使用 DI 浏览组件树link

Navigate the component tree with DIlink

应用的组件之间经常需要共享信息。你通常要用松耦合的技术来共享信息，比如数据绑定和服务共享。但是有时候让一个组件直接引用另一个组件还是很有意义的。 例如，你需要通过另一个组件的直接引用来访问其属性或调用其方法。

Application components often need to share information. You can often use loosely coupled techniques for sharing information, such as data binding and service sharing, but sometimes it makes sense for one component to have a direct reference to another component. You need a direct reference, for instance, to access values or call methods on that component.

在 Angular 中获取组件引用略微有些棘手。 Angular 组件本身并没有一棵可以用编程方式检查或浏览的树。 其父子关系是通过组件的视图对象间接建立的。

Obtaining a component reference is a bit tricky in Angular. Angular components themselves do not have a tree that you can inspect or navigate programmatically. The parent-child relationship is indirect, established through the components' view objects.

每个组件都有一个宿主视图和一些内嵌视图。 组件 A 的内嵌视图可以是组件 B 的宿主视图，而组件 B 还可以有它自己的内嵌视图。 这意味着每个组件都有一棵以该组件的宿主视图为根节点的视图树。

Each component has a host view, and can have additional embedded views. An embedded view in component A is the host view of component B, which can in turn have embedded view. This means that there is a view hierarchy for each component, of which that component's host view is the root.

有一些用于在视图树中向下导航的 API。 请到 API 参考手册中查看 Query、QueryList、ViewChildren 和 ContentChildren。

There is an API for navigating down the view hierarchy. Check out Query, QueryList, ViewChildren, and ContentChildren in the API Reference.

不存在用于获取父引用的公共 API。 不过，由于每个组件的实例都会添加到注入器的容器中，因此你可以通过 Angular 的依赖注入来访问父组件。

There is no public API for acquiring a parent reference. However, because every component instance is added to an injector's container, you can use Angular dependency injection to reach a parent component.

本节描述的就是关于这种做法的一些技巧。

This section describes some techniques for doing that.

查找已知类型的父组件link

Find a parent component of known typelink

你可以使用标准的类注入形式来获取类型已知的父组件。

You use standard class injection to acquire a parent component whose type you know.

在下面的例子中，父组件 AlexComponent 具有一些子组件，包括 CathyComponent：

In the following example, the parent AlexComponent has several children including a CathyComponent:

      
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      @Component({
  selector: 'alex',
  template: `
    <div class="a">
      <h3>{{name}}</h3>
      <cathy></cathy>
      <craig></craig>
      <carol></carol>
    </div>`,
})
export class AlexComponent extends Base
{
  name = 'Alex';
}
    

在把 AlexComponent 注入到 CathyComponent 的构造函数中之后，Cathy 可以报告她是否能访问 Alex：

Cathy reports whether or not she has access to Alex after injecting an AlexComponent into her constructor:

      
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      @Component({
  selector: 'cathy',
  template: `
  <div class="c">
    <h3>Cathy</h3>
    {{alex ? 'Found' : 'Did not find'}} Alex via the component class.<br>
  </div>`
})
export class CathyComponent {
  constructor( @Optional() public alex?: AlexComponent ) { }
}
    

注意，虽然为了安全起见我们用了 @Optional 限定符，但是现场演练 / 下载范例中仍然会确认 alex 参数是否有值。

Notice that even though the @Optional qualifier is there for safety, the现场演练 / 下载范例confirms that the alex parameter is set.

不能根据父组件的基类访问父组件link

Unable to find a parent by its base classlink

如果你不知道具体的父组件类怎么办？

What if you don't know the concrete parent component class?

可复用组件可能是多个组件的子组件。想象一个用于渲染相关金融工具的突发新闻的组件。 出于商业原因，当市场上的数据流发生变化时，这些新组件会频繁调用其父组件。

A re-usable component might be a child of multiple components. Imagine a component for rendering breaking news about a financial instrument. For business reasons, this news component makes frequent calls directly into its parent instrument as changing market data streams by.

该应用可能定义了十几个金融工具组件。理想情况下，它们全都实现了同一个基类，你的 NewsComponent 也能理解其 API。

The app probably defines more than a dozen financial instrument components. If you're lucky, they all implement the same base class whose API your NewsComponent understands.

这个设计并不怎么好。 该例子是为了验证组件是否能通过其父组件的基类来注入父组件。

This isn't necessarily good design. This example is examining whether a component can inject its parent via the parent's base class.

这个例子中的 CraigComponent 体现了此问题。往回看，你可以看到 Alex 组件扩展（继承）了基类 Base。

The sample's CraigComponent explores this question. Looking back, you see that the Alex component extends (inherits) from a class named Base.

      
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      export class AlexComponent extends Base
    

CraigComponent 试图把 Base 注入到它的构造函数参数 alex 中，并汇报这次注入是否成功了。

The CraigComponent tries to inject Base into its alex constructor parameter and reports if it succeeded.

      
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      @Component({
  selector: 'craig',
  template: `
  <div class="c">
    <h3>Craig</h3>
    {{alex ? 'Found' : 'Did not find'}} Alex via the base class.
  </div>`
})
export class CraigComponent {
  constructor( @Optional() public alex?: Base ) { }
}
    

不幸的是，这不行！现场演练 / 下载范例确认了 alex 参数为空。 因此，你不能通过父组件的基类注入它。

Unfortunately, this doesn't work. The现场演练 / 下载范例confirms that the alex parameter is null. You cannot inject a parent by its base class.

根据父组件的类接口查找它link

Find a parent by its class interfacelink

你可以通过父组件的类接口来查找它。

You can find a parent component with a class interface.

该父组件必须合作，以类接口令牌为名，为自己定义一个别名提供者。

The parent must cooperate by providing an alias to itself in the name of a class interface token.

回忆一下，Angular 总是会把组件实例添加到它自己的注入器中，因此以前你才能把 Alex 注入到 Cathy 中。

Recall that Angular always adds a component instance to its own injector; that's why you could inject Alex into Cathy earlier.

编写一个 别名提供者（一个 provide 对象字面量，其中有一个 useExisting 定义），创造了另一种方式来注入同一个组件实例，并把那个提供者添加到 AlexComponent @Component() 元数据的 providers 数组中。

Write an alias provider—a provide object literal with a useExisting definition—that creates an alternative way to inject the same component instance and add that provider to the providers array of the @Component() metadata for the AlexComponent.

      
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      providers: [{ provide: Parent, useExisting: forwardRef(() => AlexComponent) }],
    

Parent 是该提供者的类接口。 forwardRef 用于打破循环引用，因为在你刚才这个定义中 AlexComponent 引用了自身。

Parent is the provider's class interface token. The forwardRef breaks the circular reference you just created by having the AlexComponent refer to itself.

Alex 的第三个子组件 Carol，把其父组件注入到了自己的 parent 参数中 —— 和你以前做过的一样。

Carol, the third of Alex's child components, injects the parent into its parent parameter, the same way you've done it before.

      
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      export class CarolComponent {
  name = 'Carol';
  constructor( @Optional() public parent?: Parent ) { }
}
    

下面是 Alex 及其家人的运行效果。

Here's Alex and family in action.

使用 @SkipSelf() 在树中查找父组件link

Find a parent in a tree with @SkipSelf()link

想象一下组件树的一个分支：Alice -> Barry -> Carol。 无论 Alice 还是 Barry 都实现了类接口 Parent。

Imagine one branch of a component hierarchy: Alice -> Barry -> Carol. Both Alice and Barry implement the Parent class interface.

Barry 很为难。他需要访问他的母亲 Alice，同时他自己还是 Carol 的父亲。 这意味着他必须同时注入 Parent 类接口来找到 Alice，同时还要提供一个 Parent 来满足 Carol 的要求。

Barry is the problem. He needs to reach his parent, Alice, and also be a parent to Carol. That means he must both inject the Parent class interface to get Alice and provide a Parent to satisfy Carol.

Barry 的代码如下。

Here's Barry.

      
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      const templateB = `
  <div class="b">
    <div>
      <h3>{{name}}</h3>
      <p>My parent is {{parent?.name}}</p>
    </div>
    <carol></carol>
    <chris></chris>
  </div>`;

@Component({
  selector:   'barry',
  template:   templateB,
  providers:  [{ provide: Parent, useExisting: forwardRef(() => BarryComponent) }]
})
export class BarryComponent implements Parent {
  name = 'Barry';
  constructor( @SkipSelf() @Optional() public parent?: Parent ) { }
}
    

Barry 的 providers 数组看起来和 Alex 的一样。 如果你准备继续像这样编写别名提供者，就应该创建一个辅助函数。

Barry's providers array looks just like Alex's. If you're going to keep writing alias providers like this you should create a helper function.

现在，注意看 Barry 的构造函数。

For now, focus on Barry's constructor.

      
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      constructor( @SkipSelf() @Optional() public parent?: Parent ) { }
    

除增加了 @SkipSelf 装饰器之外，它和 Carol 的构造函数相同。

It's identical to Carol's constructor except for the additional @SkipSelf decorator.

使用 @SkipSelf 有两个重要原因：

@SkipSelf is essential for two reasons:

1. 它告诉注入器开始从组件树中高于自己的位置（也就是父组件）开始搜索 Parent 依赖。

   It tells the injector to start its search for a Parent dependency in a component above itself, which is what parent means.

2. 如果你省略了 @SkipSelf 装饰器，Angular 就会抛出循环依赖错误。

   Angular throws a cyclic dependency error if you omit the @SkipSelf decorator.

   NG0200: Circular dependency in DI detected for BethComponent. Dependency path: BethComponent -> Parent -> BethComponent

下面是 Alice、Barry 及其家人的运行效果。

Here's Alice, Barry, and family in action.

父类接口link

Parent class interfacelink

你已经学过，类接口是一个抽象类，它实际上用做接口而不是基类。

You learned earlier that a class interface is an abstract class used as an interface rather than as a base class.

下面的例子定义了一个类接口 Parent。

The example defines a Parent class interface.

      
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      export abstract class Parent { name: string; }
    

Parent 类接口定义了一个带类型的 name 属性，但没有实现它。 这个 name 属性是父组件中唯一可供子组件调用的成员。 这样的窄化接口帮助把子组件从它的父组件中解耦出来。

The Parent class interface defines a name property with a type declaration but no implementation. The name property is the only member of a parent component that a child component can call. Such a narrow interface helps decouple the child component class from its parent components.

一个组件想要作为父组件使用，就应该像 AliceComponent 那样实现这个类接口。

A component that could serve as a parent should implement the class interface as the AliceComponent does.

      
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      export class AliceComponent implements Parent
    

这样做可以增加代码的清晰度，但在技术上并不是必要的。 虽然 AlexComponent 像 Base 类所要求的一样具有 name 属性，但它的类签名中并没有提及 Parent。

Doing so adds clarity to the code. But it's not technically necessary. Although AlexComponent has a name property, as required by its Base class, its class signature doesn't mention Parent.

      
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      export class AlexComponent extends Base