kotlinx-coroutines-core/jvm/src/Dispatchers.kt

/*
 * Copyright 2016-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
 */

@file:Suppress("unused")

package kotlinx.coroutines

import kotlinx.coroutines.internal.*
import kotlinx.coroutines.scheduling.*
import java.util.*
import kotlin.coroutines.*

/**
 * Name of the property that defines the maximal number of threads that are used by [Dispatchers.IO] coroutines dispatcher.
 */
public const val IO_PARALLELISM_PROPERTY_NAME: String = "kotlinx.coroutines.io.parallelism"

/**
 * Groups various implementations of [CoroutineDispatcher].
 */
public actual object Dispatchers {
    /**
     * The default [CoroutineDispatcher] that is used by all standard builders like
     * [launch][CoroutineScope.launch], [async][CoroutineScope.async], etc
     * if no dispatcher nor any other [ContinuationInterceptor] is specified in their context.
     *
     * It is backed by a shared pool of threads on JVM. By default, the maximal level of parallelism used
     * by this dispatcher is equal to the number of CPU cores, but is at least two.
     * Level of parallelism X guarantees that no more than X tasks can be executed in this dispatcher in parallel.
     */
    @JvmStatic
    public actual val Default: CoroutineDispatcher = createDefaultDispatcher()

    /**
     * A coroutine dispatcher that is confined to the Main thread operating with UI objects.
     * This dispatcher can be used either directly or via [MainScope] factory.
     * Usually such dispatcher is single-threaded.
     *
     * Access to this property may throw [IllegalStateException] if no main thread dispatchers are present in the classpath.
     *
     * Depending on platform and classpath it can be mapped to different dispatchers:
     * - On JS and Native it is equivalent of [Default] dispatcher.
     * - On JVM it is either Android main thread dispatcher, JavaFx or Swing EDT dispatcher. It is chosen by
     *   [`ServiceLoader`](https://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html).
     *
     * In order to work with `Main` dispatcher, the following artifacts should be added to project runtime dependencies:
     *  - `kotlinx-coroutines-android` for Android Main thread dispatcher
     *  - `kotlinx-coroutines-javafx` for JavaFx Application thread dispatcher
     *  - `kotlinx-coroutines-swing` for Swing EDT dispatcher
     *
     * In order to set a custom `Main` dispatcher for testing purposes, add the `kotlinx-coroutines-test` artifact to 
     * project test dependencies.
     *
     * Implementation note: [MainCoroutineDispatcher.immediate] is not supported on Native and JS platforms.
     */
    @JvmStatic
    public actual val Main: MainCoroutineDispatcher get() = MainDispatcherLoader.dispatcher

    /**
     * A coroutine dispatcher that is not confined to any specific thread.
     * It executes initial continuation of the coroutine in the current call-frame
     * and lets the coroutine resume in whatever thread that is used by the corresponding suspending function, without
     * mandating any specific threading policy. Nested coroutines launched in this dispatcher form an event-loop to avoid
     * stack overflows.
     *
     * ### Event loop
     * Event loop semantics is a purely internal concept and have no guarantees on the order of execution
     * except that all queued coroutines will be executed on the current thread in the lexical scope of the outermost
     * unconfined coroutine.
     *
     * For example, the following code:
     * ```
     * withContext(Dispatchers.Unconfined) {
     *    println(1)
     *    withContext(Dispatchers.Unconfined) { // Nested unconfined
     *        println(2)
     *    }
     *    println(3)
     * }
     * println("Done")
     * ```
     * Can print both "1 2 3" and "1 3 2", this is an implementation detail that can be changed.
     * But it is guaranteed that "Done" will be printed only when both `withContext` are completed.
     *
     *
     * Note that if you need your coroutine to be confined to a particular thread or a thread-pool after resumption,
     * but still want to execute it in the current call-frame until its first suspension, then you can use
     * an optional [CoroutineStart] parameter in coroutine builders like
     * [launch][CoroutineScope.launch] and [async][CoroutineScope.async] setting it to the
     * the value of [CoroutineStart.UNDISPATCHED].
     */
    @JvmStatic
    public actual val Unconfined: CoroutineDispatcher = kotlinx.coroutines.Unconfined

    /**
     * The [CoroutineDispatcher] that is designed for offloading blocking IO tasks to a shared pool of threads.
     *
     * Additional threads in this pool are created and are shutdown on demand.
     * The number of threads used by this dispatcher is limited by the value of
     * "`kotlinx.coroutines.io.parallelism`" ([IO_PARALLELISM_PROPERTY_NAME]) system property.
     * It defaults to the limit of 64 threads or the number of cores (whichever is larger).
     *
     * Moreover, the maximum configurable number of threads is capped by the
     * `kotlinx.coroutines.scheduler.max.pool.size` system property.
     * If you need a higher number of parallel threads,
     * you should use a custom dispatcher backed by your own thread pool.
     *
     * This dispatcher shares threads with a [Default][Dispatchers.Default] dispatcher, so using
     * `withContext(Dispatchers.IO) { ... }` does not lead to an actual switching to another thread —
     * typically execution continues in the same thread.
     */
    @JvmStatic
    public val IO: CoroutineDispatcher = DefaultScheduler.IO
}