唯一的方法，你应该得到值“出”一个可观察对象/主题是与订阅!

如果你在使用getValue()你在做一些声明式的命令。它是一个逃生口，但是99.9%的情况下你不应该使用getValue()。getValue()会做一些有趣的事情:如果主题已经取消订阅，它会抛出一个错误，如果主题已经死亡，它会阻止你获得一个值，因为它是错误的，等等。但是，再一次地，它是在罕见情况下的一个逃生口。

有几种方法可以以“Rx-y”方式从Subject或Observable中获取最新的值:

Using BehaviorSubject: But actually subscribing to it. When you first subscribe to BehaviorSubject it will synchronously send the previous value it received or was initialized with. Using a ReplaySubject(N): This will cache N values and replay them to new subscribers. A.withLatestFrom(B): Use this operator to get the most recent value from observable B when observable A emits. Will give you both values in an array [a, b]. A.combineLatest(B): Use this operator to get the most recent values from A and B every time either A or B emits. Will give you both values in an array. shareReplay(): Makes an Observable multicast through a ReplaySubject, but allows you to retry the observable on error. (Basically it gives you that promise-y caching behavior). publishReplay(), publishBehavior(initialValue), multicast(subject: BehaviorSubject | ReplaySubject), etc: Other operators that leverage BehaviorSubject and ReplaySubject. Different flavors of the same thing, they basically multicast the source observable by funneling all notifications through a subject. You need to call connect() to subscribe to the source with the subject.

一个类似的答案被否决了。但我认为我可以证明我在这里提出的建议适用于有限的情况。

虽然一个可观察对象确实没有当前值，但通常它会有一个立即可用的值。例如，对于redux / flux / akita存储，您可以根据一些可观察的数据从中央存储请求数据，这些值通常会立即可用。

如果是这种情况，那么当您订阅时，该值将立即返回。

假设你有一个对服务的调用，在完成时你想从你的商店中获得一些东西的最新值，这可能不会发出:

你可以尝试这样做(你应该尽可能地把事情“放在管道里”):

 serviceCallResponse$.pipe(withLatestFrom(store$.select(x => x.customer)))
                     .subscribe(([ serviceCallResponse, customer] => {

                        // we have serviceCallResponse and customer 
                     });

这样做的问题是它会阻塞，直到第二个可观察对象发出一个值，而这可能永远不会发生。

我发现自己最近只需要在一个值立即可用的情况下评估一个可观察到的值，更重要的是，我需要能够检测到它是否可用。我最后是这样做的:

 serviceCallResponse$.pipe()
                     .subscribe(serviceCallResponse => {

                        // immediately try to subscribe to get the 'available' value
                        // note: immediately unsubscribe afterward to 'cancel' if needed
                        let customer = undefined;

                        // whatever the secondary observable is
                        const secondary$ = store$.select(x => x.customer);

                        // subscribe to it, and assign to closure scope
                        sub = secondary$.pipe(take(1)).subscribe(_customer => customer = _customer);
                        sub.unsubscribe();

                        // if there's a delay or customer isn't available the value won't have been set before we get here
                        if (customer === undefined) 
                        {
                           // handle, or ignore as needed
                           return throwError('Customer was not immediately available');
                        }
                     });

请注意，对于以上所有内容，我都使用subscribe来获取值(正如@Ben讨论的那样)。没有使用.value属性，即使我有一个行为主体。

另一种方法，如果你想/可以使用async await(必须在async函数中)，你可以在现代Rxjs中做到这一点:

 async myFunction () {
     const currentValue = await firstValueFrom(
      of(0).pipe(
        withLatestFrom(this.yourObservable$),
        map((tuple) => tuple[1]),
        take(1)
      )
    );
    // do stuff with current value

 }

 

这将发出一个值“Right away”，因为withLatestFrom，然后将解析承诺。

唯一的方法，你应该得到值“出”一个可观察对象/主题是与订阅!

如果你在使用getValue()你在做一些声明式的命令。它是一个逃生口，但是99.9%的情况下你不应该使用getValue()。getValue()会做一些有趣的事情:如果主题已经取消订阅，它会抛出一个错误，如果主题已经死亡，它会阻止你获得一个值，因为它是错误的，等等。但是，再一次地，它是在罕见情况下的一个逃生口。

有几种方法可以以“Rx-y”方式从Subject或Observable中获取最新的值:

Using BehaviorSubject: But actually subscribing to it. When you first subscribe to BehaviorSubject it will synchronously send the previous value it received or was initialized with. Using a ReplaySubject(N): This will cache N values and replay them to new subscribers. A.withLatestFrom(B): Use this operator to get the most recent value from observable B when observable A emits. Will give you both values in an array [a, b]. A.combineLatest(B): Use this operator to get the most recent values from A and B every time either A or B emits. Will give you both values in an array. shareReplay(): Makes an Observable multicast through a ReplaySubject, but allows you to retry the observable on error. (Basically it gives you that promise-y caching behavior). publishReplay(), publishBehavior(initialValue), multicast(subject: BehaviorSubject | ReplaySubject), etc: Other operators that leverage BehaviorSubject and ReplaySubject. Different flavors of the same thing, they basically multicast the source observable by funneling all notifications through a subject. You need to call connect() to subscribe to the source with the subject.

我遇到过类似的情况，在Subject的价值到达后，迟来的订阅者才订阅它。

我发现ReplaySubject类似于BehaviorSubject，在这种情况下很有魅力。 这里有一个更好的解释链接:http://reactivex.io/rxjs/manual/overview.html#replaysubject

我在子组件中遇到了同样的问题，最初它必须拥有Subject的当前值，然后订阅Subject以侦听更改。我只是在服务中维护当前值，以便组件可以访问它，例如:

import {Storage} from './storage';
import {Injectable} from 'angular2/core';
import {Subject}    from 'rxjs/Subject';

@Injectable()
export class SessionStorage extends Storage {

  isLoggedIn: boolean;

  private _isLoggedInSource = new Subject<boolean>();
  isLoggedIn = this._isLoggedInSource.asObservable();
  constructor() {
    super('session');
    this.currIsLoggedIn = false;
  }
  setIsLoggedIn(value: boolean) {
    this.setItem('_isLoggedIn', value, () => {
      this._isLoggedInSource.next(value);
    });
    this.isLoggedIn = value;
  }
}

需要当前值的组件可以直接从服务中访问它，即:

sessionStorage.isLoggedIn

不确定这是否是正确的做法:)