LiveData+combineLatestWith

fun <A, B> LiveData<A>.combineLatestWith(other: LiveData<B>): LiveData<Pair<A, B>> {
  val mediator = MediatorLiveData<Pair<A, B>>()
  mediator.addSource(this) {
    it?.let { thisValue ->
      other.value?.let { otherValue ->
        mediator.value = Pair(thisValue, otherValue)
      }
    }
  }
  mediator.addSource(other) {
    it?.let { otherValue ->
      this.value?.let { thisValue ->
        mediator.value = Pair(thisValue, otherValue)
      }
    }
  }
  return mediator
}