RxJava flatMap series of conversion operator Source introduced

Reproduced, please indicate the source as a link:
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Conversion-related operator and the official introduction

RxJavaThe flatMapseries conversion operator official description: Transforming observables

• flatMap
• flatMapCompletable
• flatMapIterable
• flatMapMaybe
• flatMapObservable
• flatMapPublisher
• flatMapSingle
• flatMapSingleElement

Here we just a specific implementation, please refer to the intermediate flow RxJava create the source operator resolved .

flatMap

• Official Example:

  Observable.just("A", "B", "C")
          .flatMap(a -> {
              return Observable.intervalRange(1, 3, 0, 1, TimeUnit.SECONDS)
                      .map(b -> '(' + a + ", " + b + ')');
          })
          .blockingSubscribe(System.out::println);
  

  Output:

  (A, 1)
  (C, 1)
  (B, 1)
  (A, 2)
  (C, 2)
  (B, 2)
  (A, 3)
  (C, 3)
  (B, 3)
  

• Returns the object ObservableFlatMap's subscribeActualmethods:

  public void subscribeActual(Observer<? super U> t) {
      if (ObservableScalarXMap.tryScalarXMapSubscribe(source, t, mapper)) {
          return;
      }
      source.subscribe(new MergeObserver<T, U>(t, mapper, delayErrors, maxConcurrency, bufferSize));
  }
  

• MergeObserverThe onNextmethod:

  public void onNext(T t) {
      // safeguard against misbehaving sources
      ...
      subscribeInner(p);
  }
  
  void subscribeInner(ObservableSource<? extends U> p) {
      for (;;) {
          if (p instanceof Callable) {
             ...
                      drain();
             ...
          } else {
              InnerObserver<T, U> inner = new InnerObserver<T, U>(this, uniqueId++);
              if (addInner(inner)) {
                  p.subscribe(inner);
              }
              break;
          }
      }
  }
  

• InnerObserverThe onNextmethod:

  public void onNext(U t) {
      if (fusionMode == QueueDisposable.NONE) {
          parent.tryEmit(t, this);
      } else {
          parent.drain();
      }
  }
  

• MergeObserverThe drain()method:

  void drain() {
      if (getAndIncrement() == 0) {
          drainLoop();
      }
  }
  
  void drainLoop() {
      final Observer<? super U> child = this.downstream;//1.0
      int missed = 1;
      for (;;) {
          ...
          SimplePlainQueue<U> svq = queue;
          if (svq != null) {
              for (;;) {
                  ...
                  U o = svq.poll();
                  if (o == null) {
                      break;
                  }
                  child.onNext(o);//1.1
              }
          }
          ...
          if (d && (svq == null || svq.isEmpty()) && n == 0 && nSources == 0) {
              Throwable ex = errors.terminate();
              if (ex != ExceptionHelper.TERMINATED) {
                  if (ex == null) {
                      child.onComplete();//1.2
                  } else {
                      child.onError(ex);//1.3
                  }
              }
              return;
          }
          ...
      }
  }
  

  Eventually call-entered the observercorresponding method.

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Reproduced in: https: //www.jianshu.com/p/191a8680b8cc