launch(Dispatchers.Main) {
    flow {
        emit(1)
        emit(2)
    }.collect {
        delay(1000)

        withContext(Dispatchers.IO) {
            Log.d("liduo", "$it")
        }

        Log.d("liduo", "$it")
    }
}

public fun <T> flow(@BuilderInference block: suspend FlowCollector<T>.() -> Unit): Flow<T> = SafeFlow(block)

public interface FlowCollector<in T> {
    // 可掛起，非線程安全
    public suspend fun emit(value: T)
}

public interface Flow<out T> {
   
    @InternalCoroutinesApi
    public suspend fun collect(collector: FlowCollector<T>)
}

public suspend inline fun <T> Flow<T>.collect(crossinline action: suspend (value: T) -> Unit): Unit =
    collect(object : FlowCollector<T> {
        override suspend fun emit(value: T) = action(value)
    })

private class SafeFlow<T>(private val block: suspend FlowCollector<T>.() -> Unit) : AbstractFlow<T>() {
    override suspend fun collectSafely(collector: FlowCollector<T>) {
        collector.block()
    }
}

@FlowPreview
public abstract class AbstractFlow<T> : Flow<T>, CancellableFlow<T> {

     // 核心方法 
    @InternalCoroutinesApi
    public final override suspend fun collect(collector: FlowCollector<T>) {
        // 創(chuàng)建SafeCollector對(duì)象，對(duì)collector進(jìn)行包裹
        val safeCollector = SafeCollector(collector, coroutineContext)
        try {
            // 調(diào)用collectSafely方法
            collectSafely(safeCollector)
        } finally {
            // 釋放攔截的續(xù)體
            safeCollector.releaseIntercepted()
        }
    }
    
    public abstract suspend fun collectSafely(collector: FlowCollector<T>)
}

internal actual class SafeCollector<T> actual constructor(
    @JvmField internal actual val collector: FlowCollector<T>,
    @JvmField internal actual val collectContext: CoroutineContext
) : FlowCollector<T>, ContinuationImpl(NoOpContinuation, EmptyCoroutineContext), CoroutineStackFrame {
    
    ...
    // 保存上下文中元素?cái)?shù)量，用于檢查上下文是否變化
    @JvmField
    internal actual val collectContextSize = collectContext.fold(0) { count, _ -> count + 1 }
    // 保存上一次的上下文
    private var lastEmissionContext: CoroutineContext? = null
    // 執(zhí)行結(jié)束后的續(xù)體
    private var completion: Continuation<Unit>? = null

    // 協(xié)程上下文
    override val context: CoroutineContext
        get() = completion?.context ?: EmptyCoroutineContext

    // 掛起的核心方法
    override fun invokeSuspend(result: Result<Any?>): Any? {
        result.onFailure { lastEmissionContext = DownstreamExceptionElement(it) }
        completion?.resumeWith(result as Result<Unit>)
        return COROUTINE_SUSPENDED
    }

    // 釋放攔截的續(xù)體
    public actual override fun releaseIntercepted() {
        super.releaseIntercepted()
    }

    // 發(fā)射數(shù)據(jù)
    override suspend fun emit(value: T) {
        // 獲取當(dāng)前suspend方法續(xù)體
        return suspendCoroutineUninterceptedOrReturn sc@{ uCont ->
            try {
                // 調(diào)用重載的方法
                emit(uCont, value)
            } catch (e: Throwable) {
                 // 出現(xiàn)異常時(shí)，將異常封裝成上下文，保存到lastEmissionContext
                lastEmissionContext = DownstreamExceptionElement(e)
                // 拋出異常
                throw e
            }
        }
    }

    // 重載的emit方法
    private fun emit(uCont: Continuation<Unit>, value: T): Any? {
        // 從續(xù)體中獲取上下文
        val currentContext = uCont.context
        // 保證當(dāng)前協(xié)程的Job是active的
        currentContext.ensureActive()
        // 獲取上次的上下文
        val previousContext = lastEmissionContext
        // 如果前后上下文發(fā)生變化
        if (previousContext !== currentContext) {
            // 檢查上下文是否發(fā)生異常
            checkContext(currentContext, previousContext, value)
        }
        // 保存續(xù)體
        completion = uCont
        // 調(diào)用emitFun方法，傳入collector，value，continuation
        return emitFun(collector as FlowCollector<Any?>, value, this as Continuation<Unit>)
    }

    // 檢查上下文變化，防止并發(fā)
    private fun checkContext(
        currentContext: CoroutineContext,
        previousContext: CoroutineContext?,
        value: T
    ) {
        // 如果上次執(zhí)行過(guò)程中發(fā)生了異常
        if (previousContext is DownstreamExceptionElement) {
            // 拋出異常
            exceptionTransparencyViolated(previousContext, value)
        }
        // 檢查上下文是否發(fā)生變化，如果變化，則拋出異常
        checkContext(currentContext)
        lastEmissionContext = currentContext
    }
    
    // 用于拋出異常
    private fun exceptionTransparencyViolated(exception: DownstreamExceptionElement, value: Any?) {
        error("""
            Flow exception transparency is violated:
                Previous 'emit' call has thrown exception ${exception.e}, but then emission attempt of value '$value' has been detected.
                Emissions from 'catch' blocks are prohibited in order to avoid unspecified behaviour, 'Flow.catch' operator can be used instead.
                For a more detailed explanation, please refer to Flow documentation.
            """.trimIndent())
    }
}

private val emitFun =
    FlowCollector<Any?>::emit as Function3<FlowCollector<Any?>, Any?, Continuation<Unit>, Any?>

@Nullable
public final Object invoke(@NotNull FlowCollector p1, @Nullable Object p2, @NotNull Continuation continuation) {
   InlineMarker.mark(0);
   // 核心執(zhí)行
   Object var10000 = p1.emit(p2, continuation);
   InlineMarker.mark(2);
   InlineMarker.mark(1);
   return var10000;
}

public suspend inline fun <T> Flow<T>.collect(crossinline action: suspend (value: T) -> Unit): Unit =
    collect(object : FlowCollector<T> {
        override suspend fun emit(value: T) = action(value)
    })

override suspend fun emit(value: T) {
    // 獲取當(dāng)前suspend方法續(xù)體
    return suspendCoroutineUninterceptedOrReturn sc@{ uCont ->
        try {
            // 調(diào)用重載的方法
            emit(uCont, value)
        } catch (e: Throwable) {
            // 出現(xiàn)異常時(shí)，將異常封裝成上下文，保存到lastEmissionContext
            lastEmissionContext = DownstreamExceptionElement(e)
            // 拋出異常
            throw e
        }
    }
}

emitFun(collector as FlowCollector<Any?>, value, this as Continuation<Unit>)

override fun invokeSuspend(result: Result<Any?>): Any? {
    // 嘗試獲取異常
    result.onFailure { lastEmissionContext = DownstreamExceptionElement(it) }
    // 如果沒(méi)有異常，則恢復(fù)flow方法續(xù)體的執(zhí)行
    completion?.resumeWith(result as Result<Unit>)
    // 返回掛起標(biāo)識(shí)，這里掛起的是消費(fèi)過(guò)程
    return COROUTINE_SUSPENDED
}