生命周期钩子link

Lifecycle hookslink

当 Angular 实例化组件类并渲染组件视图及其子视图时，组件实例的生命周期就开始了。生命周期一直伴随着变更检测，Angular 会检查数据绑定属性何时发生变化，并按需更新视图和组件实例。当 Angular 销毁组件实例并从 DOM 中移除它渲染的模板时，生命周期就结束了。当 Angular 在执行过程中创建、更新和销毁实例时，指令就有了类似的生命周期。

A component instance has a lifecycle that starts when Angular instantiates the component class and renders the component view along with its child views. The lifecycle continues with change detection, as Angular checks to see when data-bound properties change, and updates both the view and the component instance as needed. The lifecycle ends when Angular destroys the component instance and removes its rendered template from the DOM. Directives have a similar lifecycle, as Angular creates, updates, and destroys instances in the course of execution.

你的应用可以使用生命周期钩子方法来触发组件或指令生命周期中的关键事件，以初始化新实例，需要时启动变更检测，在变更检测过程中响应更新，并在删除实例之前进行清理。

Your application can use lifecycle hook methods to tap into key events in the lifecycle of a component or directive in order to initialize new instances, initiate change detection when needed, respond to updates during change detection, and clean up before deletion of instances.

先决条件link

Prerequisiteslink

在使用生命周期钩子之前，你应该对这些内容有一个基本的了解：

Before working with lifecycle hooks, you should have a basic understanding of the following:

• TypeScript 编程 。

  TypeScript programming.

• Angular 应用设计基础，就像 Angular 的基本概念中所讲的那样。

  Angular app-design fundamentals, as described in Angular Concepts.

响应生命周期事件link

Responding to lifecycle eventslink

你可以通过实现一个或多个 Angular core 库中定义的生命周期钩子接口来响应组件或指令生命周期中的事件。这些钩子让你有机会在适当的时候对组件或指令实例进行操作，比如 Angular 创建、更新或销毁这个实例时。

You can respond to events in the lifecycle of a component or directive by implementing one or more of the lifecycle hook interfaces in the Angular core library. The hooks give you the opportunity to act on a component or directive instance at the appropriate moment, as Angular creates, updates, or destroys that instance.

每个接口都有唯一的一个钩子方法，它们的名字是由接口名再加上 ng 前缀构成的。比如，OnInit 接口的钩子方法叫做 ngOnInit()。如果你在组件或指令类中实现了这个方法，Angular 就会在首次检查完组件或指令的输入属性后，紧接着调用它。

Each interface defines the prototype for a single hook method, whose name is the interface name prefixed with ng. For example, the OnInit interface has a hook method named ngOnInit(). If you implement this method in your component or directive class, Angular calls it shortly after checking the input properties for that component or directive for the first time.

      
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      @Directive()
export class PeekABooDirective implements OnInit {
  constructor(private logger: LoggerService) { }

  // implement OnInit's `ngOnInit` method
  ngOnInit() {
    this.logIt(`OnInit`);
  }

  logIt(msg: string) {
    this.logger.log(`#${nextId++} ${msg}`);
  }
}
    

你不必实现所有生命周期钩子，只要实现你需要的那些就可以了。

You don't have to implement all (or any) of the lifecycle hooks, just the ones you need.

生命周期的顺序link

Lifecycle event sequencelink

当你的应用通过调用构造函数来实例化一个组件或指令时，Angular 就会调用那个在该实例生命周期的适当位置实现了的那些钩子方法。

After your application instantiates a component or directive by calling its constructor, Angular calls the hook methods you have implemented at the appropriate point in the lifecycle of that instance.

Angular 会按以下顺序执行钩子方法。你可以用它来执行以下类型的操作。

Angular executes hook methods in the following sequence. You can use them to perform the following kinds of operations.

生命周期范例link

Lifecycle example setlink

现场演练 / 下载范例通过在受控于根组件 AppComponent 的一些组件上进行的一系列练习，演示了生命周期钩子的运作方式。 每一个例子中，父组件都扮演了子组件测试台的角色，以展示出一个或多个生命周期钩子方法。

The现场演练 / 下载范例demonstrates the use of lifecycle hooks through a series of exercises presented as components under the control of the root AppComponent. In each case a parent component serves as a test rig for a child component that illustrates one or more of the lifecycle hook methods.

下表列出了这些练习及其简介。 范例代码也用来阐明后续各节的一些特定任务。

The following table lists the exercises with brief descriptions. The sample code is also used to illustrate specific tasks in the following sections.

初始化组件或指令link

Initializing a component or directivelink

使用 ngOnInit() 方法执行以下初始化任务。

Use the ngOnInit() method to perform the following initialization tasks.

• 在构造函数外部执行复杂的初始化。组件的构造应该既便宜又安全。比如，你不应该在组件构造函数中获取数据。当在测试中创建组件时或者决定显示它之前，你不应该担心新组件会尝试联系远程服务器。

  Perform complex initializations outside of the constructor. Components should be cheap and safe to construct. You should not, for example, fetch data in a component constructor. You shouldn't worry that a new component will try to contact a remote server when created under test or before you decide to display it.

  ngOnInit() 是组件获取初始数据的好地方。比如，英雄之旅教程 。

  An ngOnInit() is a good place for a component to fetch its initial data. For an example, see the Tour of Heroes tutorial.

• 在 Angular 设置好输入属性之后设置组件。构造函数应该只把初始局部变量设置为简单的值。

  Set up the component after Angular sets the input properties. Constructors should do no more than set the initial local variables to simple values.

  请记住，只有在构造完成之后才会设置指令的数据绑定输入属性。如果要根据这些属性对指令进行初始化，请在运行 ngOnInit() 时设置它们。

  Keep in mind that a directive's data-bound input properties are not set until after construction. If you need to initialize the directive based on those properties, set them when ngOnInit() runs.

  ngOnChanges() 方法是你能访问这些属性的第一次机会。Angular 会在调用 ngOnInit() 之前调用 ngOnChanges()，而且之后还会调用多次。但它只调用一次 ngOnInit()。

  The ngOnChanges() method is your first opportunity to access those properties. Angular calls ngOnChanges() before ngOnInit(), but also many times after that. It only calls ngOnInit() once.

在实例销毁时进行清理link

Cleaning up on instance destructionlink

把清理逻辑放进 ngOnDestroy() 中，这个逻辑就必然会在 Angular 销毁该指令之前运行。

Put cleanup logic in ngOnDestroy(), the logic that must run before Angular destroys the directive.

这里是释放资源的地方，这些资源不会自动被垃圾回收。如果你不这样做，就存在内存泄漏的风险。

This is the place to free resources that won't be garbage-collected automatically. You risk memory leaks if you neglect to do so.

• 取消订阅可观察对象和 DOM 事件。

  Unsubscribe from Observables and DOM events.

• 停止 interval 计时器。

  Stop interval timers.

• 反注册该指令在全局或应用服务中注册过的所有回调。

  Unregister all callbacks that the directive registered with global or application services.

ngOnDestroy() 方法也可以用来通知应用程序的其它部分，该组件即将消失。

The ngOnDestroy() method is also the time to notify another part of the application that the component is going away.

一般性例子link

General exampleslink

下面的例子展示了各个生命周期事件的调用顺序和相对频率，以及如何在组件和指令中单独使用或同时使用这些钩子。

The following examples demonstrate the call sequence and relative frequency of the various lifecycle events, and how the hooks can be used separately or together for components and directives.

所有生命周期事件的顺序和频率link

Sequence and frequency of all lifecycle eventslink

为了展示 Angular 如何以预期的顺序调用钩子，PeekABooComponent 演示了一个组件中的所有钩子。

To show how Angular calls the hooks in the expected order, the PeekABooComponent demonstrates all of the hooks in one component.

实际上，你很少会（几乎永远不会）像这个演示中一样实现所有这些接口。

In practice you would rarely, if ever, implement all of the interfaces the way this demo does.

下列快照反映了用户单击 Create... 按钮，然后单击 Destroy... 按钮后的日志状态。

The following snapshot reflects the state of the log after the user clicked the Create... button and then the Destroy... button.

日志信息的日志和所规定的钩子调用顺序是一致的： OnChanges、OnInit、DoCheck (3x)、AfterContentInit、AfterContentChecked (3x)、 AfterViewInit、AfterViewChecked (3x)和 OnDestroy

The sequence of log messages follows the prescribed hook calling order: OnChanges, OnInit, DoCheck (3x), AfterContentInit, AfterContentChecked (3x), AfterViewInit, AfterViewChecked (3x), and OnDestroy.

如果用户点击Update Hero按钮，就会看到另一个 OnChanges 和至少两组 DoCheck、AfterContentChecked 和 AfterViewChecked 钩子。 注意，这三种钩子被触发了很多次，所以让它们的逻辑尽可能保持精简是非常重要的！

Had the user clicked the Update Hero button, the log would show another OnChanges and two more triplets of DoCheck, AfterContentChecked and AfterViewChecked. Notice that these three hooks fire often, so it is important to keep their logic as lean as possible.

使用指令来监视 DOMlink

Use directives to watch the DOMlink

这个 Spy 例子演示了如何在指令和组件中使用钩子方法。SpyDirective 实现了两个钩子 ngOnInit() 和 ngOnDestroy()，以便发现被监视的元素什么时候位于当前视图中。

The Spy example demonstrates how you can use hook method for directives as well as components. The SpyDirective implements two hooks, ngOnInit() and ngOnDestroy(), in order to discover when a watched element is in the current view.

这个模板将 SpyDirective 应用到由父组件 SpyComponent 管理的 ngFor 内的 <div> 中。

This template applies the SpyDirective to a <div> in the ngFor hero repeater managed by the parent SpyComponent.

该例子不执行任何初始化或清理工作。它只是通过记录指令本身的实例化时间和销毁时间来跟踪元素在视图中的出现和消失。

The example does not perform any initialization or clean-up. It just tracks the appearance and disappearance of an element in the view by recording when the directive itself is instantiated and destroyed.

像这样的间谍指令可以深入了解你无法直接修改的 DOM 对象。你无法触及原生 <div> 的实现，也无法修改第三方组件，但是可以用指令来监视这些元素。

A spy directive like this can provide insight into a DOM object that you cannot change directly. You can't touch the implementation of a native <div>, or modify a third party component. You can, however watch these elements with a directive.

这个指令定义了 ngOnInit() 和 ngOnDestroy() 钩子，它通过一个注入进来的 LoggerService 把消息记录到父组件中去。

The directive defines ngOnInit() and ngOnDestroy() hooks that log messages to the parent via an injected LoggerService.

      
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      let nextId = 1;

// Spy on any element to which it is applied.
// Usage: <div appSpy>...</div>
@Directive({selector: '[appSpy]'})
export class SpyDirective implements OnInit, OnDestroy {
  private id = nextId++;

  constructor(private logger: LoggerService) { }

  ngOnInit() {
    this.logger.log(`Spy #${this.id} onInit`);
  }

  ngOnDestroy() {
    this.logger.log(`Spy #${this.id} onDestroy`);
  }
}
    

你可以把这个侦探指令写到任何原生元素或组件元素上，以观察它何时被初始化和销毁。 下面是把它附加到用来重复显示英雄数据的这个 <div> 上。

You can apply the spy to any native or component element, and see that it is initialized and destroyed at the same time as that element. Here it is attached to the repeated hero <div>:

      
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      <p *ngFor="let hero of heroes" appSpy>
  {{hero}}
</p>
    

每个“侦探”的创建和销毁都可以标出英雄所在的那个 <div> 的出现和消失。钩子记录中的结构是这样的：

Each spy's creation and destruction marks the appearance and disappearance of the attached hero <div> with an entry in the Hook Log as seen here:

添加一个英雄就会产生一个新的英雄 <div>。侦探的 ngOnInit() 记录下了这个事件。

Adding a hero results in a new hero <div>. The spy's ngOnInit() logs that event.

Reset 按钮清除了这个 heroes 列表。 Angular 从 DOM 中移除了所有英雄的 div，并且同时销毁了附加在这些 div 上的侦探指令。 侦探的 ngOnDestroy() 方法汇报了它自己的临终时刻。

The Reset button clears the heroes list. Angular removes all hero <div> elements from the DOM and destroys their spy directives at the same time. The spy's ngOnDestroy() method reports its last moments.

同时使用组件和指令的钩子link

Use component and directive hooks togetherlink

在这个例子中，CounterComponent 使用了 ngOnChanges() 方法，以便在每次父组件递增其输入属性 counter 时记录一次变更。

In this example, a CounterComponent uses the ngOnChanges() method to log a change every time the parent component increments its input counter property.

这个例子将前例中的 SpyDirective 用于 CounterComponent 的日志，以便监视这些日志条目的创建和销毁。

This example applies the SpyDirective from the previous example to the CounterComponent log, in order to watch the creation and destruction of log entries.

使用变更检测钩子link

Using change detection hookslink

一旦检测到该组件或指令的输入属性发生了变化，Angular 就会调用它的 ngOnChanges() 方法。 这个 onChanges 范例通过监控 OnChanges() 钩子演示了这一点。

Angular calls the ngOnChanges() method of a component or directive whenever it detects changes to the input properties. The onChanges example demonstrates this by monitoring the OnChanges() hook.

      
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      ngOnChanges(changes: SimpleChanges) {
  for (const propName in changes) {
    const chng = changes[propName];
    const cur  = JSON.stringify(chng.currentValue);
    const prev = JSON.stringify(chng.previousValue);
    this.changeLog.push(`${propName}: currentValue = ${cur}, previousValue = ${prev}`);
  }
}
    

ngOnChanges() 方法获取了一个对象，它把每个发生变化的属性名都映射到了一个SimpleChange对象， 该对象中有属性的当前值和前一个值。这个钩子会在这些发生了变化的属性上进行迭代，并记录它们。

The ngOnChanges() method takes an object that maps each changed property name to a SimpleChange object holding the current and previous property values. This hook iterates over the changed properties and logs them.

这个例子中的 OnChangesComponent 组件有两个输入属性：hero 和 power。

The example component, OnChangesComponent, has two input properties: hero and power.

      
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      @Input() hero: Hero;
@Input() power: string;
    

宿主 OnChangesParentComponent 绑定了它们，就像这样：

The host OnChangesParentComponent binds to them as follows.

      
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      <on-changes [hero]="hero" [power]="power"></on-changes>
    

下面是此例子中的当用户做出更改时的操作演示：

Here's the sample in action as the user makes changes.

日志条目把 power 属性的变化显示为字符串。但请注意，ngOnChanges() 方法不会捕获对 hero.name 更改。这是因为只有当输入属性的值发生变化时，Angular 才会调用该钩子。在这种情况下，hero 是输入属性，hero 属性的值是对 hero 对象的引用 。当它自己的 name 属性的值发生变化时，对象引用并没有改变。

The log entries appear as the string value of the power property changes. Notice, however, that the ngOnChanges() method does not catch changes to hero.name. This is because Angular calls the hook only when the value of the input property changes. In this case, hero is the input property, and the value of the hero property is the reference to the hero object. The object reference did not change when the value of its own name property changed.

响应视图的变更link

Responding to view changeslink

当 Angular 在变更检测期间遍历视图树时，需要确保子组件中的某个变更不会尝试更改其父组件中的属性。因为单向数据流的工作原理就是这样的，这样的更改将无法正常渲染。

As Angular traverses the view hierarchy during change detection, it needs to be sure that a change in a child does not attempt to cause a change in its own parent. Such a change would not be rendered properly, because of how unidirectional data flow works.

如果你需要做一个与预期数据流反方向的修改，就必须触发一个新的变更检测周期，以允许渲染这种变更。这些例子说明了如何安全地做出这些改变。

If you need to make a change that inverts the expected data flow, you must trigger a new change detection cycle to allow that change to be rendered. The examples illustrate how to make such changes safely.

AfterView 例子展示了 AfterViewInit() 和 AfterViewChecked() 钩子，Angular 会在每次创建了组件的子视图后调用它们。

The AfterView sample explores the AfterViewInit() and AfterViewChecked() hooks that Angular calls after it creates a component's child views.

下面是一个子视图，它用来把英雄的名字显示在一个 <input> 中：

Here's a child view that displays a hero's name in an <input>:

      
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      @Component({
  selector: 'app-child-view',
  template: `<label for="hero-name">Hero name: </label>
  <input type="text" id="hero-name" [(ngModel)]="hero">`
})
export class ChildViewComponent {
  hero = 'Magneta';
}
    

AfterViewComponent 把这个子视图显示在它的模板中：

The AfterViewComponent displays this child view within its template:

      
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      template: `
  <div>child view begins</div>
    <app-child-view></app-child-view>
  <div>child view ends</div>`
    

下列钩子基于子视图中的每一次数据变更采取行动，它只能通过带@ViewChild装饰器的属性来访问子视图。

The following hooks take action based on changing values within the child view, which can only be reached by querying for the child view via the property decorated with @ViewChild.

      
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      export class AfterViewComponent implements  AfterViewChecked, AfterViewInit {
  private prevHero = '';

  // Query for a VIEW child of type `ChildViewComponent`
  @ViewChild(ChildViewComponent) viewChild: ChildViewComponent;

  ngAfterViewInit() {
    // viewChild is set after the view has been initialized
    this.logIt('AfterViewInit');
    this.doSomething();
  }

  ngAfterViewChecked() {
    // viewChild is updated after the view has been checked
    if (this.prevHero === this.viewChild.hero) {
      this.logIt('AfterViewChecked (no change)');
    } else {
      this.prevHero = this.viewChild.hero;
      this.logIt('AfterViewChecked');
      this.doSomething();
    }
  }
  // ...
}
    

在更新视图之前等待link

Wait before updating the viewlink

在这个例子中，当英雄名字超过 10 个字符时，doSomething() 方法会更新屏幕，但在更新 comment 之前会等一个节拍（tick）。

In this example, the doSomething() method updates the screen when the hero name exceeds 10 characters, but waits a tick before updating comment.

      
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      // This surrogate for real business logic sets the `comment`
private doSomething() {
  const c = this.viewChild.hero.length > 10 ? `That's a long name` : '';
  if (c !== this.comment) {
    // Wait a tick because the component's view has already been checked
    this.logger.tick_then(() => this.comment = c);
  }
}
    

在组件的视图合成完之后，就会触发 AfterViewInit() 和 AfterViewChecked() 钩子。如果你修改了这段代码，让这个钩子立即修改该组件的数据绑定属性 comment，你就会发现 Angular 抛出一个错误。

Both the AfterViewInit() and AfterViewChecked() hooks fire after the component's view has been composed. If you modify the code so that the hook updates the component's data-bound comment property immediately, you can see that Angular throws an error.

LoggerService.tick_then() 语句把日志的更新工作推迟了一个浏览器 JavaScript 周期，也就触发了一个新的变更检测周期。

The LoggerService.tick_then() statement postpones the log update for one turn of the browser's JavaScript cycle, which triggers a new change-detection cycle.

编写精简的钩子方法来避免性能问题link

Write lean hook methods to avoid performance problemslink

当你运行 AfterView 范例时，请注意当没有发生任何需要注意的变化时，Angular 仍然会频繁的调用 AfterViewChecked()。 要非常小心你放到这些方法中的逻辑或计算量。

When you run the AfterView sample, notice how frequently Angular calls AfterViewChecked()-often when there are no changes of interest. Be very careful about how much logic or computation you put into one of these methods.

响应被投影内容的变更link

Responding to projected content changeslink

内容投影是从组件外部导入 HTML 内容，并把它插入在组件模板中指定位置上的一种途径。 你可以在目标中通过查找下列结构来认出内容投影。

Content projection is a way to import HTML content from outside the component and insert that content into the component's template in a designated spot. You can identify content projection in a template by looking for the following constructs.

• 元素标签中间的 HTML。

  HTML between component element tags.

• 组件模板中的 <ng-content> 标签。

  The presence of <ng-content> tags in the component's template.

这个 AfterContent 例子探索了 AfterContentInit() 和 AfterContentChecked() 钩子。Angular 会在把外部内容投影进该组件时调用它们。

The AfterContent sample explores the AfterContentInit() and AfterContentChecked() hooks that Angular calls after Angular projects external content into the component.

对比前面的 AfterView 例子考虑这个变化。 这次不再通过模板来把子视图包含进来，而是改为从 AfterContentComponent 的父组件中导入它。下面是父组件的模板：

Consider this variation on the previous AfterView example. This time, instead of including the child view within the template, it imports the content from the AfterContentComponent's parent. The following is the parent's template.

      
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      `<after-content>
  <app-child></app-child>
</after-content>`
    

注意，<app-child> 标签被包含在 <after-content> 标签中。 永远不要在组件标签的内部放任何内容 —— 除非你想把这些内容投影进这个组件中。

Notice that the <app-child> tag is tucked between the <after-content> tags. Never put content between a component's element tags unless you intend to project that content into the component.

现在来看该组件的模板：

Now look at the component's template.

      
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      template: `
  <div>projected content begins</div>
    <ng-content></ng-content>
  <div>projected content ends</div>`
    

<ng-content> 标签是外来内容的占位符。 它告诉 Angular 在哪里插入这些外来内容。 在这里，被投影进去的内容就是来自父组件的 <app-child> 标签。

The <ng-content> tag is a placeholder for the external content. It tells Angular where to insert that content. In this case, the projected content is the <app-child> from the parent.

使用 AfterContent 钩子link

Using AfterContent hookslink

AfterContent 钩子和 AfterView 相似。关键的不同点是子组件的类型不同。

AfterContent hooks are similar to the AfterView hooks. The key difference is in the child component.

• AfterView 钩子所关心的是 ViewChildren，这些子组件的元素标签会出现在该组件的模板里面。

  The AfterView hooks concern ViewChildren, the child components whose element tags appear within the component's template.

• AfterContent 钩子所关心的是 ContentChildren，这些子组件被 Angular 投影进该组件中。

  The AfterContent hooks concern ContentChildren, the child components that Angular projected into the component.

下列 AfterContent 钩子基于子级内容中值的变化而采取相应的行动，它只能通过带有@ContentChild装饰器的属性来查询到“子级内容”。

The following AfterContent hooks take action based on changing values in a content child, which can only be reached by querying for them via the property decorated with @ContentChild.

      
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      export class AfterContentComponent implements AfterContentChecked, AfterContentInit {
  private prevHero = '';
  comment = '';

  // Query for a CONTENT child of type `ChildComponent`
  @ContentChild(ChildComponent) contentChild: ChildComponent;

  ngAfterContentInit() {
    // contentChild is set after the content has been initialized
    this.logIt('AfterContentInit');
    this.doSomething();
  }

  ngAfterContentChecked() {
    // contentChild is updated after the content has been checked
    if (this.prevHero === this.contentChild.hero) {
      this.logIt('AfterContentChecked (no change)');
    } else {
      this.prevHero = this.contentChild.hero;
      this.logIt('AfterContentChecked');
      this.doSomething();
    }
  }
  // ...
}
    

自定义变更检测逻辑link

Defining custom change detectionlink

要监控 ngOnChanges() 无法捕获的变更，你可以实现自己的变更检查逻辑，比如 DoCheck 的例子。这个例子展示了你如何使用 ngDoCheck() 钩子来检测和处理 Angular 自己没有捕捉到的变化。

To monitor changes that occur where ngOnChanges() won't catch them, you can implement your own change check, as shown in the DoCheck example. This example shows how you can use the ngDoCheck() hook to detect and act upon changes that Angular doesn't catch on its own.

DoCheck 范例使用下面的 ngDoCheck() 钩子扩展了 OnChanges 范例：

The DoCheck sample extends the OnChanges sample with the following ngDoCheck() hook:

      
        content_copy
      
      ngDoCheck() {

  if (this.hero.name !== this.oldHeroName) {
    this.changeDetected = true;
    this.changeLog.push(`DoCheck: Hero name changed to "${this.hero.name}" from "${this.oldHeroName}"`);
    this.oldHeroName = this.hero.name;
  }

  if (this.power !== this.oldPower) {
    this.changeDetected = true;
    this.changeLog.push(`DoCheck: Power changed to "${this.power}" from "${this.oldPower}"`);
    this.oldPower = this.power;
  }

  if (this.changeDetected) {
      this.noChangeCount = 0;
  } else {
      // log that hook was called when there was no relevant change.
      const count = this.noChangeCount += 1;
      const noChangeMsg = `DoCheck called ${count}x when no change to hero or power`;
      if (count === 1) {
        // add new "no change" message
        this.changeLog.push(noChangeMsg);
      } else {
        // update last "no change" message
        this.changeLog[this.changeLog.length - 1] = noChangeMsg;
      }
  }

  this.changeDetected = false;
}
    

这段代码会检查某些感兴趣的值，捕获并把它们当前的状态和之前的进行比较。当 hero 或 power 没有实质性变化时，它就会在日志中写一条特殊的信息，这样你就能看到 DoCheck() 被调用的频率。其结果很有启发性。

This code inspects certain values of interest, capturing and comparing their current state against previous values. It writes a special message to the log when there are no substantive changes to the hero or the power so you can see how often DoCheck() is called. The results are illuminating.

虽然 ngDoCheck() 钩子可以检测出英雄的 name 何时发生了变化，但却非常昂贵。无论变化发生在何处，每个变化检测周期都会以很大的频率调用这个钩子。在用户可以执行任何操作之前，本例中已经调用了20多次。

While the ngDoCheck() hook can detect when the hero's name has changed, it is very expensive. This hook is called with enormous frequency—after every change detection cycle no matter where the change occurred. It's called over twenty times in this example before the user can do anything.

这些初始化检查大部分都是由 Angular 首次在页面的其它地方渲染不相关的数据触发的。只要把光标移动到另一个 <input> 就会触发一次调用。其中的少数调用揭示了相关数据的实际变化情况。如果使用这个钩子，那么你的实现必须非常轻量级，否则会损害用户体验。

Most of these initial checks are triggered by Angular's first rendering of unrelated data elsewhere on the page. Just moving the cursor into another <input> triggers a call. Relatively few calls reveal actual changes to pertinent data. If you use this hook, your implementation must be extremely lightweight or the user experience suffers.