Angular 2 Pass Async Data

本文的主要内容是介绍父子组件通信时,如何传递异步的数据。我们将通过一个具体的示例,来介绍不同的处理方式。假设我们有一个博客组件,用来显示博主信息和分类的帖子列表信息。具体如下图所示:

图片描述

了解完具体需求后,接下来我们来一步步实现该组件。

Post Interfaces and Data

Post Interfaces

post.interface.ts

// each post will have a title and category
export interface Post {
    title: string;
    category: string;
}

// grouped posts by category
export interface GroupPosts {
    category: string;
    posts: Post[];
}

Mock Posts Data

[
  { "title": "Functional Programming", "category": "RxJS" },
  { "title": "Angular 2 Component Inheritance", "category": "NG2" },
  { "title": "RxJS Operators", "category": "RxJS" },
  { "title": "Angular 2 Http Module - HTTP", "category": "WEB" },
  { "title": "RxJS Observable", "category": "RxJS" },
  { "title": "Angular 2 AsyncPipe", "category": "NG2" }
]

Blogger Component

posts.component.ts

import { Component, Input, OnChanges, OnInit, SimpleChanges } from '@angular/core';
import { BehaviorSubject } from 'rxjs/BehaviorSubject';
import { Post, GroupPosts } from './post.interface';

@Component({
    selector: 'exe-posts',
    template: `
    <div class="list-group">
        <div *ngFor="let group of groupPosts;" class="list-group-item">
            <h4>{{ group.category }}</h4>
            <ul>
                <li *ngFor="let post of group.posts">
                    {{ post.title }}
                </li>
            </ul>
        <div>
    </div>
    `
})
export class PostsComponent implements OnInit, OnChanges {

    @Input()
    data: Post[];

    groupPosts: GroupPosts[];

    ngOnInit() {}

    ngOnChanges(changes: SimpleChanges) {
        console.dir(changes);
    }

    groupByCategory(data: Post[]): GroupPosts[] {
        if (!data) return;

        // 获取目录名,使用ES6中Set进行去重处理
        const categories = new Set(data.map(x => x.category));

        // 重新生成二维数组,用于页面显示
        const result = Array.from(categories).map(x => ({
            category: x,
            posts: data.filter(post => post.category === x)
        }));
        return result;
    }
}

blogger.component.ts

import { Component, OnInit, Input } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { Observer } from 'rxjs/Observer';
import { Post } from './post.interface';

@Component({
    selector: 'exe-bloggers',
    template: `
        <h1>Posts by: {{ blogger }}</h1>
        <div>
            <exe-posts [data]="posts"></exe-posts>
        </div>
    `
})
export class BloggerComponent implements OnInit {

    blogger = 'Semlinker';
    posts: Post[];

    ngOnInit() {
        this.getPostsByBlogger()
            .subscribe(posts => this.posts = posts);
    }

    getPostsByBlogger(): Observable<Post[]> {
        return Observable.create((observer: Observer<Post[]>) => {
            setTimeout(() => {
                const posts = [
                    { "title": "Functional Programming", "category": "RxJS" },
                    { "title": "Angular 2 Component Inheritance", "category": "NG2" },
                    { "title": "RxJS Operators", "category": "RxJS" },
                    { "title": "Angular 2 Http Module - HTTP", "category": "WEB" },
                    { "title": "RxJS Observable", "category": "RxJS" },
                    { "title": "Angular 2 AsyncPipe", "category": "NG2" }
                ];
                observer.next(posts);
            }, 2000);
        })
    }
}

app.component.ts

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

@Component({
  selector: 'exe-app',
  template: `
    <exe-bloggers></exe-bloggers>
  `
})
export class AppComponent {}

以上代码运行完后,你会发现浏览器中只显示 Posts by: Semlinker 。那我们要怎么正确显示帖子列表呢?

Solutions (方案)

Solution 1: Use *ngIf

使用 *ngIf 指令延迟 exe-posts 组件的初始化,需要调整的代码如下:

blogger.component.ts

template: `
  <h1>Posts by: {{ blogger }}</h1>
  <div *ngIf="posts">
       <exe-posts [data]="posts"></exe-posts>
  </div>
`

posts.component.ts

ngOnInit() {
  this.groupPosts = this.groupByCategory(this.data);
}

Solution 2: Use ngOnChanges

当数据绑定输入属性的值发生变化的时候,Angular 将会主动调用 ngOnChanges() 方法。它会获得一个 SimpleChanges 对象,包含绑定属性的新值和旧值,因此我们可以利用 ngOnChanges() 钩子,执行 groupPosts 的数据初始化操作。需要调整的代码如下:

1.移除 blogger.component.ts 组件模板中的 *ngIf 指令:

template: `
  <h1>Posts by: {{ blogger }}</h1>
  <div>
       <exe-posts [data]="posts"></exe-posts>
  </div>
`

2.更新 posts.component.ts

ngOnInit() {
  // this.groupPosts = this.groupByCategory(this.data);
}

ngOnChanges(changes: SimpleChanges) {
  if (changes['data']) {
     this.groupPosts = this.groupByCategory(this.data);
  }
}

Solution 3: Use RxJS BehaviorSubject

我们也可以利用 RxJS 中 BehaviorSubject 来监测变化。需要注意的是,在使用 Observable 或 Subject 时,在不使用的时候,需要及时取消订阅,以避免出现内存泄露的问题。具体代码如下:

posts.component.ts

import { Component, Input, OnChanges, OnInit, SimpleChanges } from '@angular/core';
import { BehaviorSubject } from 'rxjs/BehaviorSubject';
import 'rxjs/add/operator/takeWhile';
import { Post, GroupPosts } from './post.interface';

@Component({
    selector: 'exe-posts',
    template: `
    <div class="list-group">
        <div *ngFor="let group of groupPosts;" class="list-group-item">
            <h4>{{ group.category }}</h4>
            <ul>
                <li *ngFor="let post of group.posts">
                    {{ post.title }}
                </li>
            </ul>
        <div>
    </div>
    `
})
export class PostsComponent implements OnInit, OnChanges {

    private _data$ = new BehaviorSubject<Post[]>([]);

    @Input()
    set data(value: Post[]) {
        this._data$.next(value);
    }

    get data(): Post[] {
        return this._data$.getValue();
    }

    groupPosts: GroupPosts[];

    ngOnInit() {
        this._data$
            // 当this.groupPosts有值的时候,会自动取消订阅
            .takeWhile(() => !this.groupPosts) 
            .subscribe(x => {
                this.groupPosts = this.groupByCategory(this.data);
            });
    }

    ngOnChanges(changes: SimpleChanges) { }

    groupByCategory(data: Post[]): GroupPosts[] {
        if (!data) return;

        // 获取目录名,使用ES6中Set进行去重处理
        const categories = new Set(data.map(x => x.category));

        // 重新生成二维数组,用于页面显示
        const result = Array.from(categories).map(x => ({
            category: x,
            posts: data.filter(post => post.category === x)
        }));
        return result;
    }
}

上面示例中,我们使用了 RxJS 中 BehaviorSubject,如果想进一步了解详细信息,请参考 - RxJS Subject

需要注意的是,如果使用 takeWhile 操作符,在页面成功显示后,如果输入属性又发生变化,页面是不会随着更新的,因为我们已经取消订阅了。如果数据源会持续变化,那么可以移除 takeWhile 操作符,然后 ngOnDestroy() 钩子中执行取消订阅操作。

Solution 4: Use Observable

除了上面提到的三种方案外,我们还可以使用 Observable 对象。即设置输入属性的类型是 Observable。具体示例如下:

posts.component.ts

import { Component, Input, OnChanges, OnInit, OnDestroy, SimpleChanges } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { Subscription } from 'rxjs/Subscription';
import { Post, GroupPosts } from './post.interface';

@Component({
    selector: 'exe-posts',
    template: `
    <div class="list-group">
        <div *ngFor="let group of groupPosts;" class="list-group-item">
            <h4>{{ group.category }}</h4>
            <ul>
                <li *ngFor="let post of group.posts">
                    {{ post.title }}
                </li>
            </ul>
        <div>
    </div>
    `
})
export class PostsComponent implements OnInit, OnChanges, OnDestroy {

    @Input() data: Observable<Post[]>; // 输入属性的类型是Observable

    groupPosts: GroupPosts[];

    dataSubscription: Subscription; // 用于组件销毁时取消订阅

    ngOnInit() {
        this.dataSubscription = this.data.subscribe(posts => {
            this.groupPosts = this.groupByCategory(posts);
        });
    }

    ngOnChanges(changes: SimpleChanges) { }

    ngOnDestroy() {
        this.dataSubscription.unsubscribe();
    }

    groupByCategory(data: Post[]): GroupPosts[] {
        if (!data) return;

        // 获取目录名,使用ES6中Set进行去重处理
        const categories = new Set(data.map(x => x.category));

        // 重新生成二维数组,用于页面显示
        const result = Array.from(categories).map(x => ({
            category: x,
            posts: data.filter(post => post.category === x)
        }));

        return result;
    }
}

blogger.component.ts

import { Component, OnInit, Input } from '@angular/core';
import { Observable } from 'rxjs/Observable';
import { Observer } from 'rxjs/Observer';
import { Post } from './post.interface';

@Component({
    selector: 'exe-bloggers',
    template: `
        <h1>Posts by: {{ blogger }}</h1>
        <div>
            <exe-posts [data]="posts"></exe-posts>
        </div>
    `
})
export class BloggerComponent implements OnInit {

    blogger = 'Semlinker';

    posts: Observable<Post[]>;

    ngOnInit() {
        this.posts = this.getPostsByBlogger();
    }

    getPostsByBlogger(): Observable<Post[]> {
        return Observable.create((observer: Observer<Post[]>) => {
            setTimeout(() => {
                const posts = [
                    { "title": "Functional Programming", "category": "RxJS" },
                    { "title": "Angular 2 Component Inheritance", "category": "NG2" },
                    { "title": "RxJS Operators", "category": "RxJS" },
                    { "title": "Angular 2 Http Module - HTTP", "category": "WEB" },
                    { "title": "RxJS Observable", "category": "RxJS" },
                    { "title": "Angular 2 AsyncPipe", "category": "NG2" }
                ];
                observer.next(posts);
            }, 2000);
        })
    }
}

上面的示例是没有在模板中直接使用 Observable 类型的输入属性,若需要在模板中直接使用的话,可以使用 Angular 2 中的 AsyncPipe ,了解详细信息,请查看 - Angular 2 AsyncPipe。另外在 Angular 2 Change Detection - 2 文章中,我们也有介绍怎么利用 Observable 提高 Angular 2 变化检测的性能,有兴趣的读者可以了解一下。

我有话说

1.上面介绍的方案,应该使用哪一种?

这个还是取决于实际的应用场景,如果数据源只会发生一次变化,那么可以考虑使用 *ngIf 指令。如果数据源会持续变化,可以考虑使用其它的方案。

2.组件通信还有哪些方式?

组件通信的常用方式:@Input、@Output、@ViewChild、模板变量、MessageService、Broadcaster (Angular 1.x $rootScope 中 $on、$broadcast ) 等。

详细的信息,请参考 - Angular 2 Components Communicate

参考资源

阅读 4.9k更新于 2017-04-08
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