获取流集合中的当前值和上一个值

Question

I need to handle current and previous value in flow collect, so I need some operator that acts like that:

----A----------B-------C-----|--->

---(null+A)---(A+B)---(B+C)--|--->

One idea is something like:

fun <T: Any> Flow<T>.withPrevious(): Flow<Pair<T?, T>> = flow {
    var prev: T? = null
    this@withPrevious.collect {
        emit(prev to it)
        prev = it
    }
}

But this way there is no control over a context in which first flow will be executed. Is there more flexible solution?

Answer 1

Flow s are sequential, so you can use a variable to store the previous value:

coroutineScope.launch {
    var prevValue = null
    flow.collect { newValue ->
        // use prevValue and newValue here
        ...
        // update prevValue
        prevValue = newValue
    }
}