public static Scheduler from(Looper looper) {
        if (looper == null) {
            throw new NullPointerException("looper == null");
        }
        return new HandlerScheduler(new Handler(looper));
    }
A {@link Scheduler} which executes actions on {@code looper}.
android/android_framework/base/src/main/java/rx2/android/schedulers/AndroidSchedulers.java