Kotlin 协程探索

文章目录

Kotlin 协程是什么？
suspend 是什么？
总结

Kotlin 协程是什么？

本文只是自己经过研究后，对 Kotlin 协程的理解概括，如有偏差，还请斧正。

简要概括：

协程是 Kotlin 提供的一套线程 API 框架，可以很方便的做线程切换。而且在不用关心线程调度的情况下，能轻松的做并发编程。也可以说协程就是一种并发设计模式。

下面是使用传统线程和协程执行任务：

       Thread{//执行耗时任务}.start()val executors = Executors.newCachedThreadPool()executors.execute {//执行耗时任务}GlobalScope.launch(Dispatchers.IO) {//执行耗时任务}

在实际应用开发中，通常是在主线中去启动子线程执行耗时任务，等耗时任务执行完成，再将结果给主线程，然后刷新UI：

       Thread{//执行耗时任务runOnMainThread { //获取耗时任务结果，刷新UI}}.start()val executors = Executors.newCachedThreadPool()executors.execute {//执行耗时任务runOnMainThread {//获取耗时任务结果，刷新UI}}Observable.unsafeCreate<Unit> {//执行耗时任务}.subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread()).subscribe {//获取耗时任务结果，刷新UI}GlobalScope.launch(Dispatchers.Main) {val result = withContext(Dispatchers.IO){//执行耗时任务}//直接拿到耗时任务结果，刷新UIrefreshUI(result)}

从上面可以看到，使用Java 的 Thread 和 Executors 都需要手动去处理线程切换，这样的代码不仅不优雅，而且有一个重要问题，那就是要去处理与生命周期相关的上下文判断，这导致逻辑变复杂，而且容易出错。

RxJava 是一套优雅的异步处理框架，代码逻辑简化，可读性和可维护性都很高，很好的帮我们处理线程切换操作。这在 Java 语言环境开发下，是如虎添翼，但是在 Kotlin 语言环境中开发，如今的协程就比 RxJava 更方便，或者说更有优势。

下面看一个 Kotlin 中使用协程的例子：

        GlobalScope.launch(Dispatchers.Main) {Log.d("TestCoroutine", "launch start: ${Thread.currentThread()}")val numbersTo50Sum = withContext(Dispatchers.IO) {//在子线程中执行 1-50 的自然数和Log.d("TestCoroutine", "launch:numbersTo50Sum: ${Thread.currentThread()}")delay(1000)val naturalNumbers = generateSequence(0) { it + 1 }val numbersTo50 = naturalNumbers.takeWhile { it <= 50 }numbersTo50.sum()}val numbers50To100Sum = withContext(Dispatchers.IO) {//在子线程中执行 51-100 的自然数和Log.d("TestCoroutine", "launch:numbers50To100Sum: ${Thread.currentThread()}")delay(1000)val naturalNumbers = generateSequence(51) { it + 1 }val numbers50To100 = naturalNumbers.takeWhile { it in 51..100 }numbers50To100.sum()}val result = numbersTo50Sum + numbers50To100SumLog.d("TestCoroutine", "launch end:result=$result ${Thread.currentThread()}")}Log.d("TestCoroutine", "Hello World!,${Thread.currentThread()}")
控制台输出结果：
2023-01-02 16:05:45.846 10153-10153/com.wangjiang.example D/TestCoroutine: Hello World!,Thread[main,5,main]
2023-01-02 16:05:48.058 10153-10153/com.wangjiang.example D/TestCoroutine: launch start: Thread[main,5,main]
2023-01-02 16:05:48.059 10153-10322/com.wangjiang.example D/TestCoroutine: launch:numbersTo50Sum: Thread[DefaultDispatcher-worker-1,5,main]
2023-01-02 16:05:49.114 10153-10322/com.wangjiang.example D/TestCoroutine: launch:numbers50To100Sum: Thread[DefaultDispatcher-worker-1,5,main]
2023-01-02 16:05:50.376 10153-10153/com.wangjiang.example D/TestCoroutine: launch end:result=5050 Thread[main,5,main]

在上面的代码中：

launch 是一个函数，用于创建协程并将其函数主体的执行分派给相应的调度程序。
Dispatchers.MAIN 指示此协程应在为 UI 操作预留的主线程上执行。
Dispatchers.IO 指示此协程应在为 I/O 操作预留的线程上执行。
withContext(Dispatchers.IO) 将协程的执行操作移至一个 I/O 线程。

从控制台输出结果中，可以看出在计算 1-50 和 51-100 的自然数和的时候，线程是从主线程（Thread[main,5,main]）切换到了协程的线程（DefaultDispatcher-worker-1,5,main），这里计算 1-50 和 51-100 都是同一个子线程。

在这里有一个重要的现象，代码从逻辑上看起来是同步的，并且启动协程执行任务的时候，没有阻塞主线程继续执行相关操作，而且在协程中的异步任务执行完成之后，又自动切回了主线程。这就是 Kotlin 协程给开发做并发编程带来的好处。这也是有个概念的来源： Kotlin 协程同步非阻塞。

同步非阻塞”是真的“同步非阻塞” 吗？下面探究一下其中的猫腻，通过 Android Studio ，查看 .class 文件中的上面一段代码：

      BuildersKt.launch$default((CoroutineScope)GlobalScope.INSTANCE, (CoroutineContext)Dispatchers.getMain(), (CoroutineStart)null, (Function2)(new Function2((Continuation)null) {int I$0;int label;@Nullablepublic final Object invokeSuspend(@NotNull Object $result) {Object var10000;int numbersTo50Sum;label17: {Object var5 = IntrinsicsKt.getCOROUTINE_SUSPENDED();Function2 var10001;CoroutineContext var6;switch(this.label) {case 0:ResultKt.throwOnFailure($result);Log.d("TestCoroutine", "launch start: " + Thread.currentThread());var6 = (CoroutineContext)Dispatchers.getIO();var10001 = (Function2)(new Function2((Continuation)null) {int label;@Nullablepublic final Object invokeSuspend(@NotNull Object $result) {Object var4 = IntrinsicsKt.getCOROUTINE_SUSPENDED();switch(this.label) {case 0:ResultKt.throwOnFailure($result);Log.d("TestCoroutine", "launch:numbersTo50Sum: " + Thread.currentThread());this.label = 1;if (DelayKt.delay(1000L, this) == var4) {return var4;}break;case 1:ResultKt.throwOnFailure($result);break;default:throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");}Sequence naturalNumbers = SequencesKt.generateSequence(Boxing.boxInt(0), (Function1)null.INSTANCE);Sequence numbersTo50 = SequencesKt.takeWhile(naturalNumbers, (Function1)null.INSTANCE);return Boxing.boxInt(SequencesKt.sumOfInt(numbersTo50));}@NotNullpublic final Continuation create(@Nullable Object value, @NotNull Continuation completion) {Intrinsics.checkNotNullParameter(completion, "completion");Function2 var3 = new <anonymous constructor>(completion);return var3;}public final Object invoke(Object var1, Object var2) {return ((<undefinedtype>)this.create(var1, (Continuation)var2)).invokeSuspend(Unit.INSTANCE);}});this.label = 1;var10000 = BuildersKt.withContext(var6, var10001, this);if (var10000 == var5) {return var5;}break;case 1:ResultKt.throwOnFailure($result);var10000 = $result;break;case 2:numbersTo50Sum = this.I$0;ResultKt.throwOnFailure($result);var10000 = $result;break label17;default:throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");}numbersTo50Sum = ((Number)var10000).intValue();var6 = (CoroutineContext)Dispatchers.getIO();var10001 = (Function2)(new Function2((Continuation)null) {int label;@Nullablepublic final Object invokeSuspend(@NotNull Object $result) {Object var4 = IntrinsicsKt.getCOROUTINE_SUSPENDED();switch(this.label) {case 0:ResultKt.throwOnFailure($result);Log.d("TestCoroutine", "launch:numbers50To100Sum: " + Thread.currentThread());this.label = 1;if (DelayKt.delay(1000L, this) == var4) {return var4;}break;case 1:ResultKt.throwOnFailure($result);break;default:throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");}Sequence naturalNumbers = SequencesKt.generateSequence(Boxing.boxInt(51), (Function1)null.INSTANCE);Sequence numbers50To100 = SequencesKt.takeWhile(naturalNumbers, (Function1)null.INSTANCE);return Boxing.boxInt(SequencesKt.sumOfInt(numbers50To100));}@NotNullpublic final Continuation create(@Nullable Object value, @NotNull Continuation completion) {Intrinsics.checkNotNullParameter(completion, "completion");Function2 var3 = new <anonymous constructor>(completion);return var3;}public final Object invoke(Object var1, Object var2) {return ((<undefinedtype>)this.create(var1, (Continuation)var2)).invokeSuspend(Unit.INSTANCE);}});this.I$0 = numbersTo50Sum;this.label = 2;var10000 = BuildersKt.withContext(var6, var10001, this);if (var10000 == var5) {return var5;}}int numbers50To100Sum = ((Number)var10000).intValue();int result = numbersTo50Sum + numbers50To100Sum;Log.d("TestCoroutine", "launch end:result=" + result + ' ' + Thread.currentThread());return Unit.INSTANCE;}@NotNullpublic final Continuation create(@Nullable Object value, @NotNull Continuation completion) {Intrinsics.checkNotNullParameter(completion, "completion");Function2 var3 = new <anonymous constructor>(completion);return var3;}public final Object invoke(Object var1, Object var2) {return ((<undefinedtype>)this.create(var1, (Continuation)var2)).invokeSuspend(Unit.INSTANCE);}}), 2, (Object)null);Log.d("TestCoroutine", "Hello World!," + Thread.currentThread());

虽然上面 .class 文件中的代码比较复杂，但是从大体逻辑可以看出，Kotlin 协程也是通过回调接口来实现异步操作的，这也解释了 Kotlin 协程只是让代码逻辑是同步非阻塞，但是实际上并没有，只是 Kotlin 编译器为代码做了很多事情，这也是说 Kotlin 协程其实就是一套线程 API 框架的原因。

再看一个上面例子的变种：

        GlobalScope.launch(Dispatchers.Main) {Log.d("TestCoroutine", "launch start: ${Thread.currentThread()}")val numbersTo50Sum = async {withContext(Dispatchers.IO) {Log.d("TestCoroutine", "launch:numbersTo50Sum: ${Thread.currentThread()}")delay(2000)val naturalNumbers = generateSequence(0) { it + 1 }val numbersTo50 = naturalNumbers.takeWhile { it <= 50 }numbersTo50.sum()}}val numbers50To100Sum = async {withContext(Dispatchers.IO) {Log.d("TestCoroutine", "launch:numbers50To100Sum: ${Thread.currentThread()}")delay(500)val naturalNumbers = generateSequence(51) { it + 1 }val numbers50To100 = naturalNumbers.takeWhile { it in 51..100 }numbers50To100.sum()}}// 计算 1-50 和 51-100 的自然数和是两个并发操作val result = numbersTo50Sum.await() + numbers50To100Sum.await()Log.d("TestCoroutine", "launch end:result=$result ${Thread.currentThread()}")}Log.d("TestCoroutine", "Hello World!,${Thread.currentThread()}")控制台输出结果：
2023-01-02 16:32:12.637 13303-13303/com.wangjiang.example D/TestCoroutine: Hello World!,Thread[main,5,main]
2023-01-02 16:32:13.120 13303-13303/com.wangjiang.example D/TestCoroutine: launch start: Thread[main,5,main]
2023-01-02 16:32:14.852 13303-13444/com.wangjiang.example D/TestCoroutine: launch:numbersTo50Sum: Thread[DefaultDispatcher-worker-2,5,main]
2023-01-02 16:32:14.853 13303-13443/com.wangjiang.example D/TestCoroutine: launch:numbers50To100Sum: Thread[DefaultDispatcher-worker-1,5,main]
2023-01-02 16:32:17.462 13303-13303/com.wangjiang.example D/TestCoroutine: launch end:result=5050 Thread[main,5,main]

async 创建了一个协程，它让计算 1-50 和 51-100 的自然数和是两个并发操作。上面控制台输出结果可以看到计算 1-50 的自然数和是在线程 Thread[DefaultDispatcher-worker-2,5,main] 中，而计算 51-100 的自然数和是在另一个线程Thread[DefaultDispatcher-worker-1,5,main]中。

从上面的例子，协程在异步操作，也就是线程切换上：主线程启动子线程执行耗时操作，耗时操作执行完成将结果更新到主线程的过程中，代码逻辑简化，可读性高。

suspend 是什么？

suspend 直译就是：挂起
suspend 是 Kotlin 语言中一个关键字，用于修饰方法，当修饰方法时，表示这个方法只能被 suspend 修饰的方法调用或者在协程中被调用。
下面看一下将上面代码案例拆分成几个 suspend 方法：

    fun getNumbersTo100Sum() {GlobalScope.launch(Dispatchers.Main) {Log.d("TestCoroutine", "launch start: ${Thread.currentThread()}")val result = calcNumbers1To100Sum()Log.d("TestCoroutine", "launch end:result=$result ${Thread.currentThread()}")}Log.d("TestCoroutine", "Hello World!,${Thread.currentThread()}")}private suspend fun calcNumbers1To100Sum(): Int {return calcNumbersTo50Sum() + calcNumbers50To100Sum()}private suspend fun calcNumbersTo50Sum(): Int {return withContext(Dispatchers.IO) {Log.d("TestCoroutine", "launch:numbersTo50Sum: ${Thread.currentThread()}")delay(1000)val naturalNumbers = generateSequence(0) { it + 1 }val numbersTo50 = naturalNumbers.takeWhile { it <= 50 }numbersTo50.sum()}}private suspend fun calcNumbers50To100Sum(): Int {return withContext(Dispatchers.IO) {Log.d("TestCoroutine", "launch:numbers50To100Sum: ${Thread.currentThread()}")delay(1000)val naturalNumbers = generateSequence(51) { it + 1 }val numbers50To100 = naturalNumbers.takeWhile { it in 51..100 }numbers50To100.sum()}}
控制台输出结果：
2023-01-03 14:47:57.047 11349-11349/com.wangjiang.example D/TestCoroutine: Hello World!,Thread[main,5,main]
2023-01-03 14:47:59.311 11349-11349/com.wangjiang.example D/TestCoroutine: launch start: Thread[main,5,main]
2023-01-03 14:47:59.312 11349-11537/com.wangjiang.example D/TestCoroutine: launch:numbersTo50Sum: Thread[DefaultDispatcher-worker-3,5,main]
2023-01-03 14:48:00.336 11349-11535/com.wangjiang.example D/TestCoroutine: launch:numbers50To100Sum: Thread[DefaultDispatcher-worker-1,5,main]
2023-01-03 14:48:01.339 11349-11349/com.wangjiang.example D/TestCoroutine: launch end:result=5050 Thread[main,5,main]

suspend 关键字标记方法时，其实是告诉 Kotlin 从协程内调用方法。所以这个“挂起”，并不是说方法或函数被挂起，也不是说线程被挂起。

假设一个非 suspend 修饰的方法调用 suspend 修饰的方法会怎么样呢？

  private fun calcNumbersTo100Sum(): Int {return calcNumbersTo50Sum() + calcNumbers50To100Sum()}

此时，编译器会提示：

Suspend function 'calcNumbersTo50Sum' should be called only from a coroutine or another suspend function
Suspend function 'calcNumbers50To100' should be called only from a coroutine or another suspend function

下面查看 .class 文件中的上面方法 calcNumbers50To100Sum 代码：

   private final Object calcNumbers50To100Sum(Continuation $completion) {return BuildersKt.withContext((CoroutineContext)Dispatchers.getIO(), (Function2)(new Function2((Continuation)null) {int label;@Nullablepublic final Object invokeSuspend(@NotNull Object $result) {Object var4 = IntrinsicsKt.getCOROUTINE_SUSPENDED();switch(this.label) {case 0:ResultKt.throwOnFailure($result);Log.d("TestCoroutine", "launch:numbers50To100Sum: " + Thread.currentThread());this.label = 1;if (DelayKt.delay(1000L, this) == var4) {return var4;}break;case 1:ResultKt.throwOnFailure($result);break;default:throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");}Sequence naturalNumbers = SequencesKt.generateSequence(Boxing.boxInt(51), (Function1)null.INSTANCE);Sequence numbers50To100 = SequencesKt.takeWhile(naturalNumbers, (Function1)null.INSTANCE);return Boxing.boxInt(SequencesKt.sumOfInt(numbers50To100));}@NotNullpublic final Continuation create(@Nullable Object value, @NotNull Continuation completion) {Intrinsics.checkNotNullParameter(completion, "completion");Function2 var3 = new <anonymous constructor>(completion);return var3;}public final Object invoke(Object var1, Object var2) {return ((<undefinedtype>)this.create(var1, (Continuation)var2)).invokeSuspend(Unit.INSTANCE);}}), $completion);}

可以看到 private suspend fun calcNumbers50To100Sum() 经过 Kotlin 编译器编译后变成了private final Object calcNumbers50To100Sum(Continuation $completion)， suspend 消失了，方法多了一个参数 Continuation $completion，所以 suspend修饰 Kotlin 的方法或函数，编译器会对此方法做特殊处理。

另外，suspend 修饰的方法，也预示着这个方法是耗时方法，告诉方法调用者要使用协程。当执行 suspend 方法，也预示着要切换线程，此时主线程依然可以继续执行，而协程里面的代码可能被挂起了。

下面再稍为修改 calcNumbers50To100Sum 方法：

   private suspend fun calcNumbers50To100Sum(): Int {Log.d("TestCoroutine", "launch:numbers50To100Sum:start: ${Thread.currentThread()}")val sum= withContext(Dispatchers.Main) {Log.d("TestCoroutine", "launch:numbers50To100Sum: ${Thread.currentThread()}")delay(1000)val naturalNumbers = generateSequence(51) { it + 1 }val numbers50To100 = naturalNumbers.takeWhile { it in 51..100 }numbers50To100.sum()}Log.d("TestCoroutine", "launch:numbers50To100Sum:end: ${Thread.currentThread()}")return sum}
控制台输出结果：
2023-01-03 15:28:04.349 15131-15131/com.bilibili.studio D/TestCoroutine: Hello World!,Thread[main,5,main]
2023-01-03 15:28:04.803 15131-15131/com.bilibili.studio D/TestCoroutine: launch start: Thread[main,5,main]
2023-01-03 15:28:04.804 15131-15266/com.bilibili.studio D/TestCoroutine: launch:numbersTo50Sum: Thread[DefaultDispatcher-worker-3,5,main]
2023-01-03 15:28:06.695 15131-15131/com.bilibili.studio D/TestCoroutine: launch:numbers50To100Sum:start: Thread[main,5,main]
2023-01-03 15:28:06.696 15131-15131/com.bilibili.studio D/TestCoroutine: launch:numbers50To100Sum: Thread[main,5,main]
2023-01-03 15:28:07.700 15131-15131/com.bilibili.studio D/TestCoroutine: launch:numbers50To100Sum:end: Thread[main,5,main]
2023-01-03 15:28:07.700 15131-15131/com.bilibili.studio D/TestCoroutine: launch end:result=5050 Thread[main,5,main]

主线程不受协程线程的影响。

总结

Kotlin 协程是一套线程 API 框架，在 Kotlin 语言环境下使用它做并发编程比传统 Thread, Executors 和 RxJava 更有优势，代码逻辑上“同步非阻塞“，而且简洁，易阅读和维护。

suspend 是 Kotlin 语言中一个关键字，用于修饰方法，当修饰方法时，该方法只能被 suspend 修饰的方法和协程调用。此时，也预示着该方法是一个耗时方法，告诉调用者需要在协程中使用。

参考文档：

Android 上的 Kotlin 协程
Coroutines guide

下一篇，将研究 Kotlin Flow。