How to create a flow which detects consecutive value increments of another flow?

Question

I have a hot flow fooFlow that emits integer values. How can I construct another hot flow barFlow that only emits values when fooFlow emits a larger value than the most recent value emitted by fooFlow ? In a sense, barFlow detects consecutive value increases of fooFlow .

Example: If fooFlow emits (4, 2, 7, 3, 3, 1, 2, 4, ...), then barFlow emits (7, 2, 4, ...).

Answer 1

There might be a more natural or cleaner looking way to do this, but this is my first instinct:

val barFlow: SharedFlow<Int> = MutableSharedFlow<Int>().also { outflow ->
    var previousValue = Int.MAX_VALUE
    fooFlow.onEach { newValue ->
        if (newValue > previousValue) {
            outflow.emit(newValue)
        }
        previousValue = newValue
    }.launchIn(viewModelScope)
}

Answer 2

This is precisely what fold() can be used for:

val barFlow = fooFlow.fold(Int.MIN_VALUE) { maxValue, newValue ->
  max(maxValue, newValue)
}

It lets you set a initial value (here, Int.MIN_VALUE so that every value is greater than it), then gives you access to the previous value (the maxValue ) and the new value (the newValue ), allowing you to do whatever comparisons and use that output as the newly emitted value.