IssueTracker

Component used: Transformations.switchMap() function in Architecture Components
Version used: android.arch.lifecycle:extensions:1.1.1
Devices/Android versions reproduced on: emulators, API 8.1

LiveData created by Transformations.switchMap() retain "old" data when provided function returns null. Per the documentation, when the function returns null, there is no "backing" live data. If there is no backing live data, one would assume that there is no "data" in the live data created by switchMap.

For example:
    class Sample1 : ViewModel() {
        val personIdLiveData = MutableLiveData<String?>()
        val personLiveData =
                Transformations.switchMap(personIdLiveData) {
                    it?.let { db.getPersonDao().getById(it) }
                }
    }


If the personIdLiveData is set to null (no selection, perhaps), the previous value in personLiveData remains.

I recommend changing switchMap to something like the following (and perhaps an overload that allows a non-null default value to be passed if the caller desires)

// COPIED AND TWEAKED FROM Transformations.switchMap() IN android.arch.lifecycle:extensions:1.1.1
// NEW LINES ARE PRECEDED BY /*NEW*/
@MainThread
fun <X, Y> switchMap2(trigger: LiveData<X?>,
                      func: (X?) -> LiveData<Y?>?): LiveData<Y?> {
    val result = MediatorLiveData<Y?>()
    result.addSource(trigger, object : Observer<X?> {
        private var mSource: LiveData<Y?>? = null

        override fun onChanged(x: X?) {
            val newLiveData = func(x)
            if (mSource === newLiveData) {
                return
            }
            if (mSource != null) {
                result.removeSource(mSource!!)
            }
            mSource = newLiveData
            if (mSource != null) {
                result.addSource(mSource!!) { y -> result.value = y }
/*NEW*/     } else {
/*NEW*/         result.value = null
            }
        }
    })
    return result
}

The attached example project demonstrates the behavior before and after the fix.