merge two live data sources into a single mediator live data, returning data when either of them changed (not chained)

mergeLiveData.kt

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

fun <T1, T2> mergeLiveData(source1: LiveData<T1>, source2: LiveData<T2>): LiveData<Pair<T1, T2>> {
    val result = MediatorLiveData<Pair<T1, T2>>()

    var data1: T1? = null
    var data2: T2? = null

    fun postDataIfBothAvailable() {
        val first = data1 ?: return
        val second = data2 ?: return
        result.postValue(Pair(first, second))
    }

    result.addSource(source1) { data1 = it ; postDataIfBothAvailable() }
    result.addSource(source2) { data2 = it ; postDataIfBothAvailable() }

    return result
}

observe 2 live data sources independently (not chained) and post when either changes

val liveData = MediatorLiveData<...>()
liveData.addSource(source1) { data1 ->
    liveData.addSource(source2) {
        // do something
    }
}

fails when the source1 is changed, since it re-adds source2

based on https://stackoverflow.com/a/52306675/1016377