Kotlin 中流发出的并行处理值

Question

Kotlin code

runBlocking {
    flow {
        for (i in 0..4) {
            println("Emit $i")
            emit(i)
        }}  .onEach { if (it%2 == 0) delay(200) // Block 1
                println("A: got $it")
            }
            .onEach { println("B: got $it") } // Block 2
            .collect()
}

print in console:

Emit 0
A: got 0
B: got 0
Emit 1
A: got 1
B: got 1
Emit 2
...

How can I run parallel process both block1 & block2 to get messages from block 2 before block 1 in half cases?

Answer 1

You could try to launch separate coroutines in those blocks:

private val scope = CoroutineScope(Dispatchers.IO + SupervisorJob())

flow {
    for (i in 0..4) {
        println("Emit $i")
        emit(i)
    }
}.onEach { // Block 1
    scope.launch {
        if (it % 2 == 0) delay(200) 
        println("A: got $it")
    }

}.onEach { // Block 2
    scope.launch { 
        println("B: got $it") 
    }
}.launchIn(scope)

In this case Block 2 will be printed before Block 1 . SupervisorJob needs here to prevent cancelling launched coroutines when one of them fails.

This solution doesn't guarantee the order, eg there can be logs in the next order:

Emit 0
Emit 1
Emit 2
Emit 3
Emit 4

B: got 0
A: got 1
B: got 1
B: got 2
A: got 3
B: got 3
B: got 4
A: got 0
A: got 2
A: got 4