MediatorLiveData or switchMap transformation with multiple parameters

Question

I am using Transformations.switchMap in my ViewModel so my LiveData collection, observed in my fragment, reacts on changes of code parameter.

Answer 1

A simplification of jL4's answer, (and also in Kotlin in case it helps anybody)... no need to create a custom class for this:

class YourViewModel: ViewModel() {

    val firstLiveData: LiveData // or whatever type
    val secondLiveData: LiveData // or whatever

    // the Pair values are nullable as getting "liveData.value" can be null
    val combinedValues = MediatorLiveData>().apply {
        addSource(firstLiveData) { 
           value = Pair(it, secondLiveData.value)
        }
        addSource(secondLiveData) { 
           value = Pair(firstLiveData.value, it)
        }
    }

    val results = Transformations.switchMap(combinedValues) { pair ->
      val firstValue = pair.first
      val secondValue = pair.second
      if (firstValue != null && secondValue != null) {
         yourDataSource.yourLiveDataCall(firstValue, secondValue)
      } else null
    }

}

Explanation

Any update in firstLiveData or secondLiveData will update the value of combinedValues, and emit the two values as a pair (thanks to jL4 for this).

Calling liveData.value can be null, so this solution makes the values in Pair nullable to avoid Null Pointer Exception.

So for the actual results/datasource call, the switch map is on the combinedValues live data, and the 2 values are extracted from the Pair and null checks are performed, so you can be sure of passing non-null values to your data source.