RxJava: how to process Flowable interactively from console

Question

I've created a Flowable (RxJava v3) that parses a file. I'd like it to support backpressure. This is important because the files can be quite large, and I don't want them to be loaded into memory at once. Here is my first attempt:

public Flowable<Byte> asFlowable(InputStream is) {
    return Flowable.create(new FlowableOnSubscribe<Byte>() {

         @Override
         public void subscribe(FlowableEmitter<Byte> emitter) throws Exception {
             try (DataInputStream inputStream = new DataInputStream(is)){
                 if (inputStream.readInt() != SOME_CONSTANT) {
                     throw new IllegalArgumentException("illegal file format");
                 }

                 if (inputStream.readInt() != SOME_OTHER_CONSTANT) {
                     throw new IllegalArgumentException("illegal file format");
                 }

                 final int numItems = inputStream.readInt();

                 for(int i = 0; i < numItems; i++) {
                     if(emitter.isCancelled()) {
                         return;
                     }

                     emitter.onNext(inputStream.readByte());
                 }

                 emitter.onComplete();       
             } catch (Exception e) {
                 emitter.onError(e);
             } 
         }
     }, BackpressureStrategy.BUFFER);
}

The reason I used Flowable.create instead of Flowable.generateis because I need to validate the file, and throw errors if some magic numbers at the beginning of the file are wrong or not found. This didn't fit well with the Flowable.generate lambdas (but if you know of a better way please post it).

Ok let's assume the cold Flowable supported backpressure. Now I'd like to process it in a console-like application.

Question: I want to request a new Byte from the Flowable and print it to console each time the user presses space (similar to what more or less do in Linux). What would the best way of doing it? I intend to observe the flowable directly in the public static void main method, since I need to read and write using the console.

I've been reading the Backpressure section in RxJAva's Wiki and found this snippet:

someObservable.subscribe(new Subscriber<t>() {
    @Override
    public void onStart() {
      request(1);
    }

    @Override
    public void onCompleted() {
      // gracefully handle sequence-complete
    }

    @Override
    public void onError(Throwable e) {
      // gracefully handle error
    }

    @Override
    public void onNext(t n) {
      // do something with the emitted item "n"
      // request another item:
      request(1);
    }
});

But this confused me even more as the request method doesn't seem to exist in RxJava 3.

Answer 1

Use generate, blockingSubscribe and read a line from the console:

class State {
     DataInputStream inputStream;
     int count;
     int i;
}

BufferedReader bin = new BufferedReader(new InputStreamReader(System.in));

Flowable.generate(() -> {
    State s = new State();
    s.inputStream = new DataInputStream(is);
    try {
        if (s.inputStream.readInt() != SOME_CONSTANT) {
            throw new IllegalArgumentException("illegal file format");
        }

        if (s.inputStream.readInt() != SOME_OTHER_CONSTANT) {
            throw new IllegalArgumentException("illegal file format");
        }
        s.count = s.inputStream.readInt();
    } catch (IOException ex) {
        s.inputStream.close();
        throw ex;
    }
    return s;
}, (state, emitter) -> {
    if (state.i < s.count) {
        emitter.onNext(state.inputStream.readByte());
        s.i++;
    }
    if (state.i >= s.count) {
        emitter.onComplete();
    }
}, state -> {
    state.inputStream.close();
})
.subscribeOn(Schedulers.io())
.blockingSubscribe(b -> {
    System.out.println(b);
    bin.readLine();
}, Flowable.bufferSize());