HashMap Store and Restore onSaveInstanceState from Fragment to another Activity

Question

Need to restore markers which are stored in HashMap<String, Marker> markers; when fragment is opened back from another activity.

This is what i tried:

    HashMap<String, Marker> markers;
    //..

     @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    //..
    this.markers = new HashMap<String, Marker>();

    // Restoring the markers on configuration changes
            if (savedInstanceState != null) {
                if (savedInstanceState.containsKey("markers")) {
                    markers = (HashMap<String, Marker>) savedInstanceState.getSerializable("markers");
                    if (markers != null) {

                        for (String key : markers.keySet()) {
                            Location location =
                                    new Location(markers.get(key).getPosition().latitude, markers.get(key).getPosition().longitude);
                            addMarker(key, location);
                        }
                    }
                }
            }

    return rootView;
    }

public void addMarker(String key, Location location) {

        //if (!key.equals(strUserID)) {
        Marker marker = this.mGoogleMap.addMarker(new MarkerOptions()
                .position(new LatLng(location.latitude, location.longitude))
                .icon(BitmapDescriptorFactory.defaultMarker()));
}

    //...

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

I want to restore markers on fragment when fragment is resumed from the pause state from another activity. For Example: Activity A contains Fragment FA from which Activity B is called in foreground then on back-press again Fragment FA is opened:

     Activity A (Fragment FA)(Markers shown on map and should store hashmap `onSaveInstanceState(Bundle outState)` when while activity B is called) --->  Activity B  ---> (On Back-pressed restore markers from hashmap `if (savedInstanceState != null)`) Fragment FA.

Answer 1

Please note that if:

• A doesn't call finish when you switch to B
• A is configured to remain on back-stack
• FA is configured to remain on back-stack

Then when you switch to B and then press back, A and FA will be popped from back-stack, which means that you get them in exactly the same state they were prior to switching to B. In this case you don't need to do anything - your HashMap is still valid and holds the correct content.

However, if the system need memory it might decide to destroy "back-stacked" Activity and Fragment instances, in which case it will call onSaveInstanceState prior to destruction. If you now go back, then A and FA will be recreated, and will receive savedInstanceState parameter containing saved state. Just use it to restore whatever you want.

Practically, (assuming that Marker class implemented correctly) I think your code is fine, except one line in onCreateView:

this.markers = new HashMap<String, Marker>();

If FA is just popped from back-stack (and not re-created), this line causes you to loose the current state of HashMap. I suggest you put this line in onCreate, or simply initialize this map upon declaration:

private HashMap<String, Marker> markers = new HashMap<>();