Kotlin 协程探索

Posted 2023-01-06 麦田里的守望者-Jiang

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文章目录

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UI
            refreshUI(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 + numbers50To100Sum
            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 16:05:45.846 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;

         @Nullable
         public 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;

                     @Nullable
                     public 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));
                     

                     @NotNull
                     public 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;

                  @Nullable
                  public 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));
                  

                  @NotNull
                  public 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;
         

         @NotNull
         public 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;

         @Nullable
         public 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));
         

         @NotNull
         public 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：

下一篇，将研究 Kotlin Flow。

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