动态组件加载器link

Dynamic component loaderlink

组件的模板不会永远是固定的。应用可能会需要在运行期间加载一些新的组件。

Component templates are not always fixed. An application may need to load new components at runtime.

这本烹饪书为你展示如何使用 ComponentFactoryResolver 来动态添加组件。

This cookbook shows you how to use ComponentFactoryResolver to add components dynamically.

到现场演练 / 下载范例查看本烹饪书的源码。

See the现场演练 / 下载范例of the code in this cookbook.

动态组件加载link

Dynamic component loadinglink

下面的例子展示了如何构建动态广告条。

The following example shows how to build a dynamic ad banner.

英雄管理局正在计划一个广告活动，要在广告条中显示一系列不同的广告。几个不同的小组可能会频繁加入新的广告组件。 再用只支持静态组件结构的模板显然是不现实的。

The hero agency is planning an ad campaign with several different ads cycling through the banner. New ad components are added frequently by several different teams. This makes it impractical to use a template with a static component structure.

你需要一种新的组件加载方式，它不需要在广告条组件的模板中引用固定的组件。

Instead, you need a way to load a new component without a fixed reference to the component in the ad banner's template.

Angular 自带的 API 就能支持动态加载组件。

Angular comes with its own API for loading components dynamically.

指令link

The anchor directivelink

在添加组件之前，先要定义一个锚点来告诉 Angular 要把组件插入到什么地方。

Before you can add components you have to define an anchor point to tell Angular where to insert components.

广告条使用一个名叫 AdDirective 的辅助指令来在模板中标记出有效的插入点。

The ad banner uses a helper directive called AdDirective to mark valid insertion points in the template.

      
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      import { Directive, ViewContainerRef } from '@angular/core';

@Directive({
  selector: '[ad-host]',
})
export class AdDirective {
  constructor(public viewContainerRef: ViewContainerRef) { }
}
    

AdDirective 注入了 ViewContainerRef 来获取对容器视图的访问权，这个容器就是那些动态加入的组件的宿主。

AdDirective injects ViewContainerRef to gain access to the view container of the element that will host the dynamically added component.

在 @Directive 装饰器中，要注意选择器的名称：ad-host，它就是你将应用到元素上的指令。下一节会展示该如何做。

In the @Directive decorator, notice the selector name, ad-host; that's what you use to apply the directive to the element. The next section shows you how.

加载组件link

Loading componentslink

广告条的大部分实现代码都在 ad-banner.component.ts 中。 为了让这个例子简单点，HTML 被直接放在了 @Component 装饰器的 template 属性中。

Most of the ad banner implementation is in ad-banner.component.ts. To keep things simple in this example, the HTML is in the @Component decorator's template property as a template string.

<ng-template> 元素就是刚才制作的指令将应用到的地方。 要应用 AdDirective，回忆一下来自 ad.directive.ts 的选择器 ad-host。把它应用到 <ng-template>（不用带方括号）。 这下，Angular 就知道该把组件动态加载到哪里了。

The <ng-template> element is where you apply the directive you just made. To apply the AdDirective, recall the selector from ad.directive.ts, ad-host. Apply that to <ng-template> without the square brackets. Now Angular knows where to dynamically load components.

      
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      template: `
            <div class="ad-banner-example">
              <h3>Advertisements</h3>
              <ng-template ad-host></ng-template>
            </div>
          `
    

<ng-template> 元素是动态加载组件的最佳选择，因为它不会渲染任何额外的输出。

The <ng-template> element is a good choice for dynamic components because it doesn't render any additional output.

解析组件link

Resolving componentslink

深入看看 ad-banner.component.ts 中的方法。

Take a closer look at the methods in ad-banner.component.ts.

AdBannerComponent 接收一个 AdItem 对象的数组作为输入，它最终来自 AdService。 AdItem 对象指定要加载的组件类，以及绑定到该组件上的任意数据。 AdService 可以返回广告活动中的那些广告。

AdBannerComponent takes an array of AdItem objects as input, which ultimately comes from AdService. AdItem objects specify the type of component to load and any data to bind to the component.AdService returns the actual ads making up the ad campaign.

给 AdBannerComponent 传入一个组件数组可以在模板中放入一个广告的动态列表，而不用写死在模板中。

Passing an array of components to AdBannerComponent allows for a dynamic list of ads without static elements in the template.

通过 getAds() 方法，AdBannerComponent 可以循环遍历 AdItems 的数组，并且每三秒调用一次 loadComponent() 来加载新组件。

With its getAds() method, AdBannerComponent cycles through the array of AdItems and loads a new component every 3 seconds by calling loadComponent().

      
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      export class AdBannerComponent implements OnInit, OnDestroy {
  @Input() ads: AdItem[];
  currentAdIndex = -1;
  @ViewChild(AdDirective, {static: true}) adHost: AdDirective;
  interval: any;

  constructor(private componentFactoryResolver: ComponentFactoryResolver) { }

  ngOnInit() {
    this.loadComponent();
    this.getAds();
  }

  ngOnDestroy() {
    clearInterval(this.interval);
  }

  loadComponent() {
    this.currentAdIndex = (this.currentAdIndex + 1) % this.ads.length;
    const adItem = this.ads[this.currentAdIndex];

    const componentFactory = this.componentFactoryResolver.resolveComponentFactory(adItem.component);

    const viewContainerRef = this.adHost.viewContainerRef;
    viewContainerRef.clear();

    const componentRef = viewContainerRef.createComponent(componentFactory);
    (<AdComponent>componentRef.instance).data = adItem.data;
  }

  getAds() {
    this.interval = setInterval(() => {
      this.loadComponent();
    }, 3000);
  }
}
    

这里的 loadComponent() 方法很重要。 来一步步看看。首先，它选取了一个广告。

The loadComponent() method is doing a lot of the heavy lifting here. Take it step by step. First, it picks an ad.

在 loadComponent() 选取了一个广告之后，它使用 ComponentFactoryResolver 来为每个具体的组件解析出一个 ComponentFactory。 然后 ComponentFactory 会为每一个组件创建一个实例。

After loadComponent() selects an ad, it uses ComponentFactoryResolver to resolve a ComponentFactory for each specific component. The ComponentFactory then creates an instance of each component.

接下来，你要把 viewContainerRef 指向这个组件的现有实例。但你怎么才能找到这个实例呢？ 很简单，因为它指向了 adHost，而这个 adHost 就是你以前设置过的指令，用来告诉 Angular 该把动态组件插入到什么位置。

Next, you're targeting the viewContainerRef that exists on this specific instance of the component. How do you know it's this specific instance? Because it's referring to adHost and adHost is the directive you set up earlier to tell Angular where to insert dynamic components.

回忆一下，AdDirective 曾在它的构造函数中注入了一个 ViewContainerRef。 因此这个指令可以访问到这个你打算用作动态组件宿主的元素。

As you may recall, AdDirective injects ViewContainerRef into its constructor. This is how the directive accesses the element that you want to use to host the dynamic component.

要把这个组件添加到模板中，你可以调用 ViewContainerRef 的 createComponent()。

To add the component to the template, you call createComponent() on ViewContainerRef.

createComponent() 方法返回一个引用，指向这个刚刚加载的组件。 使用这个引用就可以与该组件进行交互，比如设置它的属性或调用它的方法。

The createComponent() method returns a reference to the loaded component. Use that reference to interact with the component by assigning to its properties or calling its methods.

对选择器的引用link

Selector referenceslink

通常，Angular 编译器会为模板中所引用的每个组件都生成一个 ComponentFactory 类。 但是，对于动态加载的组件，模板中不会出现对它们的选择器的引用。

Generally, the Angular compiler generates a ComponentFactory for any component referenced in a template. However, there are no selector references in the templates for dynamically loaded components since they load at runtime.

要想确保编译器照常生成工厂类，就要把这些动态加载的组件添加到 NgModule 的 entryComponents 数组中：

To ensure that the compiler still generates a factory, add dynamically loaded components to the NgModule's entryComponents array:

      
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      entryComponents: [ HeroJobAdComponent, HeroProfileComponent ],
    

公共的 AdComponent 接口link

The AdComponent interfacelink

在广告条中，所有组件都实现了一个公共接口 AdComponent，它定义了一个标准化的 API，来把数据传给组件。

In the ad banner, all components implement a common AdComponent interface to standardize the API for passing data to the components.

下面就是两个范例组件及其 AdComponent 接口：

Here are two sample components and the AdComponent interface for reference:

      
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      import { Component, Input } from '@angular/core';

import { AdComponent }      from './ad.component';

@Component({
  template: `
    <div class="job-ad">
      <h4>{{data.headline}}</h4>

      {{data.body}}
    </div>
  `
})
export class HeroJobAdComponent implements AdComponent {
  @Input() data: any;

}
    

最终的广告栏link

Final ad bannerlink

最终的广告栏是这样的：

The final ad banner looks like this:

参见现场演练 / 下载范例。

See the现场演练 / 下载范例.