LiveData merger that takes two live data inputs and a merger function. Merges two results using merger function, and returning result allowing null inputs and outputs. Input and out types are parametric. However only supports two live data inputs for now.

MergerLiveData.kt

class CombinedLiveData<T, K, S>(source1: LiveData<T>, source2: LiveData<K>, private val combine: (data1: T?, data2: K?) -> S) : MediatorLiveData<S>() {

    private var data1: T? = null
    private var data2: K? = null

    init {
        super.addSource(source1) {
            data1 = it
            value = combine(data1, data2)
        }
        super.addSource(source2) {
            data2 = it
            value = combine(data1, data2)
        }
    }

    override fun <T : Any?> addSource(source: LiveData<T>, onChanged: Observer<in T>) {
        throw UnsupportedOperationException()
    }

    override fun <T : Any?> removeSource(toRemote: LiveData<T>) {
        throw UnsupportedOperationException()
    }
}