[FIXED] Calling Fragment constructor caused an exception. Navigation Architecture Component

Issue

I am using navigation architecture component library and my app's starting point is this fragment:

class MainFragment : BaseFragment() {

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
        return inflater.inflate(R.layout.fragment_main, container, false)
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }
}

Which inherits from an abstract class which is BaseFragment:

abstract class BaseFragment : Fragment() {

}

When I run my app I get:

 Unable to instantiate fragment io.example.MainFragment: calling Fragment constructor caused an exception

But this does not happen if MainFragment extends Fragment instead of BaseFragment. What is the reason? Does this have something to do with how the Navigation Architecture Component works?

Solution

I had the similar issue because I had val in MyBaseFragment

protected abstract val gpsMsg: String

which I was overiding this way in other Fragment before fragment attached to context.

override val gpsMsg: String = getString(R.string.gps_not_enabled)

So the underlying error was because context was null and getString uses getResources() which returns requireContext().getResources(). And in the requireContext() source code error will be thrown.

public final Context requireContext() {
    Context context = getContext();
    if (context == null) {
        throw new IllegalStateException("Fragment " + this + " not attached to a context.");
    }
    return context;
}

So the error thrown causes fragment not to be instantiated. So I would advice being careful with context thing when override.

Answered By - Nux