如何取消/取消协程流程 [英] How to cancel/unsubscribe from coroutines Flow

问题描述

当我尝试过早地从Flow中取消时，我注意到一个奇怪的行为.看下面的例子.

I notice a strange behavior when trying to prematurely cancel from a Flow. Take a look at the following example.

这是一个发出整数值的简单流程

This is a simple flow that emits integer values

  private fun createFlow() = flow {
        repeat(10000) {
            emit(it)
        }
    }

然后我使用此代码调用createFlow函数

Then I call the createFlow function using this code

  CoroutineScope(Dispatchers.Main).launch {
            createFlow().collect {

                Log.i("Main", "$it isActive $isActive")
                if (it == 2) {
                    cancel()
                }
            }
        }

这是打印出来的

0 isActive true
1 isActive true
2 isActive true
3 isActive false
4 isActive false
etc...etc

现在，我希望流一旦达到2的值就应该停止发出整数，但是实际上它将isActive标志切换为false并保持发出而不会停止.

Now I would expect that the flow should stop emitting integers once it reaches the value of 2 but instead it actually switches the isActive flag to false and keeps emitting without otherwise stopping.

当我在两次发射之间添加延迟时，流量的行为将达到我的预期.

When I add a delay between emissions the flow behaves as I would expect.

private fun createFlow() = flow {
    repeat(10000) {
        delay(500) //add a delay
        emit(it)
    }
}

这是在再次调用流程(预期的行为)之后打印出来的内容.

This is what is printed out after calling the flow again (which is the expected behaviour).

0 isActive true
1 isActive true
2 isActive true

如何在不增加延迟的情况下精确地取消流量排放?

What can I do to cancel the flow emission exactly at the specified value without adding delay?

推荐答案

我在我在项目中用safeCollect函数替换了每个collect:

I have replaced every single collect with a safeCollect function in my project:

/**
 * Only proceed with the given action if the coroutine has not been cancelled.
 * Necessary because Flow.collect receives items even after coroutine was cancelled
 * https://github.com/Kotlin/kotlinx.coroutines/issues/1265
 */
suspend inline fun <T> Flow<T>.safeCollect(crossinline action: suspend (T) -> Unit) {
  collect {
    coroutineContext.ensureActive()
    action(it)
  }
}

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