1. launchWhenX 当处于不活跃状态时，通过pause() 修改变量 paused = true 来控制block不执行，但这会导致上游资源浪费， 在活跃状态，通过 resume() paused = false drainQueue()重新 执行协程体代码块

2. repeatOnLifecycle or flowWithLifecycle 非活跃状态 job.cancel() 销毁协程，活跃状态 ** 重新开启新协程 ** 执行block()

DispatchQueue.kt

private val queue: Queue<Runnable> = ArrayDeque<Runnable>()

    @MainThread
    fun pause() {
        paused = true
    }

    @MainThread
    fun resume() {
        if (!paused) {
            return
        }
        check(!finished) {
            "Cannot resume a finished dispatcher"
        }
        paused = false
        drainQueue()
    }

    @MainThread
    fun drainQueue() {
        if (isDraining) {
            // Block re-entrant calls to avoid deep stacks
            return
        }
        try {
            isDraining = true
            while (queue.isNotEmpty()) {
                //pasuse() canRun = true 退出当前循环
                //resume() canRun = false 执行协程体代码块
                if (!canRun()) {
                    break
                }
                queue.poll()?.run()
            }
        } finally {
            isDraining = false
        }
    }
    
    // 是否可以运行
    @MainThread
    fun canRun() = finished || !paused

flowWithLifecycle

@OptIn(ExperimentalCoroutinesApi::class)
public fun <T> Flow<T>.flowWithLifecycle(
    lifecycle: Lifecycle,
    minActiveState: Lifecycle.State = Lifecycle.State.STARTED
): Flow<T> = callbackFlow {
    lifecycle.repeatOnLifecycle(minActiveState) {
        this@flowWithLifecycle.collect {
            send(it)
        }
    }
    close()
}

repeatOnLifecycle

public suspend fun Lifecycle.repeatOnLifecycle(
    state: Lifecycle.State,
    block: suspend CoroutineScope.() -> Unit
) {
    require(state !== Lifecycle.State.INITIALIZED) {
        "repeatOnLifecycle cannot start work with the INITIALIZED lifecycle state."
    }

    if (currentState === Lifecycle.State.DESTROYED) {
        return
    }

    // This scope is required to preserve context before we move to Dispatchers.Main
    coroutineScope {
        withContext(Dispatchers.Main.immediate) {
            // Check the current state of the lifecycle as the previous check is not guaranteed
            // to be done on the main thread.
            if (currentState === Lifecycle.State.DESTROYED) return@withContext

            // Instance of the running repeating coroutine
            var launchedJob: Job? = null

            // Registered observer
            var observer: LifecycleEventObserver? = null
            try {
                // Suspend the coroutine until the lifecycle is destroyed or
                // the coroutine is cancelled
                suspendCancellableCoroutine<Unit> { cont ->
                    // Lifecycle observers that executes `block` when the lifecycle reaches certain state, and
                    // cancels when it falls below that state.
                    val startWorkEvent = Lifecycle.Event.upTo(state)
                    val cancelWorkEvent = Lifecycle.Event.downFrom(state)
                    val mutex = Mutex()
                    observer = LifecycleEventObserver { _, event ->
                        if (event == startWorkEvent) {
                            // Launch the repeating work preserving the calling context
                            launchedJob = this@coroutineScope.launch {
                                // Mutex makes invocations run serially,
                                // coroutineScope ensures all child coroutines finish
                                mutex.withLock {
                                    coroutineScope {
                                        block()
                                    }
                                }
                            }
                            return@LifecycleEventObserver
                        }
                        if (event == cancelWorkEvent) {
                            launchedJob?.cancel()
                            launchedJob = null
                        }
                        if (event == Lifecycle.Event.ON_DESTROY) {
                            cont.resume(Unit)
                        }
                    }
                    this@repeatOnLifecycle.addObserver(observer as LifecycleEventObserver)
                }
            } finally {
                launchedJob?.cancel()
                observer?.let {
                    this@repeatOnLifecycle.removeObserver(it)
                }
            }
        }
    }
}

@FlowPreview
@AndroidEntryPoint
class HIltActivity : AppCompatActivity(R.layout.activity_hilt) {
  override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val binding =
            DataBindingUtil.setContentView<ActivityHiltBinding>(this, R.layout.activity_hilt)
    
        lifecycleScope.launchWhenResumed {
            vm.getFlowTest() .collect {
                    printlnLog("collect---> $it")

                }
        }

        lifecycleScope.launch {
            vm.getFlowTest().flowWithLifecycle(lifecycle)
                .collect {
                    printlnLog("--------collect---> $it")
                }
        }

    }

  override fun onResume() {
        super.onResume()
        printlnLog("onResume")
    }

    override fun onPause() {
        super.onPause()
        printlnLog("onPPPause")
    }
}

@FlowPreview
@HiltViewModel
class HiltVm @Inject constructor(
    private val localHiltRepository: LocalHiltRepository,
    private val remoteHiltRepository: RemoteHiltRepository
) : ViewModel() {

    private var i = 0

    fun getFlowTest():Flow<Int>{
        return flow<Int> {
            while (true){
                delay(2000)
                printlnLog("-----------------$i")
                emit(i++)
            }
        }.flowOn(Dispatchers.IO)
    }

launchWhenResumed 打印如下：

launchWhenX.png

repeatOnLifecycle 打印如下：

repeatOnLifecycle.png