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.

目录

Contents

• 动态组件加载

  Dynamic component loading

• 指令

  The directive

• 加载组件

  Loading components

  • 解析组件

    Resolving Components

  • 通过选择器引用

    Selector References

• 通用的AdComponent接口

  A common AdComponent interface

• 最终的广告Banner

  Final ad banner

到在线例子 / 可下载的例子查看本烹饪书的源码。

See the live example / downloadable example of the code in this cookbook.

动态组件加载

Dynamic component loading

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.

我们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

The directive

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.

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，它就是我们将应用到元素上的指令。下一节我们会展示如何做。

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

Loading components

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，我们把HTML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.

template: `
            <div class="ad-banner">
              <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

Resolving components

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让我们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().

export class AdBannerComponent implements AfterViewInit, OnDestroy {
  @Input() ads: AdItem[];
  currentAddIndex: number = -1;
  @ViewChild(AdDirective) adHost: AdDirective;
  subscription: any;
  interval: any;

  constructor(private _componentFactoryResolver: ComponentFactoryResolver) { }

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

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

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

    let componentFactory = this._componentFactoryResolver.resolveComponentFactory(adItem.component);

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

    let 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。 我们来一步步看看。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，我们要把viewContainerRef指向这个组件的现有实例。但我们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就是我们以前设置过的指令，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被我们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，我们可以调用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

Selector references

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:

entryComponents: [ HeroJobAdComponent, HeroProfileComponent ],

公共的AdComponent接口

A common AdComponent interface

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，让我们把数据传给组件。

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:

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;
}

import { Component, Input }  from '@angular/core';
import { AdComponent }       from './ad.component';
@Component({
  template: `
    <div class="hero-profile">
      <h3>Featured Hero Profile</h3>
      <h4>{{data.name}}</h4>
      <p>{{data.bio}}</p>
      <strong>Hire this hero today!</strong>
    </div>
  `
})
export class HeroProfileComponent implements AdComponent {
  @Input() data: any;
}

export interface AdComponent {
  data: any;
}

最终的广告栏

Final ad banner

最终的广告栏是这样的：

The final ad banner looks like this:

参见在线例子 / 可下载的例子。

See the live example / downloadable example.