Android MediatorLiveData Observer


Answer :

This answer is largely reproduction of what @CommonsWare has already shared in the comment section above.

In order for the callback on MediatorLiveData's addSource method to be triggered, the MediatorLiveData object needs to be observed itself as well.

The logic behind this is that the 'mediator' mediates between a LiveData object that it observes, and a final consumer of the data. The mediator is hence an observer and observable simultaneously, and the callback on addSource won't be triggered for the mediator when there are no active observers.

As an example; according to Google's Android Architecture Components, an activity or fragment could have an observer observing a mediator on the ViewModel, which in turn may observe other LiveData objects that are handled within the ViewModel or a referenced to an utility class.

@CommonsWare pointed out the use of the Transformation class that exposes methods map and switchMap, but these were not within scope of my use case although they are worth checking out.


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