A Flow emits a pair of the original Flow's latest value and its previous value.

flowWithPrevious.kt

/**
 * Given a Flow A, returns a [Flow] that emits a pair of the latest value emitted by A (as the
 * second value of the pair), and the previous value emitted before (as the first value of the
 * pair).
 * 
 * Example
 * ```
 * val originalFlow = flowOf(1, 2, 3, 4)
 * originalFlow.flowWithPrevious().collect { (l, r) ->
 *     println("Left: $l, Right: $r")
 * }
 * ```
 *
 * Output:
 * ```
 * Left: null, Right: 1
 * Left: 1, Right: 2
 * Left: 2, Right: 3
 * Left: 3, Right: 4
 * ```
 */
fun <T : Any?> Flow<T>.flowWithPrevious(): Flow<Pair<T?, T>> = flow {
  var previousValue: T? = null
  collect { value: T ->
    val newPair = previousValue to value
    previousValue = value
    emit(newPair)
  }
}