使用明确的job来控制协程生命周期

作者: mihope | 来源:发表于2019-01-22 22:40 被阅读8次

使用明确的job来控制协程生命周期
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我们把关于contexts\children和jobs的知识融合到一起，试想我们有一个具备生命周期的对象，但这个对象不是一个协程。例如，在开发Android应用时，为了执行获取网络数据、执行动画等异步操作，我们在activity的里开启了各种协程。为了避免内存泄漏，当activity被销毁时，所有的这些协程必须取消。

通过创建一个Job的实例来控制我们协程的生命周期，把它绑定到activity的生命周期上。在activity被创建时，使用工厂方法Job()创建一个job实例;当activity被销毁时，它如下这样被取消：

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
        }
 }

这个activity实现了CoroutineScope接口，我们只需要重写CoroutineScope.coroutineContext属性来指定协程在这个scope内的执行上下文环境context。那么在这个activity里启动协程就不需要再明确指定他们的执行环境context。

这里有一段示意代码

import kotlin.coroutines.*
import kotlinx.coroutines.*

class Activity : CoroutineScope {
    lateinit var job: Job

    fun create() {
        job = Job()
    }

    fun destroy() {
        job.cancel()
    }
    // to be continued ...

    // class Activity continues
    override val coroutineContext: CoroutineContext
        get() = Dispatchers.Default + job
    // to be continued ...

    // class Activity continues
    fun doSomething() {
        // launch ten coroutines for a demo, each working for a different time
        repeat(10) { i ->
            launch {
                delay((i + 1) * 200L) // variable delay 200ms, 400ms, ... etc
                println("Coroutine $i is done")
            }
        }
    }
} // class Activity ends

fun main() = runBlocking<Unit> {
    val activity = Activity()
    activity.create() // create an activity
    activity.doSomething() // run test function
    println("Launched coroutines")
    delay(500L) // delay for half a second
    println("Destroying activity!")
    activity.destroy() // cancels all coroutines
    delay(1000) // visually confirm that they don't work    
}

//Launched coroutines
//Coroutine 0 is done
//Coroutine 1 is done
//Destroying activity!