public object EmptyCoroutineContext : CoroutineContext, Serializable {
    private const val serialVersionUID: Long = 0
    private fun readResolve(): Any = EmptyCoroutineContext
    public override fun <E : Element> get(key: Key<E>): E? = null
    public override fun <R> fold(initial: R, operation: (R, Element) -> R): R = initial
    public override fun plus(context: CoroutineContext): CoroutineContext = context
    public override fun minusKey(key: Key<*>): CoroutineContext = this
    public override fun hashCode(): Int = 0
    public override fun toString(): String = "EmptyCoroutineContext"
}

internal class CombinedContext(
    // 數(shù)據(jù)結(jié)構(gòu)左邊可能為一個(gè)Element對(duì)象或者還是一個(gè)CombinedContext對(duì)象
    private val left: CoroutineContext,
    // 數(shù)據(jù)結(jié)構(gòu)右邊只能為一個(gè)Element對(duì)象
    private val element: Element
) : CoroutineContext, Serializable {
    override fun <E : Element> get(key: Key<E>): E? {
        var cur = this
        while (true) {
            // 進(jìn)行g(shù)et操作，如果當(dāng)前CombinedContext對(duì)象中存在，則返回
            cur.element[key]?.let { return it }
            // 獲取左邊的上下文對(duì)象
            val next = cur.left
            // 如果是CombinedContext對(duì)象
            if (next is CombinedContext) {
                // 賦值，繼續(xù)循環(huán)
                cur = next
            } else { // 如果不是CombinedContext對(duì)象
                // 進(jìn)行g(shù)et操作，返回
                return next[key]
            }
        }
    }
    // 數(shù)據(jù)結(jié)構(gòu)左右分開操作，從左到右進(jìn)行fold運(yùn)算
    public override fun <R> fold(initial: R, operation: (R, Element) -> R): R =
        operation(left.fold(initial, operation), element)
    public override fun minusKey(key: Key<*>): CoroutineContext {
        // 如果右邊是指定的Element對(duì)象，則返回左邊
        element[key]?.let { return left }
        // 調(diào)用左邊的minusKey方法
        val newLeft = left.minusKey(key)
        return when {
            // 這種情況，說明左邊部分已經(jīng)是去掉指定的Element對(duì)象的，右邊也是如此，因此返回當(dāng)前對(duì)象，不需要在進(jìn)行包裹
            newLeft === left -> this
            // 這種情況，說明左邊部分包含指定的Element對(duì)象，因此返回只右邊
            newLeft === EmptyCoroutineContext -> element
            // 這種情況，返回的左邊部分是新的，因此需要和右邊部分一起包裹后，再返回
            else -> CombinedContext(newLeft, element)
        }
    }
    private fun size(): Int {
        var cur = this
        //左右各一個(gè)
        var size = 2
        while (true) {
            cur = cur.left as? CombinedContext ?: return size
            size++
        }
    }
    // 通過get方法實(shí)現(xiàn)
    private fun contains(element: Element): Boolean =
        get(element.key) == element
    private fun containsAll(context: CombinedContext): Boolean {
        var cur = context
        // 循環(huán)展開每一個(gè)CombinedContext對(duì)象，每個(gè)CombinedContext對(duì)象中的Element對(duì)象都要包含
        while (true) {
            if (!contains(cur.element)) return false
            val next = cur.left
            if (next is CombinedContext) {
                cur = next
            } else {
                return contains(next as Element)
            }
        }
    }
    ...
}

public interface Key<E : Element>
public interface Element : CoroutineContext {
    // 一個(gè)Key對(duì)應(yīng)著一個(gè)Element對(duì)象
    public val key: Key<*>
    // 相等則強(qiáng)制轉(zhuǎn)換并返回，否則則返回空
    public override operator fun <E : Element> get(key: Key<E>): E? =
        @Suppress("UNCHECKED_CAST")
        if (this.key == key) this as E else null
    // 自身與初始值進(jìn)行fold操作
    public override fun <R> fold(initial: R, operation: (R, Element) -> R): R =
        operation(initial, this)
    // 如果要去除的是當(dāng)前的Element對(duì)象，則返回空的上下文，否則返回自身
    public override fun minusKey(key: Key<*>): CoroutineContext =
        if (this.key == key) EmptyCoroutineContext else this
}

public interface CoroutineContext {
    public operator fun <E : Element> get(key: Key<E>): E?
    public fun <R> fold(initial: R, operation: (R, Element) -> R): R
    public operator fun plus(context: CoroutineContext): CoroutineContext =
        // 如果要與空上下文相加，則直接但會(huì)當(dāng)前對(duì)象，
        if (context === EmptyCoroutineContext) this else
            // 當(dāng)前Element作為初始值
            context.fold(this) { acc, element ->
                // acc:已經(jīng)加完的CoroutineContext對(duì)象
                // element:當(dāng)前要加的CoroutineContext對(duì)象
                // 獲取從acc中去掉element后的上下文removed，這步是為了確保添加重復(fù)的Element時(shí)，移動(dòng)到最右側(cè)
                val removed = acc.minusKey(element.key)
                // 去除掉element后為空上下文(說明acc中只有一個(gè)Element對(duì)象)，則返回element
                if (removed === EmptyCoroutineContext) element else {
                    // ContinuationInterceptor代表攔截器，也是一個(gè)Element對(duì)象
                    // 下面的操作是為了把攔截器移動(dòng)到上下文的最右端，為了方便快速獲取
                    // 從removed中獲取攔截器
                    val interceptor = removed[ContinuationInterceptor]
                    // 若上下文中沒有攔截器，則進(jìn)行累加(包裹成CombinedContext對(duì)象)，返回
                    if (interceptor == null) CombinedContext(removed, element) else {
                        // 若上下文中有攔截器
                        // 獲取上下文中移除到掉攔截器后的上下文left
                        val left = removed.minusKey(ContinuationInterceptor)
                        // 若移除到掉攔截器后的上下文為空上下文，說明上下文left中只有一個(gè)攔截器，
                        // 則進(jìn)行累加(包裹成CombinedContext對(duì)象)，返回
                        if (left === EmptyCoroutineContext) CombinedContext(element, interceptor) else
                            // 否則，現(xiàn)對(duì)當(dāng)前要加的element和left進(jìn)行累加，然后在和攔截器進(jìn)行累加
                            CombinedContext(CombinedContext(left, element), interceptor)
                    }
                }
            }
    public fun minusKey(key: Key<*>): CoroutineContext
    ... // (Key和Element接口)
}

public abstract class AbstractCoroutineContextElement(public override val key: Key<*>) : Element

// baseKey為衍生類的基類的Key
// safeCast用于對(duì)基類進(jìn)行轉(zhuǎn)換
// B為基類，E為衍生類
public abstract class AbstractCoroutineContextKey<B : Element, E : B>(
    baseKey: Key<B>,
    private val safeCast: (element: Element) -> E?
) : Key<E> {
    // 頂置Key，如果baseKey是AbstractCoroutineContextKey，則獲取baseKey的頂置Key
    private val topmostKey: Key<*> = if (baseKey is AbstractCoroutineContextKey<*, *>) baseKey.topmostKey else baseKey
    // 用于類型轉(zhuǎn)換
    internal fun tryCast(element: Element): E? = safeCast(element)
    // 用于判斷當(dāng)前key是否是指定key的子key
    // 邏輯為與當(dāng)前key相同，或者與當(dāng)前key的頂置key相同
    internal fun isSubKey(key: Key<*>): Boolean = key === this || topmostKey === key
}

public fun <E : Element> Element.getPolymorphicElement(key: Key<E>): E? {
    // 如果key是AbstractCoroutineContextKey
    if (key is AbstractCoroutineContextKey<*, *>) {
        // 如果key是當(dāng)前key的子key，則基類強(qiáng)制轉(zhuǎn)換成衍生類，并返回
        @Suppress("UNCHECKED_CAST")
        return if (key.isSubKey(this.key)) key.tryCast(this) as? E else null
    }
    // 如果key不是AbstractCoroutineContextKey
    // 如果key相等，則強(qiáng)制轉(zhuǎn)換，并返回
    @Suppress("UNCHECKED_CAST")
    return if (this.key === key) this as E else null
}

public fun Element.minusPolymorphicKey(key: Key<*>): CoroutineContext {
    // 如果key是AbstractCoroutineContextKey
    if (key is AbstractCoroutineContextKey<*, *>) {
        // 如果key是當(dāng)前key的子key，基類強(qiáng)制轉(zhuǎn)換后不為空，說明當(dāng)前Element需要去掉，因此返回空上下文，否則返回自身
        return if (key.isSubKey(this.key) && key.tryCast(this) != null) EmptyCoroutineContext else this
    }
    // 如果key不是AbstractCoroutineContextKey
    // 如果key相等，說明當(dāng)前Element需要去掉，因此返回空上下文，否則返回自身
    return if (this.key === key) EmptyCoroutineContext else this
}