。
如果没有 `@Input`，Angular 就会拒绝绑定到该属性。

You've bound template HTML to component properties before and never used `@Input`.
What's different?

但你以前也曾经把模板 HTML 绑定到组件的属性，而且从来没有用过 `@Input`。
差异何在？

The difference is a matter of trust.
Angular treats a component's template as _belonging_ to the component.
The component and its template trust each other implicitly.
Therefore, the component's own template may bind to _any_ property of that component,
with or without the `@Input` decorator.

差异在于信任度不同。
Angular 把组件的模板看做*从属于*该组件的。
组件和它的模板默认会相互信任。
这也就是意味着，组件自己的模板可以绑定到组件的*任意*属性，无论是否使用了 `@Input` 装饰器。

But a component or directive shouldn't blindly trust _other_ components and directives.
The properties of a component or directive are hidden from binding by default.
They are _private_ from an Angular binding perspective.
When adorned with the `@Input` decorator, the property becomes _public_ from an Angular binding perspective.
Only then can it be bound by some other component or directive.

但组件或指令不应该盲目的信任其它组件或指令。
因此组件或指令的属性默认是不能被绑定的。
从 Angular 绑定机制的角度来看，它们是*私有*的，而当添加了 `@Input` 时，Angular 绑定机制才会把它们当成*公共*的。
只有这样，它们才能被其它组件或属性绑定。

You can tell if `@Input` is needed by the position of the property name in a binding.

你可以根据属性名在绑定中出现的位置来判定是否要加 `@Input`。

* When it appears in the template expression to the ***right*** of the equals (=),
  it belongs to the template's component and does not require the `@Input` decorator.

   当它出现在等号***右侧***的模板表达式中时，它属于模板所在的组件，不需要 `@Input` 装饰器。

* When it appears in **square brackets** ([ ]) to the **left** of the equals (=),
  the property belongs to some _other_ component or directive;
  that property must be adorned with the `@Input` decorator.

   当它出现在等号**左边**的**方括号（[ ]）**中时，该属性属于*其它*组件或指令，它必须带有 `@Input` 装饰器。

Now apply that reasoning to the following example:

试用此原理分析下列范例：

<code-example path="attribute-directives/src/app/app.component.html" linenums="false" header="src/app/app.component.html (color)" region="color"></code-example>

* The `color` property in the expression on the right belongs to the template's component.
  The template and its component trust each other.
  The `color` property doesn't require the `@Input` decorator.

   `color` 属性位于右侧的绑定表达式中，它属于模板所在的组件。
    该模板和组件相互信任。因此 `color` 不需要 `@Input` 装饰器。

* The `appHighlight` property on the left refers to an _aliased_ property of the `HighlightDirective`,
  not a property of the template's component. There are trust issues.
  Therefore, the directive property must carry the `@Input` decorator.

   `appHighlight` 属性位于左侧，它引用了 `HighlightDirective` 中一个*带别名的*属性，它不是模板所属组件的一部分，因此存在信任问题。
所以，该属性必须带 `@Input` 装饰器。
