Kotlin 協(xié)程?(二）協(xié)程取消與超時(shí)
所有的協(xié)程都是在上下文中運(yùn)行,而協(xié)程上下文包含一個(gè)協(xié)程調(diào)度器.
協(xié)程調(diào)度器確定了相關(guān)的協(xié)程在哪個(gè)線程或哪些線程上執(zhí)行,協(xié)程調(diào)度器可以將協(xié)程限制在一個(gè)特定的線程執(zhí)行,或?qū)⑺峙傻揭粋€(gè)線程池,亦或是讓它不受限地運(yùn)行.
協(xié)程調(diào)度器源碼:
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
}
從源碼可以得知
Main 是運(yùn)行在主線程的及在Android中進(jìn)行UI繪制的線程.一般用于處理UI繪制及一些輕量級(jí)任務(wù)
Default 是運(yùn)行在非主線程的,一般用于CPU密集型計(jì)算型任務(wù).如數(shù)組排序,JSON解析,異常判斷等
IO 是運(yùn)行在非主線程的,一般用于磁盤(pán)操作和網(wǎng)絡(luò)IO操作任務(wù).如數(shù)據(jù)庫(kù)操作,文件讀寫(xiě),網(wǎng)絡(luò)請(qǐng)求等
Unconfined 未定義的線程,是一個(gè)特殊的調(diào)度器且似乎也運(yùn)行在 main 線程中,但實(shí)際上它是一種不同的機(jī)制.協(xié)程調(diào)度器在調(diào)用它的線程啟動(dòng)了一個(gè)協(xié)程,但它僅僅只是運(yùn)行到第一個(gè)掛起點(diǎn).掛起后,它恢復(fù)線程中的協(xié)程,而這完全由被調(diào)用的掛起函數(shù)來(lái)決定.非受限的調(diào)度器非常適用于執(zhí)行不消耗 CPU 時(shí)間的任務(wù),以及不更新局限于特定線程的任何共享數(shù)據(jù)（如UI）的協(xié)程
