如何在 Android 上使用协程取消有中断和无中断，包括根据生命周期自动取消？

Question

背景

我在从 Android

上的简单（已弃用）AsyncTask 和 Executors 迁移到 Kotlin Coroutines 时遇到问题

问题

我找不到如何使用 Kotlin Coroutines.

执行我在 AsyncTask 甚至 Executors 上可以完成的基本操作

过去，我可以选择在线程中断和不中断的情况下取消任务。现在出于某种原因，鉴于我在协程上创建的任务，它只是没有中断，这意味着如果我运行一些代码甚至在其中“睡眠”（并不总是由我），它不会被打断。

我还记得有人告诉我协程在 Android 上非常好，因为如果您在 Activity 中，它会自动取消所有任务。不过，我找不到如何操作的解释。

我已经尝试并发现了什么

对于 Coroutines 任务（据我所知称为 Deferred）我想我已经读过，当我创建它时，我必须选择它支持的取消，并且出于某种原因我不能同时拥有它们。不确定这是不是真的，但我仍然想知道，因为我想同时拥有两者以获得最佳迁移。使用 AsyncTask，我过去常常将它们添加到一个集合中（并在取消时删除），以便在 Activity 完成后，我可以遍历所有并取消它们。我什至做了一个很好的 class 来为我做这件事。

这是我创建来测试的：

class MainActivity : AppCompatActivity() {
    val uiScope = CoroutineScope(Dispatchers.Main)
    val bgDispatcher: CoroutineDispatcher = Dispatchers.IO

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        loadData()
    }

    private fun loadData(): Job = uiScope.launch {
        Log.d("AppLog", "loadData")
        val task = async(bgDispatcher) {
            Log.d("AppLog", "bg start")
            try {
                Thread.sleep(5000L) //this could be any executing of code, including things not editable
            } catch (e: Exception) {
                Log.d("AppLog", "$e")
            }
            Log.d("AppLog", "bg done this.isActive?${this.isActive}")
            return@async 123
        }
        //simulation of cancellation for any reason, sadly without the ability to cancel with interruption
        Handler(mainLooper).postDelayed({
            task.cancel()
        }, 2000L)
        val result: Int = task.await()
        Log.d("AppLog", "got result:$result") // this is called even if you change orientation, which I might not want when done in Activity
    }
}

构建gradle文件：

implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:1.4.1"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:1.4.1"

问题

协程是否可以有一个任务，我可以在有和没有线程中断的情况下取消？
应该添加什么来使它工作，以便当 Activity 结束时（例如方向改变），它会自动取消（选择有或没有中断）？我想我可以使用与 AsyncTask 类似的解决方案，但我记得有人告诉我，对于协程也有一种很好的方法。

Answer 1

默认情况下，协程不执行线程中断 - 根据 Making computation code cancellable documentation, using yield() 或检查 isActive 允许协程感知代码参与取消。

但是，当您想要线程中断与阻塞代码交互时，这正是 runInterruptible() 的用例，这将导致包含的代码当协程范围被取消时线程被中断。

这与 lifecycle-aware coroutine scopes 完美配合，后者在 Lifecycle 被销毁时自动取消：

class MainActivity : AppCompatActivity() {
    val bgDispatcher: CoroutineDispatcher = Dispatchers.IO

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        loadData()
    }

    private fun loadData(): Job = lifecycleScope.launch {
        Log.d("AppLog", "loadData")
        val result = runInterruptible(bgDispatcher) {
            Log.d("AppLog", "bg start")
            try {
                Thread.sleep(5000L) //this could be any executing of code, including things not editable
            } catch (e: Exception) {
                Log.d("AppLog", "$e")
            }
            Log.d("AppLog", "bg done this.isActive?${this.isActive}")
            return@runInterruptible 123
        }
        Log.d("AppLog", "got result:$result")
    }
}

Answer 2

协同程序并不神奇。它们是使用状态机实现的，并且可能有许多挂起点。这个在原文里都有说明Coroutines KEEP.

取消发生在那些暂停点。协同程序不能在挂起点以外的任何其他点取消（至少在正常执行中）。如果您使用 Thread.sleep，则没有暂停点。您应该使用 delay 而不是 sleep，这会引入一个挂点。如果你在做一个长操作，你可以添加几个yield()来添加挂起点并使你的协程可取消。

来自 the docs:

Coroutine cancellation is cooperative. A coroutine code has to cooperate to be cancellable. All the suspending functions in kotlinx.coroutines are cancellable. They check for cancellation of coroutine and throw CancellationException when cancelled. However, if a coroutine is working in a computation and does not check for cancellation, then it cannot be cancelled

调用 suspend 函数会自动引入挂起点。

正如@CommonsWare 所指出的，很少有理由创建协程范围。在活动和片段中，或任何与生命周期相关的组件中，您应该使用 lifecycleScope。在 ViewModels 中，有 viewModelScope.

编辑：

我已经尝试将 runInterruptible 的源调整为 在特定条件下不 中断：传递自定义异常的实例 class InterruptionException 作为取消原因将跳过线程中断。我已经用 AtomicInteger 替换了 atomicfu 结构，我假设你的目标只是 JVM。您需要通过添加 -Xopt-in=kotlinx.coroutines.InternalCoroutinesApi 编译器标志来选择加入内部协程 API。

suspend fun <T> runInterruptibleCancellable(
    context: CoroutineContext = EmptyCoroutineContext,
    block: () -> T
): T = withContext(context) {
    try {
        val threadState = ThreadState(coroutineContext.job)
        threadState.setup()
        try {
            block()
        } finally {
            threadState.clearInterrupt()
        }
    } catch (e: InterruptedException) {
        throw CancellationException("Blocking call was interrupted due to parent cancellation").initCause(e)
    }
}

private const val WORKING = 0
private const val FINISHED = 1
private const val INTERRUPTING = 2
private const val INTERRUPTED = 3

private class ThreadState(private val job: Job) : CompletionHandler {
    /*
       === States ===

       WORKING: running normally
       FINISH: complete normally
       INTERRUPTING: canceled, going to interrupt this thread
       INTERRUPTED: this thread is interrupted

       === Possible Transitions ===

       +----------------+         register job       +-------------------------+
       |    WORKING     |   cancellation listener    |         WORKING         |
       | (thread, null) | -------------------------> | (thread, cancel handle) |
       +----------------+                            +-------------------------+
               |                                                |   |
               | cancel                                  cancel |   | complete
               |                                                |   |
               V                                                |   |
       +---------------+                                        |   |
       | INTERRUPTING  | <--------------------------------------+   |
       +---------------+                                            |
               |                                                    |
               | interrupt                                          |
               |                                                    |
               V                                                    V
       +---------------+                              +-------------------------+
       |  INTERRUPTED  |                              |         FINISHED        |
       +---------------+                              +-------------------------+
    */
    private val _state = AtomicInteger(WORKING)
    private val targetThread = Thread.currentThread()

    // Registered cancellation handler
    private var cancelHandle: DisposableHandle? = null

    fun setup() {
        cancelHandle = job.invokeOnCompletion(onCancelling = true, invokeImmediately = true, handler = this)
        // Either we successfully stored it or it was immediately cancelled
        while (true) {
            when (val state = _state.get()) {
                // Happy-path, move forward
                WORKING -> if (_state.compareAndSet(state, WORKING)) return
                // Immediately cancelled, just continue
                INTERRUPTING, INTERRUPTED -> return
                else -> invalidState(state)
            }
        }
    }

    fun clearInterrupt() {
        /*
         * Do not allow to untriggered interrupt to leak
         */
        while (true) {
            when (val state = _state.get()) {
                WORKING -> if (_state.compareAndSet(state, FINISHED)) {
                    cancelHandle?.dispose()
                    return
                }
                INTERRUPTING -> {
                    /*
                     * Spin, cancellation mechanism is interrupting our thread right now
                     * and we have to wait it and then clear interrupt status
                     */
                }
                INTERRUPTED -> {
                    // Clear it and bail out
                    Thread.interrupted()
                    return
                }
                else -> invalidState(state)
            }
        }
    }

    // Cancellation handler
    override fun invoke(cause: Throwable?) {
        if (cause is InterruptionException) {
            while (true) {
                when (val state = _state.get()) {
                    // Working -> try to transite state and interrupt the thread
                    WORKING -> {
                        if (_state.compareAndSet(state, INTERRUPTING)) {
                            targetThread.interrupt()
                            _state.set(INTERRUPTED)
                            return
                        }
                    }
                    // Finished -- runInterruptible is already complete, INTERRUPTING - ignore
                    FINISHED, INTERRUPTING, INTERRUPTED -> return
                    else -> invalidState(state)
                }
            }
        }
    }

    private fun invalidState(state: Int): Nothing = error("Illegal state $state")
}

class InterruptionException(cause: Throwable?) : CancellationException() {
    init {
        initCause(cause)
    }
}

fun Job.interrupt(cause: Throwable? = null) {
    this.cancel(InterruptionException(cause))
}

suspend fun Job.interruptAndJoin() {
    interrupt()
    return join()
}

可以使用interrupt和interruptAndJoin扩展函数来触发线程中断，否则使用cancel进行非中断取消。一个例子：

val scope = CoroutineScope(Dispatchers.IO)
val job = scope.launch {
    runInterruptibleCancellable {
        // some blocking code
        Thread.sleep(1000)
        if (!isActive) {
            println("cancelled")
        } else {
            println("completed")
        }
    }
}
job.invokeOnCompletion {
    if (it is InterruptionException) {
        print("interrupted")
    }
}

runBlocking {
//  job.interruptAndJoin()  // prints "interrupted"
//  job.cancelAndJoin()     // prints "cancelled"
    job.join()              // prints "completed"
}

这个例子是我所做的唯一测试。似乎有效。我不知道它是否泄漏，我不知道它是否是线程安全的。老实说，我真的远远超出了我的专业知识。请不要在没有进一步确认它有效的情况下在生产中使用它。

如何在 Android 上使用协程取消有中断和无中断，包括根据生命周期自动取消？

How to cancel with and without interruption using Coroutines on Android, including auto-cancellation according to lifecycle?

android

android-asynctask

android-lifecycle

kotlin-coroutines

activity-lifecycle

背景

问题

我已经尝试并发现了什么

问题