Unwraps a value from an asynchronous primitive.


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{{ obj_expression | async }}



objObservable | Promise | null | undefined


The async pipe subscribes to an Observable or Promise and returns the latest value it has emitted. When a new value is emitted, the async pipe marks the component to be checked for changes. When the component gets destroyed, the async pipe unsubscribes automatically to avoid potential memory leaks.

async 管道会订阅一个 ObservablePromise,并返回它发出的最近一个值。 当新值到来时,async 管道就会把该组件标记为需要进行变更检测。当组件被销毁时,async 管道就会自动取消订阅,以消除潜在的内存泄露问题。




This example binds a Promise to the view. Clicking the Resolve button resolves the promise.

这个例子把一个 Promise 绑定到了视图中。点击 Resolve 按钮就会解析此 Promise。

@Component({ selector: 'async-promise-pipe', template: `<div> <code>promise|async</code>: <button (click)="clicked()">{{ arrived ? 'Reset' : 'Resolve' }}</button> <span>Wait for it... {{ greeting | async }}</span> </div>` }) export class AsyncPromisePipeComponent { greeting: Promise<string>|null = null; arrived: boolean = false; private resolve: Function|null = null; constructor() { this.reset(); } reset() { this.arrived = false; this.greeting = new Promise<string>((resolve, reject) => { this.resolve = resolve; }); } clicked() { if (this.arrived) { this.reset(); } else { this.resolve !('hi there!'); this.arrived = true; } } }

It's also possible to use async with Observables. The example below binds the time Observable to the view. The Observable continuously updates the view with the current time.

还可以把 async 用于 Observable。下面的例子就把 time 这个 Observable 绑定到了视图上。这个 Observable 会不断使用当前时间更新视图。

@Component({ selector: 'async-observable-pipe', template: '<div><code>observable|async</code>: Time: {{ time | async }}</div>' }) export class AsyncObservablePipeComponent { time = new Observable<string>((observer: Observer<string>) => { setInterval(() => Date().toString()), 1000); }); }