Kotlin协程到底是如何切换线程的

public interface CoroutineScope {
    public val coroutineContext: CoroutineContext
}

public object GlobalScope : CoroutineScope {
    
    override val coroutineContext: CoroutineContext
        get() = EmptyCoroutineContext
}

public val ViewModel.viewModelScope: CoroutineScope
    get() {
        val scope: CoroutineScope? = this.getTag(JOB_KEY)
        if (scope != null) {
            return scope
        }
        return setTagIfAbsent(
            JOB_KEY,
            CloseableCoroutineScope(SupervisorJob() + Dispatchers.Main.immediate)
        )
    }

	fun testOne(){
		GlobalScope.launch {
            print("1:" + Thread.currentThread().name)
            delay(1000)
            print("2:" + Thread.currentThread().name)
        }
	}
	//打印结果为：DefaultDispatcher-worker-1
    fun testTwo(){
        viewModelScope.launch {
            print("1:" + Thread.currentThread().name)
            delay(1000)
            print("2:" + Thread.currentThread().name)
        }
    }
    //打印结果为: main

interface ContinuationInterceptor : CoroutineContext.Element {
    // ContinuationInterceptor 在 CoroutineContext 中的 Key
    compaNIOn object Key : CoroutineContext.Key<ContinuationInterceptor>
    
    fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T>

    //...
}

public fun CoroutineScope.launch(
    context: CoroutineContext = EmptyCoroutineContext,
    start: CoroutineStart = CoroutineStart.DEFAULT,
    block: suspend CoroutineScope.() -> Unit
): Job {
    val newContext = newCoroutineContext(context)
    val coroutine = if (start.isLazy)
        LazyStandaloneCoroutine(newContext, block) else
        StandaloneCoroutine(newContext, active = true)
    coroutine.start(start, coroutine, block)
    return coroutine
}

public actual fun CoroutineScope.newCoroutineContext(context: CoroutineContext): CoroutineContext {
    val combined = coroutineContext + context
    val debug = if (DEBUG) combined + CoroutineId(COROUTINE_ID.incrementAndGet()) else combined
    return if (combined !== Dispatchers.Default && combined[ContinuationInterceptor] == null)
        debug + Dispatchers.Default else debug
}

val coroutine = if (start.isLazy)
        LazyStandaloneCoroutine(newContext, block) else
        StandaloneCoroutine(newContext, active = true)
    coroutine.start(start, coroutine, block)

public fun <R> start(start: CoroutineStart, receiver: R, block: suspend R.() -> T) {
    initParentJob()
    start(block, receiver, this)
}

internal fun <R, T> (suspend (R) -> T).startCoroutineCancellable(receiver: R, completion: Continuation<T>) =
    runSafely(completion) {
        // 外面再包一层 Coroutine
        createCoroutineUnintercepted(receiver, completion)
            // 如果需要，做拦截处理
            .intercepted()
            // 调用 resumeWith 方法      
            .resumeCancellableWith(Result.success(Unit))
    }

public actual fun <T> Continuation<T>.intercepted(): Continuation<T> =
    (this as? ContinuationImpl)?.intercepted() ?: this

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

//CoroutineDispatcher
public final override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> =
      DispatchedContinuation(this, continuation)  

//DispatchedContinuation
    inline fun resumeCancellableWith(
        result: Result<T>,
        noinline onCancellation: ((cause: Throwable) -> Unit)?
    ) {
        val state = result.toState(onCancellation)
        if (dispatcher.isDispatchNeeded(context)) {
            _state = state
            resumeMode = MODE_CANCELLABLE
            dispatcher.dispatch(context, this)
        } else {
            executeUnconfined(state, MODE_CANCELLABLE) {
                if (!resumeCancelled(state)) {
                    resumeUndispatchedWith(result)
                }
            }
        }
    }

internal class HandlerContext private constructor(
    private val handler: Handler,
    private val name: String?,
    private val invokeImmediately: Boolean
) : HandlerDispatcher(), Delay {
    public constructor(
        handler: Handler,
        name: String? = null
    ) : this(handler, name, false)

    //...

    override fun dispatch(context: CoroutineContext, block: Runnable) {
        // 利用主线程的 Handler 执行任务
        handler.post(block)
    }
}

public suspend fun delay(timeMillis: Long) {
    if (timeMillis <= 0) return // don't delay
    return suspendCancellableCoroutine sc@ { cont: CancellableContinuation<Unit> ->
        // if timeMillis == Long.MAX_VALUE then just wait forever like awaitCancellation, don't schedule.
        if (timeMillis < Long.MAX_VALUE) {
            cont.context.delay.scheduleResumeAfterDelay(timeMillis, cont)
        }
    }
}

internal val CoroutineContext.delay: Delay get() = get(ContinuationInterceptor) as? Delay ?: DefaultDelay

internal class HandlerContext private constructor(
    private val handler: Handler,
    private val name: String?,
    private val invokeImmediately: Boolean
) : HandlerDispatcher(), Delay {
    override fun scheduleResumeAfterDelay(timeMillis: Long, continuation: CancellableContinuation<Unit>) {
        // 利用主线程的 Handler 延迟执行任务，将完成的 continuation 放在任务中执行
        val block = Runnable {
            with(continuation) { resumeUndispatched(Unit) }
        }
        handler.postDelayed(block, timeMillis.coerceAtMost(MAX_DELAY))
        continuation.invokeOnCancellation { handler.removeCallbacks(block) }
    }

    //..
}

fun test(){
        viewModelScope.launch(Dispatchers.Main) {
            print("1:" + Thread.currentThread().name)
            withContext(Dispatchers.IO){
                delay(1000)
                print("2:" + Thread.currentThread().name)
            }
            print("3:" + Thread.currentThread().name)
        }
    }
    //1，2，3处分别输出main,DefaultDispatcher-worker-1,main

public suspend fun <T> withContext(
    context: CoroutineContext,
    block: suspend CoroutineScope.() -> T
): T {  
    return suspendCoroutineUninterceptedOrReturn sc@ { uCont ->
        // 创建新的context
        val oldContext = uCont.context
        val newContext = oldContext + context
        ....
        //使用新的Dispatcher，覆盖外层
        val coroutine = DispatchedCoroutine(newContext, uCont)
        coroutine.initParentJob()
        //DispatchedCoroutine作为了complete传入
        block.startCoroutineCancellable(coroutine, coroutine)
        coroutine.getResult()
    }
}

private class DispatchedCoroutine<in T>(
    context: CoroutineContext,
    uCont: Continuation<T>
) : ScopeCoroutine<T>(context, uCont) {
	//在complete时会会回调
    override fun afterCompletion(state: Any?) {
        afterResume(state)
    }

    override fun afterResume(state: Any?) {
        //uCont就是父协程，context仍是老版context,因此可以切换回原来的线程上
        uCont.intercepted().resumeCancellableWith(recoverResult(state, uCont))
    }
}