RuntimePermissionRequestHandler.java

    @Override
    public Single<PermissionRequestResult> requestPermission() {
        subject = AsyncSubject.create();
        
        if (activityWeakReference.get() != null) {
            ActivityCompat.requestPermissions(
                activityWeakReference.get(),
                new String[] {permission},
                requestCode);
        }
        
        return subject.firstOrError();
    }