Android kotlin协程同步代码请求 kotlin协程源码

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mob64ca14038b36 2024-07-05 04:36:55

上下文基本概念：
上下文可以理解为当前环境存储各类有用信息，再简单说就是存储数据的一个集合。
集合:
存放相同数据结构的一个容器，而容器的实现有很多种，链表类型，数组类型，树类型。
而在Kotlin中Context作为容器的实现采用一个链表的方式，而这个链表实现方式和我们所认知的有少许的不同。

首先来看Context声明

/**
 * 协程上下文对象。上下文就是个容器，这里采用了左向链表的实现
 */
public interface CoroutineContext {

    /**
     * 传入一个key对象返回对应数据
     */
    public operator fun <E : Element> get(key: Key<E>): E?


    /**
     * 传入一个初始化的数值 [initial]，[initial]其实就是CoroutineContext类型。然后遍历this的链表元素[Element],然后返回一个新的上下文
     *
     */
    public fun <R> fold(initial: R, operation: (R, Element) -> R): R

    /**
     *  将[传入的context]内容拼接当前[this的链表内容] 然后返回一个新的上下文，新的上下文包含
     *  插入的新元素（不可变来实现线程安全，[this的链表内容]原有的链表元素还在）。[传入的context]的内容如果和[this的链表内容]有重复那么去除[this的链表内容]重复内容
     */
    public operator fun plus(context: CoroutineContext): CoroutineContext =
        //传入的元素是空那么必要拼接直接返回
        if (context === EmptyCoroutineContext) this else // fast path -- avoid lambda creation
            //[this的链表内容]作为acc参数启动fold参数，然后遍历[传入的context]所有元素
            context.fold(this) { acc, element ->
                //element元素是[传入的context]中存储的个对象
                //minusKey函数用于去除某个上下文链表中一个元素，然后返回一个新的上下文，这里不会改变原来element这个对象的链表（不可变的线程安全模式）
                val removed = acc.minusKey(element.key)
                //EmptyCoroutineContext 返回时表示链表没有任何元素
                if (removed === EmptyCoroutineContext) element else {
                    //获取一个拦截器对象，由于拦截器对象太常用了，所以这里是做优化，把拦截器放在链表末尾
                    //链表放末尾容易更快速取得。如果当前element是ContinuationInterceptor对象话，这里一定返回null（上一步就去除了重复类）
                    val interceptor = removed[ContinuationInterceptor]
                    //如果拦截器对象为空的话，直接拼接CombinedContext对象中。CombinedContext对象可以视为一个节点对象
                    if (interceptor == null) CombinedContext(removed, element) else {
                        //拦截器不为空，在[传入的context]去除它，返回一个不包含的它的上下文
                        val left = removed.minusKey(ContinuationInterceptor)
                        //为了严谨起见在判断一次left是不是为空（minusKey和get函数有可能子类实现机制不统一导致问题，如果根据规范判空可以省略）
                        if (left === EmptyCoroutineContext) CombinedContext(element, interceptor) else
                            //链表放最后
                            CombinedContext(CombinedContext(left, element), interceptor)
                    }
                }
            }

    /**
     *返回一个去除链表中key的对象的上下文。
     */
    public fun minusKey(key: Key<*>): CoroutineContext

    /**
     *  Key 标识接口
     */
    public interface Key<E : Element>

    /**
     * 上下文链表中元素标识接口
     */
    public interface Element : CoroutineContext {
        /**
         * 这个元素对应的键
         */
        public val key: Key<*>

        /**
         * 
         */
        public override operator fun <E : Element> get(key: Key<E>): E? =
            @Suppress("UNCHECKED_CAST")
            if (this.key == key) this as E else null

        /**
         * 进行累加的高阶函数。由于element对象不是一个集合对象，所以这里不会递归，只会执行一次 operation
         */
        public override fun <R> fold(initial: R, operation: (R, Element) -> R): R =
            operation(initial, this)

        /**
         * 如果当前key相等那么返回EmptyCoroutineContext。这个函数寓意就是用来去除key
         */
        public override fun minusKey(key: Key<*>): CoroutineContext =
            if (this.key == key) EmptyCoroutineContext else this
    }
}

链表节点对象CombinedContext，CombinedContext可以理解为上下文中每个节点，而元素存储属性element，上一个节点用属性left引用
由于都是简单的数据结构我这里不在做注释

// this class is not exposed, but is hidden inside implementations
// this is a left-biased list, so that `plus` works naturally
@SinceKotlin("1.3")
internal class CombinedContext(
    private val left: CoroutineContext,
    private val element: Element
) : CoroutineContext, Serializable {

    override fun <E : Element> get(key: Key<E>): E? {
        var cur = this
        while (true) {
            cur.element[key]?.let { return it }
            val next = cur.left
            if (next is CombinedContext) {
                cur = next
            } else {
                return next[key]
            }
        }
    }

    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 {
        element[key]?.let { return left }
        val newLeft = left.minusKey(key)
        return when {
            newLeft === left -> this
            newLeft === EmptyCoroutineContext -> element
            else -> CombinedContext(newLeft, element)
        }
    }

    private fun size(): Int {
        var cur = this
        var size = 2
        while (true) {
            cur = cur.left as? CombinedContext ?: return size
            size++
        }
    }

    private fun contains(element: Element): Boolean =
        get(element.key) == element

    private fun containsAll(context: CombinedContext): Boolean {
        var cur = context
        while (true) {
            if (!contains(cur.element)) return false
            val next = cur.left
            if (next is CombinedContext) {
                cur = next
            } else {
                return contains(next as Element)
            }
        }
    }

    override fun equals(other: Any?): Boolean =
        this === other || other is CombinedContext && other.size() == size() && other.containsAll(this)

    override fun hashCode(): Int = left.hashCode() + element.hashCode()

    override fun toString(): String =
        "[" + fold("") { acc, element ->
            if (acc.isEmpty()) element.toString() else "$acc, $element"
        } + "]"

    private fun writeReplace(): Any {
        val n = size()
        val elements = arrayOfNulls<CoroutineContext>(n)
        var index = 0
        fold(Unit) { _, element -> elements[index++] = element }
        check(index == n)
        @Suppress("UNCHECKED_CAST")
        return Serialized(elements as Array<CoroutineContext>)
    }

    private class Serialized(val elements: Array<CoroutineContext>) : Serializable {
        companion object {
            private const val serialVersionUID: Long = 0L
        }

        private fun readResolve(): Any = elements.fold(EmptyCoroutineContext, CoroutineContext::plus)
    }
}

来个个案例吧：

public class My4CoroutineName(
    val name: String
) : AbstractCoroutineContextElement(My4CoroutineName) {

    public companion object Key : CoroutineContext.Key<My4CoroutineName>
    override fun toString(): String = "CoroutineName($name)"
}

public class My5CoroutineName(  
    val name: String
) : AbstractCoroutineContextElement(My5CoroutineName) {
    public companion object Key : CoroutineContext.Key<My5CoroutineName>
    override fun toString(): String = "CoroutineName($name)"
}


public class My3CoroutineName(
    val name: String
) : AbstractCoroutineContextElement(My3CoroutineName) {
    public companion object Key : CoroutineContext.Key<My3CoroutineName>
    override fun toString(): String = "CoroutineName($name)"
}


fun studyContext() {

    val my3CoroutineName = My3CoroutineName("hello")
    val my4CoroutineName = My4CoroutineName("world")
    val my5CoroutineName = My5CoroutineName("world")

    val newElement = my3CoroutineName + my4CoroutineName + my5CoroutineName

    println("<top>.studyContext")
}

Android kotlin协程同步代码请求 kotlin协程源码_拦截器

看下删除操作吧：

fun studyContext() {

    val my3CoroutineName = My3CoroutineName("hello")
    val my4CoroutineName = My4CoroutineName("world")
    val my5CoroutineName = My5CoroutineName("world")

    val newElement = my3CoroutineName + my4CoroutineName + my5CoroutineName


    val minusKey = newElement.minusKey(My3CoroutineName)
    println("<top>.studyContext")

}

Android kotlin协程同步代码请求 kotlin协程源码_拦截器_02