Kotlin協(xié)程的啟動方式介紹

更新時間：2022年09月07日 14:56:34 作者：且聽真言

這篇文章我們來講協(xié)程的啟動，畢竟協(xié)程是一個很強大的設計模式，深入了解需要花很多的時間，我們先從簡單開始，其實學會了簡單的使用，基本已經可以滿足我們平時的開發(fā)需要了，話不多說，開始

1.GlobalScope.launch

代碼示例：

fun testGlobalScope() {
    GlobalScope.launch {
        println("Coroutinue started!")
        delay(1000L)
        println("Hello World!")
    }
    println("After launch!")
    Thread.sleep(2000L)
    println("Process end!")
}
/**
 * After launch!
 * Coroutinue started!
 * Hello World!
 * Process end!
 */

@DelicateCoroutinesApi
public object GlobalScope : CoroutineScope {
    /**
     * Returns [EmptyCoroutineContext].
     */
    override val coroutineContext: CoroutineContext
        get() = EmptyCoroutineContext
}

public fun CoroutineScope.launch(
    context: CoroutineContext = EmptyCoroutineContext,
    start: CoroutineStart = CoroutineStart.DEFAULT,
    block: suspend CoroutineScope.() -> Unit
): Job {
    val newContext = newCoroutineContext(context)
    val coroutine = if (start.isLazy)
        LazyStandaloneCoroutine(newContext, block) else
        StandaloneCoroutine(newContext, active = true)
    coroutine.start(start, coroutine, block)
    return coroutine
}

launch函數(shù)是CoroutineScope的擴展函數(shù)，它有三個參數(shù)：

context: CoroutineContext = EmptyCoroutineContext, 第一個參數(shù)是協(xié)程上下文，它的默認值是 EmptyCoroutineContext，如果不傳這個參數(shù)，默認就會使用 EmptyCoroutineContext。也可以傳入 Kotlin 官方為我們提供的 Dispatchers，來指定協(xié)程運行的線程池。（Dispatchers.IO、Dispatchers.Unconfined、Dispatchers.Main）
start: CoroutineStart = CoroutineStart.DEFAULT，第二個參數(shù)是協(xié)程的啟動模式，默認值是CoroutineStart.DEFAULT，CoroutineStart 是一個枚舉類，一共有：DEFAULT、LAZY、ATOMIC、UNDISPATCHED。
block: suspend CoroutineScope.() -> Unit，第三個參數(shù)是函數(shù)類型block，它的類型是suspend CoroutineScope.() -> Unit。本質是一個掛起函數(shù)。
函數(shù)的返回值是一個 Job，它其實代表的是協(xié)程的句柄，并不能返回協(xié)程的執(zhí)行結果。

2.runBlocking 啟動協(xié)程

代碼示例

fun testRunBlocking2() {
    runBlocking {
        println("Coroutinue started!")
        delay(1000L)
        println("Hello World!")
    }
    println("After Launch")
    Thread.sleep(2000L)
    println("Process end")
}
/**
 * Coroutinue started!
 * Hello World!
 * After Launch
 * Process end
 */

@Throws(InterruptedException::class)
public actual fun <T> runBlocking(context: CoroutineContext, block: suspend CoroutineScope.() -> T): T {
    contract {
        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
    }
    val currentThread = Thread.currentThread()
    val contextInterceptor = context[ContinuationInterceptor]
    val eventLoop: EventLoop?
    val newContext: CoroutineContext
    if (contextInterceptor == null) {
        // create or use private event loop if no dispatcher is specified
        eventLoop = ThreadLocalEventLoop.eventLoop
        newContext = GlobalScope.newCoroutineContext(context + eventLoop)
    } else {
        // See if context's interceptor is an event loop that we shall use (to support TestContext)
        // or take an existing thread-local event loop if present to avoid blocking it (but don't create one)
        eventLoop = (contextInterceptor as? EventLoop)?.takeIf { it.shouldBeProcessedFromContext() }
            ?: ThreadLocalEventLoop.currentOrNull()
        newContext = GlobalScope.newCoroutineContext(context)
    }
    val coroutine = BlockingCoroutine<T>(newContext, currentThread, eventLoop)
    coroutine.start(CoroutineStart.DEFAULT, coroutine, block)
    return coroutine.joinBlocking()
}

runBlocking是普通函數(shù)，第一個參數(shù)：context: CoroutineContext,協(xié)程上下文。第二個參數(shù)是函數(shù)類型，block: suspend CoroutineScope.() -> T,函數(shù)類型是有返回值類型 T 的，與 runBlocking 的返回值類型是一樣的，runBlocking 其實是可以從協(xié)程當中返回執(zhí)行結果的。

fun testRunBlocking() {
    val runBlockingResult = runBlocking {
        delay(500L)
        return@runBlocking "HaHa"
    }
    println("result:$runBlockingResult")
}
result:HaHa

runBlocking特點：

runBlocking 啟動的協(xié)程會阻塞當前線程的執(zhí)行。

3.async啟動協(xié)程

使用 async{} 創(chuàng)建協(xié)程，可以通過它返回的Deferred拿到協(xié)程的執(zhí)行結果。

代碼示例

fun testAsync() {
    runBlocking {
        val deferred = async {
            println("do async:${Thread.currentThread().name}")
            delay(1000L)
            return@async "do completed"
        }
        println("After async:${Thread.currentThread().name}")
        val result = deferred.await()
        println("Result is: $result")
    }
}
After async:main @coroutine#1
do async:main @coroutine#2
Result is: do completed

public fun <T> CoroutineScope.async(
    context: CoroutineContext = EmptyCoroutineContext,
    start: CoroutineStart = CoroutineStart.DEFAULT,
    block: suspend CoroutineScope.() -> T
): Deferred<T> {
    val newContext = newCoroutineContext(context)
    val coroutine = if (start.isLazy)
        LazyDeferredCoroutine(newContext, block) else
        DeferredCoroutine<T>(newContext, active = true)
    coroutine.start(start, coroutine, block)
    return coroutine
}

async注意點