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Kotlin关于协程是什么的探究

作者:麦田里的守望者-Jiang

Android官方对协程的定义-协程是一种并发设计模式,您可以在Android平台上使用它来简化异步执行的代码。协程是在版本1.3中添加到Kotlin的,它基于来自其他语言的既定概念

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 ThreadExecutors 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:

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:

Android 上的 Kotlin 协程

Coroutines guide

下一篇,将研究 Kotlin Flow

到此这篇关于Kotlin关于协程是什么的探究的文章就介绍到这了,更多相关Kotlin协程内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!

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