コルーチン階層におけるキャンセルの伝播

親子関係

import kotlinx.coroutines.*
fun main() = runBlocking {
    val parent = launch {
        launch { delay(1000); println("child 1") }
        launch { delay(1000); println("child 2") }
        delay(1000)
        println("parent")
    }
    delay(50)
    parent.cancel()
    parent.join()
    println("All cancelled")
}

子の失敗による親のキャンセル

import kotlinx.coroutines.*
fun main() = runBlocking {
    try {
        coroutineScope {
            launch {
                delay(50)
                throw RuntimeException("child failed")
            }
            launch {
                delay(1000)
                println("sibling — never prints")
            }
        }
    } catch (e: RuntimeException) {
        println("Caught: ${e.message}")
    }
}

キャンセルは上方向には伝播しない

import kotlinx.coroutines.*
fun main() = runBlocking {
    val parent = launch {
        val child = launch {
            delay(1000)
            println("child done")
        }
        delay(50)
        child.cancel()  // cancels child
        child.join()
        println("parent still running")  // parent is fine
    }
    parent.join()
}

coroutineScope と GlobalScope の違い

import kotlinx.coroutines.*
fun main() = runBlocking {
    try {
        coroutineScope {
            launch { delay(50); throw RuntimeException("error") }
        }
    } catch (e: Exception) {
        println("coroutineScope propagated: ${e.message}")
    }
    // GlobalScope.launch { } would NOT propagate to runBlocking
}

サブツリーのキャンセル

import kotlinx.coroutines.*
fun main() = runBlocking {
    val root = launch {
        launch {
            launch { delay(1000); println("deep") }
            delay(1000)
        }
    }
    delay(50)
    root.cancel()
    root.join()
    println("Whole subtree cancelled")
}

cancel() 後の join()

import kotlinx.coroutines.*
fun main() = runBlocking {
    val job = launch {
        try { delay(1000) }
        finally { println("cleanup ran") }
    }
    delay(50)
    job.cancel()
    job.join()  // waits for finally to complete
    println("Proceeded after join")
}

cancelAndJoin()

import kotlinx.coroutines.*
fun main() = runBlocking {
    val job = launch {
        try { delay(1000) }
        finally { println("cleanup") }
    }
    delay(50)
    job.cancelAndJoin()  // cancel + join in one call
    println("Done")
}

async を通じた伝播

import kotlinx.coroutines.*
fun main() = runBlocking {
    try {
        coroutineScope {
            val d = async { throw RuntimeException("async fail") }
            d.await()  // rethrows here
        }
    } catch (e: RuntimeException) {
        println("Caught from async: ${e.message}")
    }
}

構造化された並行性の保証

import kotlinx.coroutines.*
suspend fun doWork() = coroutineScope {
    launch { delay(100); println("work 1") }
    launch { delay(200); println("work 2") }
    // both children complete before doWork returns
}
fun main() = runBlocking {
    doWork()
    println("All work done")
}

キャンセルの伝播図

import kotlinx.coroutines.*
fun main() = runBlocking {
    // Parent
    launch {
        val c1 = launch { delay(1000); println("c1") }  // child 1
        val c2 = launch { delay(1000); println("c2") }  // child 2
        delay(50)
        c1.cancel()  // only c1 cancelled
        c1.join()
        println("c2 still active: ${c2.isActive}")
        c2.cancelAndJoin()
    }.join()
}

Android における CoroutineScope のライフサイクル

import kotlinx.coroutines.*
// Pseudocode:
// class MyViewModel : ViewModel() {
//     fun load() = viewModelScope.launch {
//         val data = repo.fetch() // cancelled if VM cleared
//         _state.value = data
//     }
// }
fun main() = runBlocking { println("viewModelScope cancels on ViewModel.onCleared()") }

確認問題

まとめ