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.

CombinedLiveData.java

import androidx.annotation.NonNull;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.MediatorLiveData;
import androidx.lifecycle.Observer;

import java.util.function.BiFunction;

public class CombinedLiveData<T, K, S> extends MediatorLiveData<S> {

    private T data1;
    private K data2;

    public CombinedLiveData(LiveData<T> source1, LiveData<K> source2, BiFunction<T, K, S> combine) {
        super.addSource(source1, it -> {
            data1 = it;
            setValue(combine.apply(data1, data2));
        });

        super.addSource(source2, it -> {
            data2 = it;
            setValue(combine.apply(data1, data2));
        });
    }

    @Override
    public <S1> void addSource(@NonNull LiveData<S1> source, @NonNull Observer<? super S1> onChanged) {
        throw new UnsupportedOperationException();
    }

    @Override
    public <S1> void removeSource(@NonNull LiveData<S1> toRemote) {
        throw new UnsupportedOperationException();
    }
}

CombinedLiveData.kt

import androidx.lifecycle.LiveData
import androidx.lifecycle.MediatorLiveData
import androidx.lifecycle.Observer

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()
    }
}

The MediatorLiveData can be observed with lifecycle owner, so don't need the lifecycle owner for create it
yes, that's true. Sorry, my bad )