Is Kotlin Flow's Collect is only internal kotlinx.coroutines API?

Issue

Taking the direct example from https://kotlinlang.org/docs/reference/coroutines/flow.html#flows-are-cold

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) } 
}

I got the error on collect.

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

When I add @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) }
}

I get an error in the collect's lambda (function of value -> println(value) as below

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

I am using Kotlin version 1.4.21.

    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'

Did I do anything wrong that I cannot compile the example code in Android Studio?

Solution

The answer is, NO, collect is not only internal kotlinx.coroutines API. The error message is misleading.

As per @ir42's comment, add import kotlinx.coroutines.flow.collect solve the problem.

Additional info, why I didn't pick collectLatest as the answer

collect and collectLatest is different.

Using this example

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) } 
}

Collect will produce

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

as per https://kotlinlang.org/docs/reference/coroutines/flow.html

But 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) } 
}

will produce

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

Answered By - Elye