IllegalArgumentException when using Otto with a retained Fragment

Issue

I am using Otto 1.3.3 and when I resume my application sometimes I get an IllegalArgumentException with the following stacktrace:

Caused by: java.lang.IllegalArgumentException: Producer method for type class 
com.couchsurfing.mobile.ui.setup
        .SessionProviderFragment$SessionConnectionStateChangeEvent found on 
        type class com.couchsurfing.mobile.ui.setup.SessionProviderFragment, 
        but already registered by type class 
        com.couchsurfing.mobile.ui.setup.SessionProviderFragment.
    at com.squareup.otto.Bus.register(Bus.java:194)
    at com.couchsurfing.mobile.ui.BaseRetainedFragment
       .onCreate(BaseRetainedFragment.java:20)

The SessionProviderFragment has its instance retained, please find below the extended class:

public abstract class BaseRetainedFragment extends SherlockFragment {

    @Inject
    Bus bus;

    @Override
    public void onCreate(final Bundle state) {
        super.onCreate(state);
        ((CouchsurfingApplication) getActivity().getApplication()).inject(this);
        setRetainInstance(true);
        bus.register(this);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        bus.unregister(this);
        bus = null;
    }
}

I tried both using bus.register(this) in onAttach() or onCreate(), that didn't change the issue.

Solution

I am using one "Retained Fragment" per activity to save the state of an HTTP session request. My issue was that I didn't instantiate my "Retained Fragment" the proper way.

Before I had in onCreate():

if (savedInstanceState == null) {
    sessionProviderFragment = new SessionProviderFragment();
    getSupportFragmentManager().beginTransaction().add(sessionProviderFragment,
        SessionProviderFragment.TAG).commit();
}

Apparently the code above could create several SessionProviderFragment when quitting the activity is reopening it later. It seams that the correct way is :

sessionProviderFragment = (SessionProviderFragment) getSupportFragmentManager()
    .findFragmentByTag(SessionProviderFragment.TAG);

// If not retained (or first time running), we need to create it.
if (sessionProviderFragment == null) {
    sessionProviderFragment = new SessionProviderFragment();
    getSupportFragmentManager().beginTransaction().add(sessionProviderFragment,
            SessionProviderFragment.TAG).commit();
}
if (savedInstanceState == null) {
    initUiFragment();
}

I also moved the bus register/unregister in onResume/onPause in my BaseFragment to be sure that I will always have one SessionProviderFragment registered on the bus at a time.

Answered By - Niqo