Android Observing Multiple MutableLiveData

Question

I have an MVVM setup, with two fragments interacting via a ViewModel. My SourceFragment is updating two MutableLiveData LatLngs in the VM. My second is a MapFragment in which I am drawing a route with the two LatLng LiveData. Both fragments are loaded as the Activity starts up. The SourceFragment updates the LatLngs. I need to wait for the map to load before requesting the route being plotted and I have a third MutableLiveData Boolean that I update once the map is loaded. Currently the way I achieve this is by setting up three separate viewModel Observers for the three MutableLiveData in my MapFragment with a set of if statements to check when all three MutableLiveData are set. I have a feeling that this could be done way better with something like MediatorLiveData but I have not managed to find an example I can work off. Is MediatorLiveData or something else applicable here?

My Source Fragment

viewModel.setPickupLocation(pickupLatLng);
viewModel.setDestinationLatLng(destinationLatLng());

My Map Fragment

@Override
public void onActivityCreated(@NonNull Bundle savedInstanceState) {
    Log.d(TAG, "onActivityCreated: called");
    super.onActivityCreated(savedInstanceState);

    getCustomerOrDriverViewModel();

}

private void initViewModels() { 

    viewModel.getPickupLocation().observe(getViewLifecycleOwner(), new Observer<LatLng>() {
        @Override
        public void onChanged(@NonNull LatLng pickupLocation) {
            Log.d(TAG, "onChanged: from onActivityCreated: called for getPickupLocation");

            if(viewModel.getMapReady().getValue() != null) {
                if(viewModel.getMapReady().getValue()) {
                    resetRoute();

                    setSingleMarker(pickupLocation);
                    if(viewModel.getDestinationLatLng().getValue() != null) {
                        plotRoute(viewModel.getDestinationLatLng().getValue(), pickupLocation);
                    }
                }
            }
        }
    });

    viewModel.getDestinationLatLng().observe(getViewLifecycleOwner(), new Observer<LatLng>() {
        @Override
        public void onChanged(@NonNull LatLng destinationLatLng) {
            Log.d(TAG, "onChanged: from onActivityCreated: called for getPickupLocation");
            if(viewModel.getMapReady().getValue() != null) {
                if(viewModel.getMapReady().getValue()) {
                    resetRoute();

                    if(viewModel.getPickupLocation().getValue() != null) {
                        plotRoute(destinationLatLng, viewModel.getPickupLocation().getValue());
                    }
                }
            }
        }
    });

    viewModel.getMapReady().observe(getViewLifecycleOwner(), new Observer<Boolean>() {
        @Override
        public void onChanged(@NonNull Boolean ready) {
            Log.d(TAG, "onChanged: from onActivityCreated: called for getMapReady");
            if(ready) {
                if(viewModel.getPickupLocation().getValue() != null && viewModel.getDestinationLatLng().getValue() != null) {
                    Log.d(TAG, "onChanged: from onActivityCreated: called for getMapReady: plotting route");
                    resetRoute();
                    plotRoute(viewModel.getDestinationLatLng().getValue(), viewModel.getPickupLocation().getValue());
                }
            }
        }
    });
}

mMap.setOnMapLoadedCallback(new GoogleMap.OnMapLoadedCallback() {
    @Override
    public void onMapLoaded() {
        viewModel.setMapReady(true);
    }
});

My ViewModel

private MutableLiveData<LatLng> pickupLocation = new MutableLiveData<>();
private MutableLiveData<LatLng> destinationLatLng = new MutableLiveData<>();
private MutableLiveData<Boolean> mapReady = new MutableLiveData<>();

public void setPickupLocation(LatLng input) {
    Log.d(TAG, "setPickupLocation: ");
    pickupLocation.setValue(input);
}

public void setDestinationLatLng(LatLng input) {
    Log.d(TAG, "setPickupLocation: ");
    destinationLatLng.setValue(input);
}

public void setMapReady(Boolean input) {
    Log.d(TAG, "setPickupLocation: ");
    mapReady.setValue(input);
}

public MutableLiveData<LatLng> getPickupLocation() {
    Log.d(TAG, "getPickupLocation: ");
    return pickupLocation;
}

public MutableLiveData<LatLng> getDestinationLatLng() {
    Log.d(TAG, "getDestinationLatLng: ");
    return destinationLatLng;
}

public MutableLiveData<Boolean> getMapReady() {
    Log.d(TAG, "getMapReady: ");
    return mapReady;
}

score 0 · Answer 1 · answered Feb 27 '20 at 00:25

Have 3 variables in MyMapFragment keeping track of each view model request. Make all 3 variables null at fragment creation. When each viewModel request completes, change the corresponding variable.

Then make a helper method

private void setupData(){
    if(variable1 == null || variable2 == null || variable3 == null){
        return;
    }
    plotRoute(xxxxxx);
}

Then call setupData() in each of your viewmodel responses in place of the triple nested if statement checks

Android Observing Multiple MutableLiveData

1 Answers1