Question

I have a Singleton Data class, which I use to store data. I’m accessing it in different Fragments.

When the first Fragment is loaded, it’s no problem that all fields in the Singleton are null. When the second Fragment is shown, it depends on these fields to show its data. The first Fragment ensures these fields are initialized.

However, when the user presses the home button in the second Fragment, and opens it again after like an hour or so, the Singleton has lost all its data, and the Fragment tries to access null fields.

I wanted to implement the onSaveInstanceState method, but I’m stumped on how this works – I have no instance of the data to assign it to.

@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putSerializable("DATA", Data.getInstance());
}

@Override
public void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);

    savedInstanceState.getSerializable("DATA"); //What to do with this?
}

Any help is welcome.

Answer 1

Are the fragments both hosted in the same Activity? If so, why not store the shared state in a member of the Activity, and use the Activity’s onSaveInstanceState() and onCreate() to save and restore it. In your fragments you can then do…

((MyActivityClass)getActivity()).getSharedState()

Otherwise, you could make your singleton object manage a member object that can be serialized and deserialized:

MySingleton.instance().saveTo(outState);
MySingleton.instance().restoreFrom(savedInstanceState);
MySingleton.instance().getState();

Where

public void restoreFrom(Bundle savedInstanceState) {
    mState = savedInstanceState.getSerializable("DATA");
}