Blog post: http://adelnizamutdinov.github.io/blog/2015/01/23/using-rxjavas-observable-semantics-for-greater-good/

RxMediaRecorder.java

import android.media.MediaRecorder;
import android.support.annotation.NonNull;
import com.f2prateek.rx.android.schedulers.AndroidSchedulers;
import java.io.File;
import java.io.IOException;
import java.util.concurrent.TimeUnit;
import rx.Observable;
import rx.subscriptions.Subscriptions;

public class RxMediaRecorder {
  public static @NonNull MediaRecorder into(@NonNull File file) {
    final MediaRecorder recorder = new MediaRecorder();
    recorder.setAudioSource(MediaRecorder.AudioSource.MIC);
    recorder.setOutputFormat(MediaRecorder.OutputFormat.AMR_WB);
    recorder.setOutputFile(file.getAbsolutePath());
    recorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_WB);
    return recorder;
  }

  public static @NonNull Observable<Long> record(@NonNull MediaRecorder recorder) {
    return prepare(recorder).flatMap(RxMediaRecorder::stream);
  }

  static @NonNull Observable<MediaRecorder> prepare(@NonNull MediaRecorder recorder) {
    return Observable.create(subscriber -> {
      try {
        Threads.assertBg();

        recorder.prepare();
        subscriber.onNext(recorder);
        subscriber.onCompleted();
      } catch (IOException e) {
        subscriber.onError(e);
      }
    });
  }

  static @NonNull Observable<Long> stream(@NonNull MediaRecorder recorder) {
    return Observable.create(sub -> {
      sub.add(Subscriptions.create(() -> {
        Threads.assertBg();

        recorder.stop();
        recorder.reset();
        recorder.release();
      }));

      recorder.start();

      final long start = System.currentTimeMillis();
      sub.add(Observable.interval(16, TimeUnit.MILLISECONDS, AndroidSchedulers.mainThread())
                  .map(x -> System.currentTimeMillis() - start)
                  .subscribe(sub));
    });
  }
}

Brilliant!

Can you put license on this code so people can use it?

Thanks!