MediatorLiveData or switchMap transformation with multiple parameters

Question

I am using Transformations.switchMap in my ViewModel so my LiveData collection, observed in my fragment, reacts on changes of code parameter.

Answer 1

Source : https://plus.google.com/+MichielPijnackerHordijk/posts/QGXF9gRomVi

To have multiple triggers for switchMap(), you need to use a custom MediatorLiveData to observe the combination of the LiveData objects -

class CustomLiveData extends MediatorLiveData> {
    public CustomLiveData(LiveData code, LiveData nbDays) {
        addSource(code, new Observer() {
            public void onChanged(@Nullable String first) {
                setValue(Pair.create(first, nbDays.getValue()));
            }
        });
        addSource(nbDays, new Observer() {
            public void onChanged(@Nullable Integer second) {
                setValue(Pair.create(code.getValue(), second));
            }
        });
    }
}

Then you can do this -

CustomLiveData trigger = new CustomLiveData(code, nbDays);
LiveData dayPrices = Transformations.switchMap(trigger, 
    value -> dbManager.getDayPriceData(value.first, value.second));

If you use Kotlin and want to work with generics:

class DoubleTrigger(a: LiveData, b: LiveData) : MediatorLiveData>() {
    init {
        addSource(a) { value = it to b.value }
        addSource(b) { value = a.value to it }
    }
}

Then:

val dayPrices = Transformations.switchMap(DoubleTrigger(code, nbDays)) {
    dbManager.getDayPriceData(it.first, it.second)
}