Kotlin挂起函数的详细介绍

Kotlin 协程的优势：

• 解决回调地狱的问题。
• 以同步的方式完成异步任务。

示例：

fun main() {
    runBlocking {
        val a = getA()
        println(a)
        val b = getB(a)
        println(b)
        val c = getC(b)
        println(c)
    }
}
suspend fun getA(): String {
    withContext(Dispatchers.IO) {
        delay(2000L)
    }
    return "A content"
}
suspend fun getB(a: String): String {
    withContext(Dispatchers.IO) {
        delay(2000L)
    }
    return "$a B content"
}
suspend fun getC(b: String): String {
    withContext(Dispatchers.IO) {
        delay(2000L)
    }
    return "$b C content"
}

输出
A content
A content B content
A content B content C content

suspend关键字修饰的方法 就是 挂起函数。挂起函数具备挂起和恢复的能力。挂起就是将程序执行流程转移到其他线程，主线程不阻塞。挂起函数的本质是Callback。

Kotlin编译器检测到suspend关键字修饰的函数，会将挂起函数转换成带有CallBack的函数。

suspend fun getA(): String {
    withContext(Dispatchers.IO) {
        delay(5000L)
        println("now in A process:" + Thread.currentThread())
    }
    /**
     * 这里的代码涉及挂起函数中的操作。
     */
    println("finish A process:" + Thread.currentThread())
    return "A content"
}

将上述Kotlin代码转换成java代码。

 @Nullable
   public static final Object getA(@NotNull Continuation var0) {
      Object $continuation;
      label20: {
         if (var0 instanceof <undefinedtype>) {
            $continuation = (<undefinedtype>)var0;
            if ((((<undefinedtype>)$continuation).label & Integer.MIN_VALUE) != 0) {
               ((<undefinedtype>)$continuation).label -= Integer.MIN_VALUE;
               break label20;
            }
         }
         $continuation = new ContinuationImpl(var0) {
            // $FF: synthetic field
            Object result;
            int label;

            @Nullable
            public final Object invokeSuspend(@NotNull Object $result) {
               this.result = $result;
               this.label |= Integer.MIN_VALUE;
               return TestCoroutinue2Kt.getA(this);
            }
         };
      }
      Object $result = ((<undefinedtype>)$continuation).result;
      Object var4 = IntrinsicsKt.getCOROUTINE_SUSPENDED();
      switch(((<undefinedtype>)$continuation).label) {
      case 0:
         ResultKt.throwOnFailure($result);
         CoroutineContext var10000 = (CoroutineContext)Dispatchers.getIO();
         Function2 var10001 = (Function2)(new Function2((Continuation)null) {
            int label;
            @Nullable
            public final Object invokeSuspend(@NotNull Object $result) {
               Object var3 = IntrinsicsKt.getCOROUTINE_SUSPENDED();
               switch(this.label) {
               case 0:
                  ResultKt.throwOnFailure($result);
                  this.label = 1;
                  if (DelayKt.delay(5000L, this) == var3) {
                     return var3;
                  }
                  break;
               case 1:
                  ResultKt.throwOnFailure($result);
                  break;
               default:
                  throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");
               }
               String var2 = "now in A process:" + Thread.currentThread();
               System.out.println(var2);
               return Unit.INSTANCE;
            }
            @NotNull
            public final Continuation create(@Nullable Object value, @NotNull Continuation completion) {
               Intrinsics.checkNotNullParameter(completion, "completion");
               Function2 var3 = new <anonymous constructor>(completion);
               return var3;
            }
            public final Object invoke(Object var1, Object var2) {
               return ((<undefinedtype>)this.create(var1, (Continuation)var2)).invokeSuspend(Unit.INSTANCE);
            }
         });
         ((<undefinedtype>)$continuation).label = 1;
         if (BuildersKt.withContext(var10000, var10001, (Continuation)$continuation) == var4) {
            return var4;
         }
         break;
      case 1:
         ResultKt.throwOnFailure($result);
         break;
      default:
         throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine");
      }
      String var1 = "finish A process:" + Thread.currentThread();
      System.out.println(var1);
      return "A content";
   }

注意：runBlocking 的第二个参数 也是 传入一个 suspend修饰的函数 即挂起函数。

public actual fun <T> runBlocking(context: CoroutineContext, block: suspend CoroutineScope.() -> T): T {

可以发现上面的Continuation 是一个带有泛型参数的Callback,这里的转换称为CPS转换，将原本的同步挂起函数转换成CallBack异步代码。

/**
 * Interface representing a continuation after a suspension point that returns a value of type `T`.
 */
@SinceKotlin("1.3")
public interface Continuation<in T> {
    /**
     * The context of the coroutine that corresponds to this continuation.
     */
    public val context: CoroutineContext
    /**
     * Resumes the execution of the corresponding coroutine passing a successful or failed [result] as the
     * return value of the last suspension point.
     */
    public fun resumeWith(result: Result<T>)
}

注意：挂起函数，只能在协程中被调用，或者被其他挂起函数调用。

为什么挂起函数可以调用挂起函数，而普通函数不能调用挂起函数?

fun main() {
    doA() //这里会报错
}
suspend fun doA() {
}

 public static final void main() {
   }
   // $FF: synthetic method
   public static void main(String[] var0) {
      main();
   }

 @Nullable
   public static final Object doA(@NotNull Continuation $completion) {
      return Unit.INSTANCE;
   }

被调用的挂起函数需要传入一个Continuation, 没有被suspend修饰的函数是没有Continuation参数的,所以没法在普通函数中调用挂起函数，普通函数没有Continuation。

挂起函数最终都是在协程中被调用，协程提供了挂起函数运行的环境。

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