Convert/ wrap async listener to Observable (RxJava2)

Question

I want to wrap a real listener to Observable object. For starters here is a test case, with him everything is fine.

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    getObservablePhoneState()
        // Run on a background thread
        .subscribeOn(Schedulers.io())
        // Be notified on the main thread
        .observeOn(AndroidSchedulers.mainThread())
        .subscribe(integer -> Log.i(TAG, "----- subscribe onNext = " + integer));
}

private Flowable<Integer> getObservablePhoneState() {
    return Flowable.create(emitter -> {

        Log.i(TAG, "Emitting 1");
        emitter.onNext(1);

        Log.i(TAG, "Emitting 2");
        emitter.onNext(2);

    }, BackpressureStrategy.BUFFER);
}



*** logcat ***
Emitting 1
Emitting 2
----- subscribe onNext = 1
----- subscribe onNext = 2

This code generates an error:

private Flowable<Integer> getObservablePhoneState() {
    return Flowable.create(emitter -> {

        PhoneStateListener phoneStateListener = new PhoneStateListener() {
            @Override
            public void onCallStateChanged(int state, String incomingNumber) {
                Log.i(TAG, "onCallStateChanged = " + state);
                emitter.onNext(state);
            }
        };
        TelephonyManager telephonyManager = (TelephonyManager) getActivity().getSystemService(Context.TELEPHONY_SERVICE);
        telephonyManager.listen(phoneStateListener, PhoneStateListener.LISTEN_CALL_STATE);

    }, BackpressureStrategy.BUFFER);
}

*** logcat ***
io.reactivex.exceptions.OnErrorNotImplementedException: 
Attempt to read from field 'android.os.MessageQueue 
android.os.Looper.mQueue' on a null object reference

With Observable.create() the same error. Perhaps this is due to the fact that RxJava2 does not support emitting a null value.

How to do it right?

Is it possible you're calling thing before onCreate() in your activity? I tried your code and it worked fine for me. — James McCracken
– James McCracken, Commented Sep 20, 2017 at 22:13

gyosida · Accepted Answer · 2017-09-21 00:28:45Z

3

You should remove the subscribeOn(Schedulers.io()) to avoid creating the PhoneStateListener in a different thread, because under the hood is trying to send messages using a Handler which mQueue is null. Just call

getObservablePhoneState()
     .subscribe { integer -> Log.i("", "----- subscribe onNext = " + integer) }

answered Sep 21, 2017 at 0:28

gyosida

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6 Comments

tim4dev Over a year ago

Thx, your comment was useful. the code worked. but... onNext occurs only 1--3 times, and further - nothing. I'm testing under Android 8, and I'm using the latest versions of the libraries. I will post the complete code for tests later. Perhaps I chose the too complicated Listener.

gyosida Over a year ago

onNext is being called 1-3 times but your listener was called more times?

tim4dev Over a year ago

Listener dead and (of course) onNext. Android 4.2 - all OK. Android 8 - :( Native Listener working fine. Full code here github.com/tim4dev/dirty_code/tree/master/…

tim4dev Over a year ago

Native phoneStateListener() working OK everywhere. On Android 4.2 "Observable Listener" (Observable, Flowable) worked fine. On Android 8.0 "Observable Listener" (Observable, Flowable) called 2--3 times and dead. Steps to reproduce the problem: - set PHONE_NUMBER to your own phone (yes, you will phone call to yourself) - run app on Android 8 - call - press disconnect (off hook) - if you do it fast enough - everything will be ok - if you listen to short beeps and wait, the Listener dies and does not react any more.

gyosida Over a year ago

I'm not sure but, have you tried passing a different state to the listen method? check the changelog for 8.0 the API could have changed

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