订阅Observable时如何获得最后的价值? [英] How to get last value when subscribing to an Observable?

问题描述

我有两个Angular2组件，需要通过服务共享数据:

I have two Angular2 components which need to share data via a service:

@Injectable()
export class SearchService {
  private searchResultSource = new Subject<string>()
  searchResult$ = this.searchResultSource.asObservable()

  setSearchResults(_searchResult: string): void {
    this.searchResultSource.next(_searchResult)
  }
}

假定呈现ComponentA并通过SearchService.setSearchResults发出事件.然后，用户导航到ComponentB，它也订阅了searchResult$.但是，ComponentB永远不会观察到ComponentA发出的事件，因为在ComponentA发出事件时它尚未订阅searchResult$，因为它不存在.

Suppose ComponentA is rendered and it emits an event via SearchService.setSearchResults. Then the user navigates to ComponentB, which also subscribes to searchResult$. However, ComponentB will never observe the event emitted by ComponentA because it was not subscribed to searchResult$ at the time ComponentA emitted an event, as it did not exist.

如何创建一个Observable，它向每个新订户发出最后一个事件?

How can I create an Observable which emits the last event to every new subscriber?

推荐答案

BehaviorSubject立即向新订户发出最后一个值:

BehaviorSubject immediately emits the last value to new subscribers:

@Injectable()
export class SearchService {

  private searchResultSource = new BehaviorSubject<string>('');

  setSearchResults(_searchResult: string): void {
      this.searchResultSource.next(_searchResult);
  }
}

ReplaySubject将所有先前的事件发送给新订阅者.

ReplaySubject emits all previous events to new subscribers.

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