Kotlin Flow 的 Collect 是否只是内部 kotlinx.coroutines API？

问题描述

以https://kotlinlang.org/docs/reference/coroutines/flow.html#flows-are-cold中的直接例子

fun simple(): Flow<Int> = flow { 
    println("Flow started")
    for (i in 1..3) {
        delay(100)
        emit(i)
    }
}

fun main() = runBlocking<Unit> {
    println("Calling simple function...")
    val flow = simple()
    println("Calling collect...")
    flow.collect { value -> println(value) } 
    println("Calling collect again...")
    flow.collect { value -> println(value) } 
}

我在 collect 上遇到错误。

This is an internal kotlinx.coroutines API that should not be used from outside of kotlinx.coroutines. No compatibility guarantees are provided.It is recommended to report your use-case of internal API to kotlinx.coroutines issue tracker,so stable API Could be provided instead

当我添加 @InternalCoroutinesApi

@InternalCoroutinesApi
fun main() = runBlocking<Unit> {
    println("Calling simple function...")
    val flow = simple()
    println("Calling collect...")
    flow.collect { value -> println(value) }
    println("Calling collect again...")
    flow.collect { value -> println(value) }
}

我在 collect 的 lambda（value -> println(value 的函数）中出现错误，如下所示

Type mismatch.
required:
FlowCollector<Int>
Found:
([ERROR :  ]) → Unit
Cannot infer a type for this parameter. Please specify it explicitly.

我使用的是 Kotlin 1.4.21 版。

    implementation "org.jetbrains.kotlin:kotlin-stdlib:1.4.2"
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.4.2'
    implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.4.1'
    testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-test:1.4.2'

我是不是做错了什么，无法在 Android Studio 中编译示例代码？

解决方法

答案是，不，collect 不仅仅是内部 kotlinx.coroutines API。错误消息具有误导性。

根据@ir42 的评论，add import kotlinx.coroutines.flow.collect 解决了问题。

附加信息，为什么我没有选择collectLatest作为答案

collect 和 collectLatest 是不同的。

使用这个例子

fun simple(): Flow<Int> = flow { // flow builder
    for (i in 1..3) {
        delay(100) // pretend we are doing something useful here
        emit(i) // emit next value
    }
}

fun main() = runBlocking<Unit> {
    // Launch a concurrent coroutine to check if the main thread is blocked
    launch {
        for (k in 1..3) {
            println("I'm not blocked $k")
            delay(100)
        }
    }
    // Collect the flow
    simple().collect { value -> println(value) } 
}

收集会产生

I'm not blocked 1
1
I'm not blocked 2
2
I'm not blocked 3
3

根据https://kotlinlang.org/docs/reference/coroutines/flow.html

但是collectLatest

fun simple(): Flow<Int> = flow { // flow builder
    for (i in 1..3) {
        delay(100) // pretend we are doing something useful here
        emit(i) // emit next value
    }
}

fun main() = runBlocking<Unit> {
    // Launch a concurrent coroutine to check if the main thread is blocked
    launch {
        for (k in 1..3) {
            println("I'm not blocked $k")
            delay(100)
        }
    }
    // Collect the flow
    simple().collectLatest { value -> println(value) } 
}

会产生

I'm not blocked 1
I'm not blocked 2
1
I'm not blocked 3
2

,

将 collect 替换为 collectLatest。