Kotlin CoroutineScope 无法在 android 视图中取消

Question

例如，这个视图。当onDetachedFromWindow调用范围被取消，但启动的作业仍然处于活动状态。

class TestView : FrameLayout,CoroutineScope {
    val TAG = "TestView"
    override val coroutineContext: CoroutineContext
        get() = Dispatchers.Main + Job()

    constructor(context: Context) : super(context)
    constructor(context: Context, attrs: AttributeSet?) : super(context, attrs) {
        launch {
            while (true) {
                Log.i(TAG,"is in launch coroutine....${coroutineContext} ${this@TestView.coroutineContext}")
                delay(1000)
            }
        }
    }
    constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(
        context,
        attrs,
        defStyleAttr
    )

    override fun onDetachedFromWindow() {
        super.onDetachedFromWindow()
        cancel()
        Log.i(TAG,"onDetachedFromWindow")
    }
}

日志是

2019-09-19 21:32:26.652 22912-22912/com.ymr.myapplication I/TestView: is in launch coroutine....[StandaloneCoroutine{Active}@7f9d20f, Main] [JobImpl{Active}@2f3fde2, Main]
2019-09-19 21:32:27.655 22912-22912/com.ymr.myapplication I/TestView: is in launch coroutine....[StandaloneCoroutine{Active}@7f9d20f, Main] [JobImpl{Active}@80a2573, Main]
2019-09-19 21:32:28.656 22912-22912/com.ymr.myapplication I/TestView: is in launch coroutine....[StandaloneCoroutine{Active}@7f9d20f, Main] [JobImpl{Active}@dad2c30, Main]
2019-09-19 21:32:29.649 22912-22912/com.ymr.myapplication I/TestView: onDetachedFromWindow
2019-09-19 21:32:29.665 22912-22912/com.ymr.myapplication I/TestView: is in launch coroutine....[StandaloneCoroutine{Active}@7f9d20f, Main] [JobImpl{Active}@dab39f4, Main]
2019-09-19 21:32:30.666 22912-22912/com.ymr.myapplication I/TestView: is in launch coroutine....[StandaloneCoroutine{Active}@7f9d20f, Main] [JobImpl{Active}@448351d, Main]
2019-09-19 21:32:31.668 22912-22912/com.ymr.myapplication I/TestView: is in launch coroutine....[StandaloneCoroutine{Active}@7f9d20f, Main] [JobImpl{Active}@45ba392, Main]
2019-09-19 21:32:32.669 22912-22912/com.ymr.myapplication I/TestView: is in launch coroutine....[StandaloneCoroutine{Active}@7f9d20f, Main] [JobImpl{Active}@bc75163, Main]

那么为什么不能取消范围启动的作业呢？

Answer 1

你可以这样试试，

class TestView : FrameLayout, CoroutineScope {
val TAG = "TestView"
private lateinit var job: Job

override val coroutineContext: CoroutineContext
    get() = Dispatchers.Main + job

constructor(context: Context) : super(context)

constructor(context: Context, attrs: AttributeSet?) : super(context, attrs) {
    job = Job()

    launch {
        while (true) {
            Log.i(TAG,"is in launch coroutine....${coroutineContext} ${this@TestView.coroutineContext}")
            delay(1000)
        }
    }
}
constructor(context: Context, attrs: AttributeSet?, defStyleAttr: Int) : super(
    context,
    attrs,
    defStyleAttr
)

override fun onDetachedFromWindow() {
    super.onDetachedFromWindow()
    job.cancel()
    Log.i(TAG,"onDetachedFromWindow")
}
}

Answer 2

问题是您每次都在coroutineContext 中创建一个新的Job。您可以使用

轻松解决此问题

override val coroutineContext = Dispatchers.Main + Job()

但是，请记住，如果您的视图在分离后附加到窗口，coroutineContext 将已被取消。

查看以下示例以更好地了解其工作原理：

class MyActivity : AppCompatActivity(), CoroutineScope {
    lateinit var job: Job
    override val coroutineContext: CoroutineContext
        get() = Dispatchers.Main + job

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        job = Job()
    }

    override fun onDestroy() {
        super.onDestroy()
        job.cancel() // Cancel job on activity destroy. After destroy all children jobs will be cancelled automatically
    }

    /*
     * Note how coroutine builders are scoped: if activity is destroyed or any of the launched coroutines
     * in this method throws an exception, then all nested coroutines are cancelled.
     */
    fun loadDataFromUI() = launch { // <- extension on current activity, launched in the main thread
       val ioData = async(Dispatchers.IO) { // <- extension on launch scope, launched in IO dispatcher
           // blocking I/O operation
       }
       // do something else concurrently with I/O
       val data = ioData.await() // wait for result of I/O
       draw(data) // can draw in the main thread
    }
}