具有Backpressure的RxJava主题 - 只有在下游完成消耗后才允许最后一个值发出

Question

我有一个调用onNext()一些UI事件的PublishSubject .订户通常需要2秒钟才能完成其工作.onNext()在订户忙时,我需要忽略除最后一个之外的所有呼叫.我尝试了以下,但是我无法控制流量.请求似乎排队等待,每个请求都得到处理(因此背压似乎不起作用).如何让它忽略所有请求,但最后一个？(我不想使用,debounce因为代码需要立即做出反应,任何合理的小超时都不起作用).

而且我意识到使用subscribeOn一个主题没有任何影响,因此我正在使用observeOn其中一个运算符进行异步工作.这是正确的方法吗？

Subject<Boolean> loadingQueue = PublishSubject.<Boolean>create().toSerialized();

loadingQueue
  .toFlowable(BackpressureStrategy.LATEST)
  .observeOn(AndroidSchedulers.mainThread())
  .map(discarded -> {
    // PRE-LOADING
    Log.d("RXLOADING", "PRE-LOADING: " + Thread.currentThread().getName());
    return discarded;
   })
   .observeOn(Schedulers.computation())
   .map(b -> {
     Log.d("RXLOADING", "LOADING: " + Thread.currentThread().getName());
     Thread.sleep(2000);
     return b;
   })
   .observeOn(AndroidSchedulers.mainThread())
   .subscribe(b -> {
      Log.d("RXLOADING", "FINISHED: " + Thread.currentThread().getName() + "\n\n");
   });


loadingQueue.onNext(true);
loadingQueue.onNext(true);
loadingQueue.onNext(true);
....

我看到的输出是:

PRE-LOADING: main
PRE-LOADING: main
LOADING: RxComputationThreadPool-1
PRE-LOADING: main
PRE-LOADING: main
PRE-LOADING: main
PRE-LOADING: main
PRE-LOADING: main
PRE-LOADING: main
LOADING: RxComputationThreadPool-1
FINISHED: main
LOADING: RxComputationThreadPool-1
FINISHED: main
LOADING: RxComputationThreadPool-1
FINISHED: main
LOADING: RxComputationThreadPool-1
FINISHED: main
LOADING: RxComputationThreadPool-1
FINISHED: main
LOADING: RxComputationThreadPool-1
FINISHED: main
LOADING: RxComputationThreadPool-1
FINISHED: main
FINISHED: main

相反,我希望代码执行以下操作(即加载一次,当它加载时,背压以阻止所有请求并发出最后一个请求,一旦第一个观察者完成 - 所以总的来说理想情况下应该加载两次最多):

PRE-LOADING: main
LOADING: RxComputationThreadPool-1
FINISHED: main

PRE-LOADING: main
LOADING: RxComputationThreadPool-1
FINISHED: main

Answer 1

您无法执行此操作,observeOn因为它将缓冲至少1个元素,因此如果已经发生一次"加载",则始终执行"PRE-LOADING"阶段.

但是你可以这样做,delay因为它不会操纵链上的请求数量,并在调度程序上单独调度每个onNext而不自行排队:

public static void main(String[] args) throws Exception {
    Subject<Boolean> loadingQueue = 
         PublishSubject.<Boolean>create().toSerialized();

    loadingQueue
      .toFlowable(BackpressureStrategy.LATEST)
      .delay(0, TimeUnit.MILLISECONDS, Schedulers.single())        // <-------
      .map(discarded -> {
        // PRE-LOADING
        System.out.println("PRE-LOADING: " 
             + Thread.currentThread().getName());
        return discarded;
       })
       .delay(0, TimeUnit.MILLISECONDS, Schedulers.computation())  // <-------
       .map(b -> {
           System.out.println("LOADING: " 
             + Thread.currentThread().getName());
         Thread.sleep(2000);
         return b;
       })
       .delay(0, TimeUnit.MILLISECONDS, Schedulers.single())       // <-------
       .rebatchRequests(1)             // <----------------------------------- one-by-one
       .subscribe(b -> {
           System.out.println("FINISHED: " 
               + Thread.currentThread().getName() + "\n\n");
       });


    loadingQueue.onNext(true);
    loadingQueue.onNext(true);
    loadingQueue.onNext(true);

    Thread.sleep(10000);
}