public interface Continuation<in T> {
    // 續(xù)體的上下文
    public val context: CoroutineContext
    // 該方法用于恢復(fù)續(xù)體的執(zhí)行
    // result為掛起點(diǎn)執(zhí)行完成的返回值，T為返回值的類(lèi)型
    public fun resumeWith(result: Result<T>)
}

public interface ContinuationInterceptor : CoroutineContext.Element {
    // 攔截器的Key
    companion object Key : CoroutineContext.Key<ContinuationInterceptor>
    // 攔截器對(duì)續(xù)體進(jìn)行攔截時(shí)會(huì)調(diào)用該方法，并對(duì)continuation進(jìn)行緩存
    // 攔截判斷：根據(jù)傳入的continuation對(duì)象與返回的continuation對(duì)象是否相同
    public fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T>
    // 當(dāng)interceptContinuation方法攔截的協(xié)程執(zhí)行完畢后，會(huì)調(diào)用該方法
    public fun releaseInterceptedContinuation(continuation: Continuation<*>) {
        /* do nothing by default */
    }
    // get方法多態(tài)實(shí)現(xiàn)
    public override operator fun <E : CoroutineContext.Element> get(key: CoroutineContext.Key<E>): E? {
        @OptIn(ExperimentalStdlibApi::class)
        if (key is AbstractCoroutineContextKey<*, *>) {
            @Suppress("UNCHECKED_CAST")
            return if (key.isSubKey(this.key)) key.tryCast(this) as? E else null
        }
        @Suppress("UNCHECKED_CAST")
        return if (ContinuationInterceptor === key) this as E else null
    }
    // minusKey方法多態(tài)實(shí)現(xiàn)
    public override fun minusKey(key: CoroutineContext.Key<*>): CoroutineContext {
        @OptIn(ExperimentalStdlibApi::class)
        if (key is AbstractCoroutineContextKey<*, *>) {
            return if (key.isSubKey(this.key) && key.tryCast(this) != null) EmptyCoroutineContext else this
        }
        return if (ContinuationInterceptor === key) EmptyCoroutineContext else this
    }
}

public abstract class CoroutineDispatcher :
    AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {
    // ContinuationInterceptor的多態(tài)實(shí)現(xiàn)，調(diào)度器本質(zhì)上就是攔截器
    @ExperimentalStdlibApi
    public companion object Key : AbstractCoroutineContextKey<ContinuationInterceptor, CoroutineDispatcher>(
        ContinuationInterceptor,
        { it as? CoroutineDispatcher })
    // 用于判斷調(diào)度器是否要調(diào)用dispatch方法進(jìn)行調(diào)度，默認(rèn)為true
    public open fun isDispatchNeeded(context: CoroutineContext): Boolean = true
    // 調(diào)度的核心方法，在這里進(jìn)行調(diào)度，執(zhí)行block
    public abstract fun dispatch(context: CoroutineContext, block: Runnable)
    // 如果調(diào)度是由Yield方法觸發(fā)的，默認(rèn)通過(guò)dispatch方法實(shí)現(xiàn)
    @InternalCoroutinesApi
    public open fun dispatchYield(context: CoroutineContext, block: Runnable): Unit = dispatch(context, block)
    // ContinuationInterceptor接口的方法，將續(xù)體包裹成DispatchedContinuation，并傳入當(dāng)前調(diào)度器
    public final override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> =
        DispatchedContinuation(this, continuation)
    // 釋放父協(xié)程與子協(xié)程的關(guān)聯(lián)。
    @InternalCoroutinesApi
    public override fun releaseInterceptedContinuation(continuation: Continuation<*>) {
        (continuation as DispatchedContinuation<*>).reusableCancellableContinuation?.detachChild()
    }
    // 重載了"+"操作，直接返回others
    // 因?yàn)閮蓚€(gè)調(diào)度器相加沒(méi)有意義，同一個(gè)上下文中只能有一個(gè)調(diào)度器
    // 如果需要加的是調(diào)度器對(duì)象，則直接替換成最新的，因此直接返回
    public operator fun plus(other: CoroutineDispatcher): CoroutineDispatcher = other
    override fun toString(): String = "$classSimpleName@$hexAddress"
}

internal abstract class EventLoop : CoroutineDispatcher() {
    // 用于記錄使用當(dāng)前EventLoop的runBlocking方法和Dispatchers.Unconfined調(diào)度器的數(shù)量
    private var useCount = 0L
    // 表示當(dāng)前的EventLoop是否被暴露給其他的線程
    // runBlocking會(huì)將EventLoop暴露給其他線程
    // 因此，當(dāng)runBlocking使用時(shí)，shared必須為true
    private var shared = false
    // Dispatchers.Unconfined調(diào)度器的任務(wù)執(zhí)行隊(duì)列
    private var unconfinedQueue: ArrayQueue<DispatchedTask<*>>? = null
    // 處理任務(wù)隊(duì)列的下一個(gè)任務(wù)，該方法只能在EventLoop所在的線程調(diào)用
    // 返回值<=0，說(shuō)明立刻執(zhí)行下一個(gè)任務(wù)
    // 返回值>0，說(shuō)明等待這段時(shí)間后，執(zhí)行下一個(gè)任務(wù)
    // 返回值為L(zhǎng)ong.MAX_VALUE，說(shuō)明隊(duì)列里沒(méi)有任務(wù)了 
    public open fun processNextEvent(): Long {
        if (!processUnconfinedEvent()) return Long.MAX_VALUE
        return 0
    }
    // 隊(duì)列是否為空
    protected open val isEmpty: Boolean get() = isUnconfinedQueueEmpty
    // 下一個(gè)任務(wù)多長(zhǎng)時(shí)間后執(zhí)行
    protected open val nextTime: Long
        get() {
            val queue = unconfinedQueue ?: return Long.MAX_VALUE
            return if (queue.isEmpty) Long.MAX_VALUE else 0L
        }
    // 任務(wù)的核心處理方法
    public fun processUnconfinedEvent(): Boolean {
        // 若隊(duì)列為空，則返回
        val queue = unconfinedQueue ?: return false
        // 從隊(duì)首取出一個(gè)任務(wù)，如果為空，則返回
        val task = queue.removeFirstOrNull() ?: return false
        // 執(zhí)行
        task.run()
        return true
    }
    // 表示當(dāng)前EventLoop是否可以在協(xié)程上下文中被調(diào)用
    // EventLoop本質(zhì)上也是協(xié)程上下文
    // 如果EventLoop在runBlocking方法中使用，必須返回true
    public open fun shouldBeProcessedFromContext(): Boolean = false
    // 向隊(duì)列中添加一個(gè)任務(wù)
    public fun dispatchUnconfined(task: DispatchedTask<*>) {
        // 若隊(duì)列為空，則創(chuàng)建一個(gè)新的隊(duì)列
        val queue = unconfinedQueue ?:
            ArrayQueue<DispatchedTask<*>>().also { unconfinedQueue = it }
        queue.addLast(task)
    }
    // EventLoop當(dāng)前是否還在被使用
    public val isActive: Boolean
        get() = useCount > 0
    // EventLoop當(dāng)前是否還在被Unconfined調(diào)度器使用
    public val isUnconfinedLoopActive: Boolean
        get() = useCount >= delta(unconfined = true)
    // 判斷隊(duì)列是否為空
    public val isUnconfinedQueueEmpty: Boolean
        get() = unconfinedQueue?.isEmpty ?: true
    // 下面三個(gè)方法用于計(jì)算使用當(dāng)前的EventLoop的runBlocking方法和Unconfined調(diào)度器的數(shù)量
    // useCount是一個(gè)64位的數(shù)，
    // 它的高32位用于記錄Unconfined調(diào)度器的數(shù)量，低32位用于記錄runBlocking方法的數(shù)量
    private fun delta(unconfined: Boolean) =
        if (unconfined) (1L shl 32) else 1L
    fun incrementUseCount(unconfined: Boolean = false) {
        useCount += delta(unconfined)
        // runBlocking中使用，shared為true
        if (!unconfined) shared = true 
    }
    fun decrementUseCount(unconfined: Boolean = false) {
        useCount -= delta(unconfined)
        // 如果EventLoop還在被使用
        if (useCount > 0) return
        assert { useCount == 0L }
        // 如果EventLoop不被使用了，并且在EventLoop中使用過(guò)
        if (shared) {
            // 關(guān)閉相關(guān)資源，并在ThreadLocal中移除
            shutdown()
        }
    }
    protected open fun shutdown() {}
}