让 ReplaySubject 只返回订阅的最后一个值

Question

我有一个奇怪的用例，我需要跟踪所有以前发出的事件。

感谢 ReplaySubject，到目前为止它运行良好。在每个新订阅者上，此主题都会重新发出之前的所有事件。

现在，对于特定场景，我需要能够只提供最新发布的事件（有点像 BehaviorSubject），但保持源相同的事件。

这是我正在努力实现的目标：stackblitz

import { ReplaySubject, BehaviorSubject, from } from "rxjs";

class EventManager {
  constructor() {
    this.mySubject = new ReplaySubject();
  }

  publish(value) {
    this.mySubject.next(value);
  }

  fullSubscribe(next, error, complete) {
    return this.mySubject.subscribe(next, error, complete);
  }

  subscribe(next, error, complete) {
    return this.mySubject.pipe(/* an operator to get the last one on new subscribe */).subscribe(next, error, complete);
  }
}

const myEventManager = new EventManager();

myEventManager.publish("Data 1");
myEventManager.publish("Data 2");
myEventManager.publish("Data 3");

myEventManager.fullSubscribe(v => {
  console.log("SUB 1", v);
});

myEventManager.subscribe(v => {
  console.log("SUB 2", v);
});

谢谢

Answer 1

如果您跟踪已发布的事件数量，您可以使用skip：

  subscribe(next, error?, complete?) {
    return this.mySubject.pipe(
      skip(this.publishCount - 1)
    ).subscribe(next, error, complete);
  }

这是一个StackBlitz 演示。

Answer 2

您可以通过操作BehaviorSubject 来实现类似于ReplaySubject 的行为，而不是强制ReplaySubject 表现得像BehaviorSubject。

import { BehaviorSubject, from, concat } from 'rxjs';
import { scan, shareReplay } from 'rxjs/operators';

class EventManager {
  constructor() {
    this.mySubject = new BehaviorSubject();
    this.allEmittedValues = this.mySubject.pipe(
      scan((xs, x) => [...xs, x], []),
      shareReplay(1)
    );

    // Necessary since we need to start accumulating allEmittedValues
    // immediately.
    this.allEmittedValues.subscribe();
  }

  dispose() {
    // ends all subscriptions
    this.mySubject.complete();
  }

  publish(value) {
    this.mySubject.next(value);
  }

  fullSubscribe(next, error, complete) {
    // First, take the latest value of the accumulated array of emits and
    // unroll it into an observable
    const existingEmits$ = this.allEmittedValues.pipe(
      take(1),
      concatMap((emits) => from(emits))
    );
    // Then, subscribe to the main subject, skipping the replayed value since
    // we just got it at the tail end of existingEmits$
    const futureEmits$ = this.mySubject.pipe(skip(1));

    return concat(existingEmits$, futureEmits$).subscribe(
      next,
      error,
      complete
    );
  }

  subscribe(next, error, complete) {
    return this.mySubject.subscribe(next, error, complete);
  }
}

Answer 3

为什么不在EventManager 上有一个ReplaySubject 和BehaviorSubject 的实例？

import { ReplaySubject, BehaviorSubject, from } from "rxjs";

class EventManager {
  constructor() {
    this.replaySubject = new ReplaySubject();
    this.behaviorSubject = new BehaviorSubject();
  }

  publish(value) {
    this.replaySubject.next(value);
    this.behaviorSubject.next(value);
  }

  fullSubscribe(next, error, complete) {
    return this.replaySubject.subscribe(next, error, complete);
  }

  subscribe(next, error, complete) {
    return this.behaviorSubject.subscribe(next, error, complete);
  }
}