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Coroutines support libraries for Kotlin
/*
* Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
*/
@file:OptIn(ExperimentalContracts::class)
package kotlinx.coroutines
import kotlinx.coroutines.internal.*
import kotlinx.coroutines.intrinsics.*
import kotlin.contracts.*
import kotlin.coroutines.*
import kotlin.coroutines.intrinsics.*
/**
* Defines a scope for new coroutines. Every **coroutine builder** (like [launch], [async], etc.)
* is an extension on [CoroutineScope] and inherits its [coroutineContext][CoroutineScope.coroutineContext]
* to automatically propagate all its elements and cancellation.
*
* The best ways to obtain a standalone instance of the scope are [CoroutineScope()] and [MainScope()] factory functions,
* taking care to cancel these coroutine scopes when they are no longer needed (see section on custom usage below for
* explanation and example).
*
* Additional context elements can be appended to the scope using the [plus][CoroutineScope.plus] operator.
*
* ### Convention for structured concurrency
*
* Manual implementation of this interface is not recommended, implementation by delegation should be preferred instead.
* By convention, the [context of a scope][CoroutineScope.coroutineContext] should contain an instance of a
* [job][Job] to enforce the discipline of **structured concurrency** with propagation of cancellation.
*
* Every coroutine builder (like [launch], [async], and others)
* and every scoping function (like [coroutineScope] and [withContext]) provides _its own_ scope
* with its own [Job] instance into the inner block of code it runs.
* By convention, they all wait for all the coroutines inside their block to complete before completing themselves,
* thus enforcing the structured concurrency. See [Job] documentation for more details.
*
* ### Android usage
*
* Android has first-party support for coroutine scope in all entities with the lifecycle.
* See [the corresponding documentation](https://developer.android.com/topic/libraries/architecture/coroutines#lifecyclescope).
*
* ### Custom usage
*
* `CoroutineScope` should be declared as a property on entities with a well-defined lifecycle that are
* responsible for launching children coroutines. The corresponding instance of `CoroutineScope` shall be created
* with either `CoroutineScope()` or `MainScope()` functions. The difference between them is only in the
* [CoroutineDispatcher]:
*
* * `CoroutineScope()` uses [Dispatchers.Default] for its coroutines.
* * `MainScope()` uses [Dispatchers.Main] for its coroutines.
*
* **The key part of custom usage of `CustomScope` is cancelling it at the end of the lifecycle.**
* The [CoroutineScope.cancel] extension function shall be used when the entity that was launching coroutines
* is no longer needed. It cancels all the coroutines that might still be running on behalf of it.
*
* For example:
*
* ```
* class MyUIClass {
* val scope = MainScope() // the scope of MyUIClass, uses Dispatchers.Main
*
* fun destroy() { // destroys an instance of MyUIClass
* scope.cancel() // cancels all coroutines launched in this scope
* // ... do the rest of cleanup here ...
* }
*
* /*
* * Note: if this instance is destroyed or any of the launched coroutines
* * in this method throws an exception, then all nested coroutines are cancelled.
* */
* fun showSomeData() = scope.launch { // launched in the main thread
* // ... here we can use suspending functions or coroutine builders with other dispatchers
* draw(data) // draw in the main thread
* }
* }
* ```
*/
public interface CoroutineScope {
/**
* The context of this scope.
* Context is encapsulated by the scope and used for implementation of coroutine builders that are extensions on the scope.
* Accessing this property in general code is not recommended for any purposes except accessing the [Job] instance for advanced usages.
*
* By convention, should contain an instance of a [job][Job] to enforce structured concurrency.
*/
public val coroutineContext: CoroutineContext
}
/**
* Adds the specified coroutine context to this scope, overriding existing elements in the current
* scope's context with the corresponding keys.
*
* This is a shorthand for `CoroutineScope(thisScope + context)`.
*/
public operator fun CoroutineScope.plus(context: CoroutineContext): CoroutineScope =
ContextScope(coroutineContext + context)
/**
* Creates the main [CoroutineScope] for UI components.
*
* Example of use:
* ```
* class MyAndroidActivity {
* private val scope = MainScope()
*
* override fun onDestroy() {
* super.onDestroy()
* scope.cancel()
* }
* }
* ```
*
* The resulting scope has [SupervisorJob] and [Dispatchers.Main] context elements.
* If you want to append additional elements to the main scope, use [CoroutineScope.plus] operator:
* `val scope = MainScope() + CoroutineName("MyActivity")`.
*/
@Suppress("FunctionName")
public fun MainScope(): CoroutineScope = ContextScope(SupervisorJob() + Dispatchers.Main)
/**
* Returns `true` when the current [Job] is still active (has not completed and was not cancelled yet).
*
* Check this property in long-running computation loops to support cancellation:
* ```
* while (isActive) {
* // do some computation
* }
* ```
*
* This property is a shortcut for `coroutineContext.isActive` in the scope when
* [CoroutineScope] is available.
* See [coroutineContext][kotlin.coroutines.coroutineContext],
* [isActive][kotlinx.coroutines.isActive] and [Job.isActive].
*/
@Suppress("EXTENSION_SHADOWED_BY_MEMBER")
public val CoroutineScope.isActive: Boolean
get() = coroutineContext[Job]?.isActive ?: true
/**
* A global [CoroutineScope] not bound to any job.
* Global scope is used to launch top-level coroutines which are operating on the whole application lifetime
* and are not cancelled prematurely.
*
* Active coroutines launched in `GlobalScope` do not keep the process alive. They are like daemon threads.
*
* This is a **delicate** API. It is easy to accidentally create resource or memory leaks when
* `GlobalScope` is used. A coroutine launched in `GlobalScope` is not subject to the principle of structured
* concurrency, so if it hangs or gets delayed due to a problem (e.g. due to a slow network), it will stay working
* and consuming resources. For example, consider the following code:
*
* ```
* fun loadConfiguration() {
* GlobalScope.launch {
* val config = fetchConfigFromServer() // network request
* updateConfiguration(config)
* }
* }
* ```
*
* A call to `loadConfiguration` creates a coroutine in the `GlobalScope` that works in background without any
* provision to cancel it or to wait for its completion. If a network is slow, it keeps waiting in background,
* consuming resources. Repeated calls to `loadConfiguration` will consume more and more resources.
*
* ### Possible replacements
*
* In many cases uses of `GlobalScope` should be removed, marking the containing operation with `suspend`, for example:
*
* ```
* suspend fun loadConfiguration() {
* val config = fetchConfigFromServer() // network request
* updateConfiguration(config)
* }
* ```
*
* In cases when `GlobalScope.launch` was used to launch multiple concurrent operations, the corresponding
* operations shall be grouped with [coroutineScope] instead:
*
* ```
* // concurrently load configuration and data
* suspend fun loadConfigurationAndData() {
* coroutinesScope {
* launch { loadConfiguration() }
* launch { loadData() }
* }
* }
* ```
*
* In top-level code, when launching a concurrent operation from a non-suspending context, an appropriately
* confined instance of [CoroutineScope] shall be used instead of a `GlobalScope`. See docs on [CoroutineScope] for
* details.
*
* ### GlobalScope vs custom scope
*
* Do not replace `GlobalScope.launch { ... }` with `CoroutineScope().launch { ... }` constructor function call.
* The latter has the same pitfalls as `GlobalScope`. See [CoroutineScope] documentation on the intended usage of
* `CoroutineScope()` constructor function.
*
* ### Legitimate use-cases
*
* There are limited circumstances under which `GlobalScope` can be legitimately and safely used, such as top-level background
* processes that must stay active for the whole duration of the application's lifetime. Because of that, any use
* of `GlobalScope` requires an explicit opt-in with `@OptIn(DelicateCoroutinesApi::class)`, like this:
*
* ```
* // A global coroutine to log statistics every second, must be always active
* @OptIn(DelicateCoroutinesApi::class)
* val globalScopeReporter = GlobalScope.launch {
* while (true) {
* delay(1000)
* logStatistics()
* }
* }
* ```
*/
@DelicateCoroutinesApi
public object GlobalScope : CoroutineScope {
/**
* Returns [EmptyCoroutineContext].
*/
override val coroutineContext: CoroutineContext
get() = EmptyCoroutineContext
}
/**
* 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 coroutineScope(block: suspend CoroutineScope.() -> R): R {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
return suspendCoroutineUninterceptedOrReturn { uCont ->
val coroutine = ScopeCoroutine(uCont.context, uCont)
coroutine.startUndispatchedOrReturn(coroutine, block)
}
}
/**
* 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, failure of any child coroutine in this scope or [cancellation][CoroutineScope.cancel] of the scope itself
* cancels all the scope's children, just like inside [coroutineScope] block.
*/
@Suppress("FunctionName")
public fun CoroutineScope(context: CoroutineContext): CoroutineScope =
ContextScope(if (context[Job] != null) context else context + Job())
/**
* Cancels this scope, including its job and all its children with an optional cancellation [cause].
* A cause can be used to specify an error message or to provide other details on
* a cancellation reason for debugging purposes.
* Throws [IllegalStateException] if the scope does not have a job in it.
*/
public fun CoroutineScope.cancel(cause: CancellationException? = null) {
val job = coroutineContext[Job] ?: error("Scope cannot be cancelled because it does not have a job: $this")
job.cancel(cause)
}
/**
* Cancels this scope, including its job and all its children with a specified diagnostic error [message].
* A [cause] can be specified to provide additional details on a cancellation reason for debugging purposes.
* Throws [IllegalStateException] if the scope does not have a job in it.
*/
public fun CoroutineScope.cancel(message: String, cause: Throwable? = null): Unit = cancel(CancellationException(message, cause))
/**
* Ensures that current scope is [active][CoroutineScope.isActive].
*
* If the job is no longer active, throws [CancellationException].
* If the job was cancelled, thrown exception contains the original cancellation cause.
* This function does not do anything if there is no [Job] in the scope's [coroutineContext][CoroutineScope.coroutineContext].
*
* This method is a drop-in replacement for the following code, but with more precise exception:
* ```
* if (!isActive) {
* throw CancellationException()
* }
* ```
*
* @see CoroutineContext.ensureActive
*/
public fun CoroutineScope.ensureActive(): Unit = coroutineContext.ensureActive()
/**
* Returns the current [CoroutineContext] retrieved by using [kotlin.coroutines.coroutineContext].
* This function is an alias to avoid name clash with [CoroutineScope.coroutineContext] in a receiver position:
*
* ```
* launch { // this: CoroutineScope
* val flow = flow {
* coroutineContext // Resolves into the context of outer launch, which is incorrect, see KT-38033
* currentCoroutineContext() // Retrieves actual context where the flow is collected
* }
* }
* ```
*/
public suspend inline fun currentCoroutineContext(): CoroutineContext = coroutineContext