Kotlin挂起函数的详细介绍 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。

@Nullablepublic static final Object getA(@NotNull Continuation var0) {Object $continuation; label20: {if (var0 instanceof ) {$continuation = ()var0; if (((()$continuation).label & Integer.MIN_VALUE) != 0) {(()$continuation).label -= Integer.MIN_VALUE; break label20; }}$continuation = new ContinuationImpl(var0) {// $FF: synthetic fieldObject result; int label; @Nullablepublic final Object invokeSuspend(@NotNull Object $result) {this.result = $result; this.label |= Integer.MIN_VALUE; return TestCoroutinue2Kt.getA(this); }}; }Object $result = (()$continuation).result; Object var4 = IntrinsicsKt.getCOROUTINE_SUSPENDED(); switch((()$continuation).label) {case 0:ResultKt.throwOnFailure($result); CoroutineContext var10000 = (CoroutineContext)Dispatchers.getIO(); Function2 var10001 = (Function2)(new Function2((Continuation)null) {int label; @Nullablepublic 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($result); break; default:throw new IllegalStateException("call to 'resume' before 'invoke' with coroutine"); }String var2 = "now in A process:" + Thread.currentThread(); System.out.println(var2); return Unit.INSTANCE; }@NotNullpublic final Continuation create(@Nullable Object value, @NotNull Continuation completion) {Intrinsics.checkNotNullParameter(completion, "completion"); Function2 var3 = new (completion); return var3; }public final Object invoke(Object var1, Object var2) {return (()this.create(var1, (Continuation)var2)).invokeSuspend(Unit.INSTANCE); }}); (()$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 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 {/*** 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)}

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 methodpublic static void main(String[] var0) {main(); }

@Nullablepublic 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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