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Kotlin掛起函數(shù)的詳細(xì)介紹_Android

作者:且聽(tīng)真言 ? 更新時(shí)間: 2022-11-02 編程語(yǔ)言

Kotlin 協(xié)程的優(yōu)勢(shì):

  • 解決回調(diào)地獄的問(wèn)題。
  • 以同步的方式完成異步任務(wù)。

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"
}

輸出
A content
A content B content
A content B content C content

suspend關(guān)鍵字修飾的方法 就是 掛起函數(shù)。掛起函數(shù)具備掛起和恢復(fù)的能力。掛起就是將程序執(zhí)行流程轉(zhuǎn)移到其他線程,主線程不阻塞。掛起函數(shù)的本質(zhì)是Callback。

Kotlin編譯器檢測(cè)到suspend關(guān)鍵字修飾的函數(shù),會(huì)將掛起函數(shù)轉(zhuǎn)換成帶有CallBack的函數(shù)。

suspend fun getA(): String {
    withContext(Dispatchers.IO) {
        delay(5000L)
        println("now in A process:" + Thread.currentThread())
    }
    /**
     * 這里的代碼涉及掛起函數(shù)中的操作。
     */
    println("finish A process:" + Thread.currentThread())
    return "A content"
}

將上述Kotlin代碼轉(zhuǎn)換成java代碼。

 @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 的第二個(gè)參數(shù) 也是 傳入一個(gè) suspend修飾的函數(shù) 即掛起函數(shù)。

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

可以發(fā)現(xiàn)上面的Continuation 是一個(gè)帶有泛型參數(shù)的Callback,這里的轉(zhuǎn)換稱(chēng)為CPS轉(zhuǎn)換,將原本的同步掛起函數(shù)轉(zhuǎn)換成CallBack異步代碼。

/**
 * 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:掛起函數(shù),只能在協(xié)程中被調(diào)用,或者被其他掛起函數(shù)調(diào)用。

為什么掛起函數(shù)可以調(diào)用掛起函數(shù),而普通函數(shù)不能調(diào)用掛起函數(shù)?

fun main() {
    doA() //這里會(huì)報(bào)錯(cuò)
}
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;
   }

被調(diào)用的掛起函數(shù)需要傳入一個(gè)Continuation, 沒(méi)有被suspend修飾的函數(shù)是沒(méi)有Continuation參數(shù)的,所以沒(méi)法在普通函數(shù)中調(diào)用掛起函數(shù),普通函數(shù)沒(méi)有Continuation。

掛起函數(shù)最終都是在協(xié)程中被調(diào)用,協(xié)程提供了掛起函數(shù)運(yùn)行的環(huán)境。

原文鏈接:https://blog.csdn.net/zhangying1994/article/details/126691885

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