RXJava/RXAndroid - Can't create handler inside thread that has not called Looper.prepare()

Question

When you received an error with this message

rx.exceptions.OnErrorFailedException: Error occurred when trying to propagate error to Observer.onError

but your subscription is already handling onError

MyMethodThatRetunsAnObservable(string)
        .subscribe(
            response -> handleResponse(response),
            throwable -> handleError(throwable));

In case that is caused by: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

Note that onError notifications will cut ahead of onNext notifications on the emission thread if Scheduler is truly asynchronous. So it's needed to specify the UI Thread in which we are going to observe

public static void shortToast(String msg) {
    Observable.just(msg)
            .subscribeOn(AndroidSchedulers.mainThread())
            .subscribe(message -> {
                Toast.makeText(App.getInstance(), message, Toast.LENGTH_SHORT).show();
            });
}

Answer 1

yes, you got the error. You should call subscribeOn(AndroidSchedulers.mainThread() instead of observerOn. The show log function was called before the observerOn and called in the onSubcribe method.