将coroutine函数挂起，直到收到结果，然后继续

Question

我正在慢慢地学习kotlin-coroutines，我想从简单的情况开始，在这种情况下，我遍历列表，每次调用一个API调用，等待响应，处理它，然后每次迭代都重复这个过程。

viewModelScope.launch {
     list.forEach { item ->
          Log.d("TESTING", "forEach iteration before calling test(): ${item.id}")

          test()

          Log.d("TESTING", "forEach iteration after calling test(): ${item.id}")
     }
}

suspend fun test(item.id: String) = coroutineScope {
    Log.d("TESTING", "suspend test before receiving response: ${item.id}")

    val apiRepsonse = async {
        repository.doingApiCall() {
            Log.d("TESTING", "response: ${item.id}")
        }
    }.await()

    Log.d("TESTING", "suspend test after receiving response: ${item.id}")
}

所以，如果我们想象一下，list包含：{ 10, 15, 25, 35 } (id's)

我希望在logcat中收到以下信息：

Log.d("TESTING", "forEach iteration before calling test(): 10")
Log.d("TESTING", "suspend test before receiving response: 10")
Log.d("TESTING", "response: 10")
Log.d("TESTING", "suspend test after receiving response: 10")
Log.d("TESTING", "forEach iteration after calling test(): 10")
___________________
Log.d("TESTING", "forEach iteration before calling test(): 15")
Log.d("TESTING", "suspend test before receiving response: 15")
Log.d("TESTING", "response: 15")
Log.d("TESTING", "suspend test after receiving response: 15")
Log.d("TESTING", "forEach iteration after calling test(): 15")
...

但是，现在我得到的是这样的东西：

Log.d("TESTING", "forEach iteration before calling test(): 10")
Log.d("TESTING", "suspend test before receiving response: 10")
Log.d("TESTING", "suspend test after receiving response: 10")
Log.d("TESTING", "forEach iteration after calling test(): 10")
Log.d("TESTING", "forEach iteration before calling test(): 15")
Log.d("TESTING", "suspend test before receiving response: 15")
Log.d("TESTING", "suspend test after receiving response: 15")
Log.d("TESTING", "forEach iteration after calling test(): 15")

Log.d("TESTING", "response: 10")
Log.d("TESTING", "response: 15")

...

基本上，每个循环都会移动，而不会等待API异步响应。我以为.await()是为了这个场景而设计的，但我可能在这里遗漏了一些东西，因为它显然不像我想象的那样起作用。

Answer 1

变化

val apiRepsonse = async {
        repository.doingApiCall() {
            Log.d("TESTING", "response: ${item.id}")
        }
    }.await()

至

val apiRepsonse = async {
        Log.d("TESTING", "Start work in another 'thread'.")
        repository.doingApiCall() {
            Log.d("TESTING", "Finish work in another 'thread'.")
        }
    }.await()
Log.d("TESTING", "response: ${apiRepsonse.id}")

这样，您等待传递的价值就被实际使用了。

我想这能消除你对LogCat的困惑。

我试着自己解决你的问题，我已经把它写成了一个可以运行的测试，可以理解：

class TestFoo {

    @Test
    fun foo() {
        val job = GlobalScope.launch {
            listOf("10", "15").forEach { item ->
                println("TESTING forEach iteration before calling test(): ${item}")

                test(item)

                println("TESTING forEach iteration after calling test(): ${item}")
            }
        }
        while(job.isActive) {
            // wait
        }
    }

    private suspend fun test(item: String) = coroutineScope {
        println("TESTING suspend test before receiving response: ${item}")

        val apiRepsonse = async {
            delay(1000)
            println("TESTING response: ${item}")
            "Hello"
        }.await()

        println("TESTING suspend test after receiving response: ${item}")
    }

}

下面是"LogCat"：

​

​

您是否因为repository.doingApiCall()中正在发生的线程处理而获得不同的日志？

Answer 2

最有可能的是，repository.doingApiCall()不是一个适当的挂起函数，而是在不挂起协同线的情况下触发一些异步操作。看起来它需要一个lambda作为回调，这是另一个提示，即它不是一个挂起函数。

如果它是一个挂起函数，则不需要使用async来分派它。

顺便说一句，使用async并立即在其上调用await是使用withContext(Dispatchers.Default)的一种更复杂、更不优化的方法，它还可以默默地吞噬异常。

Answer 3

没有测试，这应该有效：

// same code here
viewModelScope.launch {
     list.forEach { item ->
          Log.d("TESTING", "forEach iteration before calling test(): ${item.id}")

          test()

          Log.d("TESTING", "forEach iteration after calling test(): ${item.id}")
     }
}

suspend fun test(item.id: String) = withContext(Dispatchers.IO) {
    Log.d("TESTING", "suspend test before receiving response: ${item.id}")

    repository.doingApiCall() {
        Log.d("TESTING", "response: ${item.id}")
    }

    Log.d("TESTING", "suspend test after receiving response: ${item.id}")
}

有两件事改变了：

不要启动新的async.await，而是使用withContext()来挂起withContext()中的调用者

• ，您不需要使用withContext()