LiveDataPairs.kt

fun <A : Any, B : Any> LiveData<A>.pairLatest(b: LiveData<B>): LiveData<Pair<A, B>> =
        object : MediatorLiveData<Pair<A, B>>() {
            private var aReceived = false
            private var lastA: A? = null
            private var bReceived = false
            private var lastB: B? = null

            init {
                addSource(this@pairLatest) {
                    aReceived = true
                    lastA = it
                    if (aReceived && bReceived) value = Pair(lastA!!, lastB!!)
                }

                addSource(b) {
                    bReceived = true
                    lastB = it
                    if (aReceived && bReceived) value = Pair(lastA!!, lastB!!)
                }
            }
        }


fun <A, B> LiveData<A>.pairLatestNullable(b: LiveData<B>): LiveData<Pair<A?, B?>> =
        object : MediatorLiveData<Pair<A?, B?>>() {
            private var aReceived = false
            private var lastA: A? = null
            private var bReceived = false
            private var lastB: B? = null

            init {
                addSource(this@pairLatestNullable) {
                    aReceived = true
                    lastA = it
                    if (aReceived && bReceived) value = Pair(lastA, lastB)
                }

                addSource(b) {
                    bReceived = true
                    lastB = it
                    if (aReceived && bReceived) value = Pair(lastA, lastB)
                }
            }
        }