前言

协程拦截器ContinuationInterceptor作用：

  1. 线程切换
  2. hook结果（一般用不到）

我们开发Swing或者Android更新UI需要在特定的线程上进行操作，那么这里就涉及线程的切换问题，那么我们看看ContinuationInterceptor在本章中如何使用和源码分析。

简单案例


class MyContinuation() : Continuation<String> {
    override val context: CoroutineContext = EmptyCoroutineContext
    override fun resumeWith(result: Result<String>) {
        log("MyContinuation resumeWith 结果 = ${result.getOrNull()}")
    }

}

suspend fun demo() = suspendCoroutine<String> { c ->

    thread(name = "demo1创建的线程") {
        log("demo 调用resume回调")
        c.resume("hello")
    }

}

suspend fun demo2() = suspendCoroutine<String> { c ->
    thread(name = "demo2创建的线程") {
        log("demo2 调用resume回调")
        c.resume("world")
    }
}

fun testInterceptor() {


    // 假设下面的lambda需要在UI线程运行
    val suspendLambda = suspend {
        log("demo 运行前")
        val resultOne = demo()
        log("demo 运行后")
        val resultTwo = demo2()
        log("demo2 运行后")
        //拼接结果
        resultOne + resultTwo
    }

    val myContinuation = MyContinuation()

    thread(name = "一个新的线程") {
        suspendLambda.startCoroutine(myContinuation)

    }
}

fun log(msg: String) {

    println("[${Thread.currentThread().name}] ${msg}")
}

输出：

[一个新的线程] demo 运行前
[demo1创建的线程] demo 调用resume回调
[demo1创建的线程] demo 运行后
[demo2创建的线程] demo2 调用resume回调
[demo2创建的线程] demo2 运行后
[demo2创建的线程] MyContinuation resumeWith 结果 = helloworld

首先我们先我们先明白上面的输出打印的时所在的线程状态。
suspendLambda会在编译时创建一个状态机函数，而这个函数的调用依靠外部调用Continuation.resume函数进行运行，那么这个状态机运行的线程自然会恢复在Continuation.resume调用线程，如果你看完前几章节的内容理解这个并不难。

假设我们期望suspendLambda只运行在Ui线程该如何实现?这里直接使用ContinuationInterceptor即可。

我们这里创建如下对象：

class MyCoroutineDispatch : AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {

    override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> {
        log("interceptContinuation")
        return MyInterceptorContinuation<T>(continuation.context, continuation)
    }

    override fun releaseInterceptedContinuation(continuation: Continuation<*>) {
        super.releaseInterceptedContinuation(continuation)

        log("releaseInterceptedContinuation " + continuation::class.java.simpleName)
    }


    class MyInterceptorContinuation<T>(
        override val context: CoroutineContext,
        val continuation: Continuation<T>
    ) : Continuation<T> {


        override fun resumeWith(result: Result<T>) {
            //获取Android主线程的Looper，进而切换主线程
            Handler(Looper.getMainLooper()).post {
                log("MyInterceptorContinuation resume")
                continuation.resumeWith(result)
            }

        }

    }
}

然后MyContinuation上下文改用MyCoroutineDispatch即可

class MyContinuation() : Continuation<String> {
   //这里不在使用空上下文
   override val context: CoroutineContext = MyCoroutineDispatch()
   override fun resumeWith(result: Result<String>) {
       log("MyContinuation resumeWith 结果 = ${result.getOrNull()}")
   }

}

最后看下完整的代码


class MyCoroutineDispatch : AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {

    override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> {
        log("interceptContinuation")
        return MyInterceptorContinuation<T>(continuation.context, continuation)
    }

    override fun releaseInterceptedContinuation(continuation: Continuation<*>) {
        super.releaseInterceptedContinuation(continuation)

        log("releaseInterceptedContinuation " + continuation::class.java.simpleName)
    }


    class MyInterceptorContinuation<T>(
        override val context: CoroutineContext,
        val continuation: Continuation<T>
    ) : Continuation<T> {


        override fun resumeWith(result: Result<T>) {
            //获取Android主线程的Looper，进而切换主线程
            Handler(Looper.getMainLooper()).post {
                log("MyInterceptorContinuation resume")
                continuation.resumeWith(result)
            }

        }

    }
}

class MyContinuation() : Continuation<String> {
    //这里不在使用空上下文
    override val context: CoroutineContext = MyCoroutineDispatch()
    override fun resumeWith(result: Result<String>) {
        log("MyContinuation resumeWith 结果 = ${result.getOrNull()}")
    }

}

suspend fun demo() = suspendCoroutine<String> { c ->

    thread(name = "demo1创建的线程") {
        log("demo 调用resume回调")
        c.resume("hello")
    }

}

suspend fun demo2() = suspendCoroutine<String> { c ->
    thread(name = "demo2创建的线程") {
        log("demo2 调用resume回调")
        c.resume("world")
    }
}

fun testInterceptor() {


    // 假设下面的lambda需要在UI线程运行
    val suspendLambda = suspend {
        log("demo 运行前")
        val resultOne = demo()
        log("demo 运行后")
        val resultTwo = demo2()
        log("demo2 运行后")
        //拼接结果
        resultOne + resultTwo
    }

    val myContinuation = MyContinuation()

    thread(name = "一个新的线程") {
        suspendLambda.startCoroutine(myContinuation)

    }
}

fun log(msg: String) {

    Log.e("TAG","[${Thread.currentThread().name}] ${msg}")
}

对应输出：

[一个新的线程] interceptContinuation
[main] MyInterceptorContinuation resume
[main] demo 运行前
[demo1创建的线程] demo 调用resume回调
[main] MyInterceptorContinuation resume
[main] demo 运行后
[demo2创建的线程] demo2 调用resume回调
[main] MyInterceptorContinuation resume
[main] demo2 运行后
[main] releaseInterceptedContinuation MyInterceptorContinuation
[main] MyContinuation resumeWith 结果 = helloworld

看到输出后总算看到我们期待的那样suspendLambda输出在main/ui线程

分析源码

MyCoroutineDispatch分析

我们来看MyCoroutineDispatch 类声明

class MyCoroutineDispatch : 
AbstractCoroutineContextElement(ContinuationInterceptor),
ContinuationInterceptor {

}

我们继承AbstractCoroutineContextElement类，并实现了ContinuationInterceptor接口，我们分别看看各自的用处。
AbstractCoroutineContextElement 的声明：

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

可以看到了有实现了Element接口，Element接口是什么？

  public interface Element : CoroutineContext {
   
        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

        public override fun <R> fold(initial: R, operation: (R, Element) -> R): R =
            operation(initial, this)

        public override fun minusKey(key: Key<*>): CoroutineContext =
            if (this.key == key) EmptyCoroutineContext else this
    }

原来这个Element是前面文章介绍上下文相关的东西，简单来说就是可以放入某个协程上下文中的链表存储的对象。而Element本身也是一个上下文对象。在上下文中可以用get函数或者[]操作符获取对应的存储对象。

所以这个MyCoroutineDispatch可以当做上下文使用,并且也可以放入其他上下文存储，自身的key是ContinuationInterceptor。所以他可以放入MyContinuation中做上下文对象。

再看看ContinuationInterceptor

public interface ContinuationInterceptor : CoroutineContext.Element {
  
    companion object Key : CoroutineContext.Key<ContinuationInterceptor>
    
    public fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T>

   
    public fun releaseInterceptedContinuation(continuation: Continuation<*>) {
        /* do nothing by default */
    }

ContinuationInterceptor是一个拦截规范，interceptContinuation传入一个原始continuation对象，然后返回一个代理的Continuation，然后在代理Continuation中进行现场切换。如果不返回代理continuation，直接返回原始continuation 即可。当状态机结束的时候releaseInterceptedContinuation会被调用，参数是interceptContinuation返回的对象。

获取拦截器流程

回过头再看看下面的代码

fun testInterceptor() {


    // 假设下面的lambda需要在UI线程运行
    val suspendLambda = suspend {
        log("demo 运行前")
        val resultOne = demo()
        log("demo 运行后")
        val resultTwo = demo2()
        log("demo2 运行后")
        //拼接结果
        resultOne + resultTwo
    }

    val myContinuation = MyContinuation()

    thread(name = "一个新的线程") {
        suspendLambda.startCoroutine(myContinuation)

    }
}

suspendLambda会被转化为SuspendLambda，SuspendLambda有继承ContinuationImpl

那么来看看这个ContinuationImpl类


internal abstract class ContinuationImpl(
    completion: Continuation<Any?>?,
    private val _context: CoroutineContext?
) : BaseContinuationImpl(completion) {
    constructor(completion: Continuation<Any?>?) : this(completion, completion?.context)
	
    public override val context: CoroutineContext
        get() = _context!!

    @Transient
    private var intercepted: Continuation<Any?>? = null

    public fun intercepted(): Continuation<Any?> =
        intercepted
            ?: (context[ContinuationInterceptor]?.interceptContinuation(this) ?: this)
                .also { intercepted = it }

    protected override fun releaseIntercepted() {
        val intercepted = intercepted
        if (intercepted != null && intercepted !== this) {
            context[ContinuationInterceptor]!!.releaseInterceptedContinuation(intercepted)
        }
        this.intercepted = CompletedContinuation // just in case
    }
}

我们注意下context获取方式

internal abstract class ContinuationImpl(
    completion: Continuation<Any?>?,
    private val _context: CoroutineContext?
) : BaseContinuationImpl(completion) {
	//使用传入completion的上下文作为ContinuationImpl的上下文。
	//MyContinuation是completion，而MyContinuation的上下文MyCoroutineDispatch
	//MyCoroutineDispatch就是我们创建的拦截器
    constructor(completion: Continuation<Any?>?)
     : this(completion, completion?.context)//we
	
    public override val context: CoroutineContext
        get() = _context!!
  }

我们在来看看个函数intercepted

//ContinuationImpl.kt
 @Transient
    private var intercepted: Continuation<Any?>? = null

    public fun intercepted(): Continuation<Any?> =
    //如果拦截器为空那么会做如下三步
    //1.上下文中获取可以为ContinuationInterceptor的拦截器
    //2.调用拦截器interceptContinuation函数获取一个代理Continuation对象。所以拦截器的interceptContinuation只会调用一次
    //3.保存拦截器返回的代理Continuation对象后面方便再次获取就不需要再次调用interceptContinuation
        intercepted
            ?: (context[ContinuationInterceptor]?.interceptContinuation(this) ?: this)
                .also {
                 //保存获取的拦截器
                 intercepted = it 
                 }

我们我们最后看看什么时候第一次调用intercepted的代码。

public fun <T> (suspend () -> T).startCoroutine(
    completion: Continuation<T>
) {
    createCoroutineUnintercepted(completion).intercepted().resume(Unit)
}

启动协程的时候回获取一次拦截器，然后用拦截器返回代理Continuation 的resume

再来看看我们的写的拦截器：


class MyCoroutineDispatch : AbstractCoroutineContextElement(ContinuationInterceptor), ContinuationInterceptor {
		//intercepted()第一次调用的会调用到这里
    override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> {
        log("interceptContinuation")
        //返回一个代理Continuation对象
        return MyInterceptorContinuation<T>(continuation.context, continuation)
    }

    override fun releaseInterceptedContinuation(continuation: Continuation<*>) {
        super.releaseInterceptedContinuation(continuation)

        log("releaseInterceptedContinuation " + continuation::class.java.simpleName)
    }


    class MyInterceptorContinuation<T>(
        override val context: CoroutineContext,
        val continuation: Continuation<T>
    ) : Continuation<T> {


        override fun resumeWith(result: Result<T>) {
            //获取Android主线程的Looper，进而切换主线程
            Handler(Looper.getMainLooper()).post {
                log("MyInterceptorContinuation resume")
                //回调原始的Continuation对象
                continuation.resumeWith(result)
            }

        }

    }
}

当调用启动协程的时候回调用拦截器的代理Continuation对象的resumeWith，然后在Ui线程回调原始Continuation对象。

我们再看看我的挂起函数demo又是怎么切换回ui线程

suspend fun demo() = suspendCoroutine<String> { c ->

    thread(name = "demo1创建的线程") {
        log("demo 调用resume回调")
        c.resume("hello")
    }
}

在正常不启用拦截器的情况会回调suspendLambda在demo1创建的线程线程回调。但是我们发现启用拦截器后被在ui线程回调。而真正做切换的逻辑在suspendCoroutine这个lambda表达式上。

public suspend inline fun <T> suspendCoroutine(crossinline block: (Continuation<T>) -> Unit): T =
    suspendCoroutineUninterceptedOrReturn { c: Continuation<T> ->
        val safe = SafeContinuation(c.intercepted())//返回拦截器的代理的Continuation对象
        block(safe)
        safe.getOrThrow()
    }

这里我们便知道答案。demo()函数拿到的Continuation会经过一层拦截器代理对象，一切便自然解释的通了。

我们画个图来总结下。
在这里插入图片描述

总的来说：拦截器返回一个代理Continuation对象给挂起函数，当挂起函数恢复的时候，恢复代理Continuation的resume函数,最后代理Continuation对象切换指定的线程在回调原始的Continuation对象

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