Kotlin 中 CoroutineScope 和 coroutineScope 的区别

Question

谁能在函数CoroutineScope() 和coroutineScope() 之间明确说明？

我在查源码的时候发现都是CoroutineScope.kt的函数。此外，coroutineScope() 是 suspend 函数，而另一个是 normal 函数

以下是我能找到的文档：

/**
 * Creates a [CoroutineScope] that wraps the given coroutine [context].
 *
 * If the given [context] does not contain a [Job] element, then a default `Job()` is created.
 * This way, cancellation or failure or any child coroutine in this scope cancels all the other children,
 * just like inside [coroutineScope] block.
 */
@Suppress("FunctionName")
public fun CoroutineScope(context: CoroutineContext): CoroutineScope =
    ContextScope(if (context[Job] != null) context else context + Job())

和

/**
 * Creates a [CoroutineScope] and calls the specified suspend block with this scope.
 * The provided scope inherits its [coroutineContext][CoroutineScope.coroutineContext] from the outer scope, but overrides
 * the context's [Job].
 *
 * This function is designed for _parallel decomposition_ of work. When any child coroutine in this scope fails,
 * this scope fails and all the rest of the children are cancelled (for a different behavior see [supervisorScope]).
 * This function returns as soon as the given block and all its children coroutines are completed.
 * A usage example of a scope looks like this:
 *
 * ```
 * suspend fun showSomeData() = coroutineScope {
 *
 *   val data = async(Dispatchers.IO) { // <- extension on current scope
 *      ... load some UI data for the Main thread ...
 *   }
 *
 *   withContext(Dispatchers.Main) {
 *     doSomeWork()
 *     val result = data.await()
 *     display(result)
 *   }
 * }
 * ```
 *
 * The scope in this example has the following semantics:
 * 1) `showSomeData` returns as soon as the data is loaded and displayed in the UI.
 * 2) If `doSomeWork` throws an exception, then the `async` task is cancelled and `showSomeData` rethrows that exception.
 * 3) If the outer scope of `showSomeData` is cancelled, both started `async` and `withContext` blocks are cancelled.
 * 4) If the `async` block fails, `withContext` will be cancelled.
 *
 * The method may throw a [CancellationException] if the current job was cancelled externally
 * or may throw a corresponding unhandled [Throwable] if there is any unhandled exception in this scope
 * (for example, from a crashed coroutine that was started with [launch][CoroutineScope.launch] in this scope).
 */
public suspend fun <R> coroutineScope(block: suspend CoroutineScope.() -> R): R =
    suspendCoroutineUninterceptedOrReturn { uCont ->
        val coroutine = ScopeCoroutine(uCont.context, uCont)
        coroutine.startUndispatchedOrReturn(coroutine, block)
    }

我想弄清楚它们之间的区别。如果有人能回答何时使用哪一个，那将很有帮助。

Answer 1

CoroutineScope（大写 C 版本）与 coroutineScope（较小的 c 版本）之间的最佳区别，我可以弄清楚，并且很容易理解是将它们与 非结构化 与 结构化相关联并发

让我分享一个例子：

class MainActivity : AppCompatActivity() {
    private lateinit var btn: Button
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        btn = findViewById(R.id.start_btn)
        btn.setOnClickListener {
            CoroutineScope(Dispatchers.Main).launch {
                val result = downloadUserData()
                Toast.makeText(applicationContext, "Result : $result", Toast.LENGTH_LONG).show()
            }
        }
    }

    private suspend fun downloadUserData(): Int {
        var result = 0
        // Here, we use CoroutineScope (Capital C version) which will start a new scope and
        // launch coroutine in new scope Dispatchers.IO, Not In Parent Scope which is Dispatchers.Main
        // Thus, this function would directly return without waiting for loop completion and will return 0
        CoroutineScope(Dispatchers.IO).launch {
            for (i in 0 until 100) {
                kotlinx.coroutines.delay(10)
                result++
            }
        }
        return result
    }
}

输出： Result : 0

这是一个非结构化并发的例子，它不能保证子协程在返回之前完成。因此，调用者/父协程会得到子协程返回的错误值。甚至，当子协程已经返回时，子协程可能正在后台运行（处于活动状态），这在某些情况下可能导致 内存泄漏。

解决方案：

当我们需要在多个协程之间进行通信时，我们需要确保结构化并发（推荐）

这可以通过在子/被调用者协程内重用父/调用者协程范围来完成。这可以通过子/被调用协程内的coroutineScope {}（较小的c）版本来实现。

private suspend fun downloadUserData(): Int {
    var result = 0
    // By using coroutineScope (Smaller c version) below, we ensure that this coroutine would execute in the
    // parent/caller coroutine's scope, so it would make sure that the for loop would complete
    // before returning from this suspended function. This will return 20000 properly
    coroutineScope {
        for (i in 0 until 100) {
            kotlinx.coroutines.delay(10)
            result++
        }
    }
    return result
}

输出：Result : 100

Answer 2

CoroutineScope() 只不过是 CoroutineScope 对象的工厂，CoroutineScope 对象只不过是 CoroutineContext 的持有者。它在协程中没有积极作用，但它是基础架构的重要组成部分，可以轻松正确地进行结构化并发。这是因为所有协程构建器，如 launch 或 async 都是 CoroutineScope 上的扩展函数并继承其上下文。

如果有的话，您很少需要调用 CoroutineScope()，因为通常您要么选择一个现有的协程范围，要么通过其他便利函数（如 Android 上的 MainScope）或 Kotlin 内部为您创建一个协程范围。

另一方面，coroutineScope() 是一个函数，它执行您在子协程中传递的块。它基本上是withContext(this.coroutineContext) 的别名，当你想在前台继续一些工作时启动一个或多个后台协程时，你应该主要使用它，然后在完成块时加入后台协程。

Answer 3

它们是完全不同的两个东西。

CoroutineScope 是定义 Coroutine Scope 概念的接口：要启动和创建协程，您需要一个。

GlobalScope 是一个例如全局范围的实例。

CoroutineScope() 是一个全局函数，它创建一个CoroutineScope

当你有一个作用域时，你可以使用launch() 或async() 或任何其他与执行协程相关的方法。

// create a context
val myContext = Dispacher.IO
// you can combine dispachers, parent jobs etc.
// create the new scope
val myScope: CoroutineScope = CoroutineScope(myContext)
// returns immediately (unless you specify a start mode that run immediately)
val job = myScope.launch {
  // suspend calls are allowed here cause this is a coroutine
}
// this code is executed right away

您可以从协程外部执行此操作（纯代码）。

另一方面，

coroutineScope() 是一个全局挂起函数，它在底层创建一个新的CoroutineScope，然后执行你在正文中传递的挂起函数，并等待它（及其所有子项）在返回之前完成。它是一个挂起函数，所以你不能在协程之外调用它。

// must be inside a coroutine here!

// this create a new CoroutineScope,
// then launch the given coroutine,
// then wait for it to complete
val result = coroutineScope {
   // your coroutine here, which run immediately
   return@coroutineScope "my result"
}
// this code is executed after the coroutine above is completed
// I can use "result" here

与coroutineScope 类似，supervisedScope 仅有 1 个区别：在其中执行的多个子协程（启动/异步/...）不会取消其他子协程，如果其中一个失败导致它使用SupervisorJob

Answer 4

CoroutineScope() 是将Context 作为输入并为您提供Context 和Job 作为CoroutineScope 接口对象的方法。

您可以使用此对象启动协程作业，如下所示：

suspend fun doNotDoThis() {
  CoroutineScope(coroutineContext).launch {
      println("I'm confused")
  }
}

而coroutineScope() 需要block/labmda 作为协程job 执行：

   fun main() = runBlocking { // this: CoroutineScope
    launch { 
        delay(200L)
        println("Task from runBlocking")
    }

    coroutineScope { // Creates a new coroutine scope
        launch {
            delay(500L) 
            println("Task from nested launch")
        }

        delay(100L)
        println("Task from coroutine scope") // This line will be printed before nested launch
    }

    println("Coroutine scope is over") // This line is not printed until nested launch completes
}

参考文献：

Article-1

Article-2

Kotlin-Docs

我希望这能回答你的问题。

Answer 5

在非结构化并发示例中，如果您将 Launch builder 替换为 Async 并在 deferred 上等待，它将与您在结构化并发中使用的示例相同。你的答案还不清楚。解释结构化并发的实际使用（这在异常和错误处理中很有用，当其中一个子作业抛出异常时，应该不会影响其他子作业（作业））