Analog of RxJava's Observable.scan operator for Android Architecture Components LiveData

LiveDataScanOperator.kt

/**
 * Analog of RxJava's Observable.scan operator.
 *
 * Returns a LiveData that applies the [accumulator] function to the fist item in the source LiveData,
 * and an initial value, then feeds the result of that function along with the second item emitted by
 * the source LiveData into the same function, and so on until all items have been emitted by the source
 * LiveData, emitting the result of each of these iterations.
 */
fun <T, R> LiveData<T>.scan(initialValue: R, accumulator: (R, T) -> R): LiveData<R> {
    val returnLiveData = MediatorLiveData<R>()
    returnLiveData.value = initialValue

    returnLiveData.addSource(this) {
        //TODO: get rid of the !!
        val result = accumulator.invoke(returnLiveData.value!!, this.value!!)
        returnLiveData.value = result
    }

    return returnLiveData
}