const Lazy = React.lazy(() => import("./LazyComponent"))
 <Suspense fallback={"loading"}>
        <Lazy/> // lazy 包裝的組件
 </Suspense>

function LazyComp(){
  console.info("sus", "render lazy")
  return "i am a lazy man"
}
function delay(ms){
  return new Promise((resolve, reject) => {
    setTimeout(resolve, ms)
  })
}
// 模擬動(dòng)態(tài)加載組件
const Lazy = lazy(() => delay(5000).then(x => ({"default": LazyComp})))
function App() {
  const context = useContext(Context)
  console.info("outer context")
  return (
      <Suspense fallback={"loading"}>
        <Lazy/>
      </Suspense>
  )
}

  const props = workInProgress.pendingProps;
  const lazyComponent: LazyComponentType<any, any> = elementType;
  const payload = lazyComponent._payload;
  const init = lazyComponent._init;
  let Component = init(payload); // 如果未加載完成，則會(huì)拋出異常，否則會(huì)返回加載完成的組件

export function lazy<T>(
  ctor: () => Thenable<{default: T, ...}>,
): LazyComponent<T, Payload<T>> {
  const payload: Payload<T> = {
    // We use these fields to store the result.
    _status: Uninitialized,
    _result: ctor,
  };
  const lazyType: LazyComponent<T, Payload<T>> = {
    $$typeof: REACT_LAZY_TYPE,
    _payload: payload,
    _init: lazyInitializer,
  };

function lazyInitializer<T>(payload: Payload<T>): T {
  if (payload._status === Uninitialized) {
    console.info("sus", "payload status", "Uninitialized")
    const ctor = payload._result;
    const thenable = ctor(); // 這里的 ctor 就是我們返回 promise 的函數(shù)，執(zhí)行之后得到一個(gè)加載組件的 promise
    // 加載完成后修改狀態(tài)，并將結(jié)果掛載到 _result 上
    thenable.then(
      moduleObject => {
        if (payload._status === Pending || payload._status === Uninitialized) {
          // Transition to the next state.
          const resolved: ResolvedPayload<T> = (payload: any);
          resolved._status = Resolved;
          resolved._result = moduleObject;
        }
      },
      error => {
        if (payload._status === Pending || payload._status === Uninitialized) {
          // Transition to the next state.
          const rejected: RejectedPayload = (payload: any);
          rejected._status = Rejected;
          rejected._result = error;
        }
      },
    );
    if (payload._status === Uninitialized) {
      // In case, we're still uninitialized, then we're waiting for the thenable
      // to resolve. Set it as pending in the meantime.
      const pending: PendingPayload = (payload: any);
      pending._status = Pending;
      pending._result = thenable;
    }
  }
  // 如果已經(jīng)加載完成，則直接返回組件
  if (payload._status === Resolved) {
    const moduleObject = payload._result;
    console.info("sus", "get lazy resolved result")
    return moduleObject.default; // 注意這里返回的是 moduleObject.default 而不是直接返回 moduleObject
  } else {
    // 否則拋出異常
    console.info("sus, raise a promise", payload._result)
    throw payload._result;
  }
}

  do {
    try {
      workLoopSync();
      break;
    } catch (thrownValue) {
      handleError(root, thrownValue);
    }
  } while (true);

throwException(
    root,
    erroredWork.return,
    erroredWork,
    thrownValue,
    workInProgressRootRenderLanes,
);
completeUnitOfWork(erroredWork);

// 首先判斷是否是為 promise
if (
    value !== null &&
    typeof value === 'object' &&
    typeof value.then === 'function'
  ) {
    const wakeable: Wakeable = (value: any);
    resetSuspendedComponent(sourceFiber, rootRenderLanes);
    // 獲取到 Suspens 父組件
    const suspenseBoundary = getNearestSuspenseBoundaryToCapture(returnFiber);
    if (suspenseBoundary !== null) {
      suspenseBoundary.flags &= ~ForceClientRender;
      // 給 Suspens 父組件 打上一些標(biāo)記，讓 Suspens 父組件知道已經(jīng)有異常拋出，需要渲染 fallback
      markSuspenseBoundaryShouldCapture(
        suspenseBoundary,
        returnFiber,
        sourceFiber,
        root,
        rootRenderLanes,
      );
      // We only attach ping listeners in concurrent mode. Legacy Suspense always
      // commits fallbacks synchronously, so there are no pings.
      if (suspenseBoundary.mode & ConcurrentMode) {
        attachPingListener(root, wakeable, rootRenderLanes);
      }
      // 將拋出的 promise 放入Suspens 父組件的 updateQueue 中，后續(xù)會(huì)遍歷這個(gè) queue 進(jìn)行回調(diào)綁定
      attachRetryListener(suspenseBoundary, root, wakeable, rootRenderLanes);
      return;
    } 
  }

 if (flags & Update) {
        try {
          commitSuspenseCallback(finishedWork);
        } catch (error) {
          captureCommitPhaseError(finishedWork, finishedWork.return, error);
        }
        attachSuspenseRetryListeners(finishedWork);
      }
      return;

function attachSuspenseRetryListeners(finishedWork: Fiber) {
  const wakeables: Set<Wakeable> | null = (finishedWork.updateQueue: any);
  if (wakeables !== null) {
    finishedWork.updateQueue = null;
    let retryCache = finishedWork.stateNode;
    if (retryCache === null) {
      retryCache = finishedWork.stateNode = new PossiblyWeakSet();
    }
    wakeables.forEach(wakeable => {
      // Memoize using the boundary fiber to prevent redundant listeners.
      const retry = resolveRetryWakeable.bind(null, finishedWork, wakeable);
      // 判斷一下這個(gè) promise 是否已經(jīng)綁定過一次了，如果綁定過則可以忽略
      if (!retryCache.has(wakeable)) {
        retryCache.add(wakeable);
        if (enableUpdaterTracking) {
          if (isDevToolsPresent) {
            if (inProgressLanes !== null && inProgressRoot !== null) {
              // If we have pending work still, associate the original updaters with it.
              restorePendingUpdaters(inProgressRoot, inProgressLanes);
            } else {
              throw Error(
                'Expected finished root and lanes to be set. This is a bug in React.',
              );
            }
          }
        }
        // 將 retry 綁定 promise 的 then 回調(diào)
        wakeable.then(retry, retry);
      }
    });
  }
}

 if (retryLane === NoLane) {
    // TODO: Assign this to `suspenseState.retryLane`? to avoid
    // unnecessary entanglement?
    retryLane = requestRetryLane(boundaryFiber);
  }
  // TODO: Special case idle priority?
  const eventTime = requestEventTime();
  const root = markUpdateLaneFromFiberToRoot(boundaryFiber, retryLane);
  if (root !== null) {
    markRootUpdated(root, retryLane, eventTime);
    ensureRootIsScheduled(root, eventTime);
  }

function mountSuspensePrimaryChildren(
  workInProgress,
  primaryChildren,
  renderLanes,
) {
  const mode = workInProgress.mode;
  const primaryChildProps: OffscreenProps = {
    mode: 'visible',
    children: primaryChildren,
  };
  const primaryChildFragment = mountWorkInProgressOffscreenFiber(
    primaryChildProps,
    mode,
    renderLanes,
  );
  primaryChildFragment.return = workInProgress;
  workInProgress.child = primaryChildFragment; // 子節(jié)點(diǎn)為 primaryChildFragment，下一次訪問會(huì)拋出異常
  return primaryChildFragment;
}

function mountSuspenseFallbackChildren(
  workInProgress,
  primaryChildren,
  fallbackChildren,
  renderLanes,
) {
  const mode = workInProgress.mode;
  const progressedPrimaryFragment: Fiber | null = workInProgress.child;
  const primaryChildProps: OffscreenProps = {
    mode: 'hidden',
    children: primaryChildren,
  };
  let primaryChildFragment;
  let fallbackChildFragment;
  primaryChildFragment.return = workInProgress;
  fallbackChildFragment.return = workInProgress;
  primaryChildFragment.sibling = fallbackChildFragment;
  workInProgress.child = primaryChildFragment; // 注意這里的子節(jié)點(diǎn)是 primaryChildFragment
  return fallbackChildFragment; // 但返回的卻是 fallbackChildFragment，目的是為了跳過 primaryChild 的遍歷
}

function updateSuspensePrimaryChildren(
  current,
  workInProgress,
  primaryChildren,
  renderLanes,
) {
const currentPrimaryChildFragment: Fiber = (current.child: any);
  const currentFallbackChildFragment: Fiber | null =
    currentPrimaryChildFragment.sibling;
  const primaryChildFragment = updateWorkInProgressOffscreenFiber(
    currentPrimaryChildFragment,
    {
      mode: 'visible',
      children: primaryChildren,
    },
  );
  if ((workInProgress.mode & ConcurrentMode) === NoMode) {
    primaryChildFragment.lanes = renderLanes;
  }
  primaryChildFragment.return = workInProgress;
  primaryChildFragment.sibling = null;
  // 如果 currentFallbackChildFragment 存在，需要添加到 deletions 中
  if (currentFallbackChildFragment !== null) {
    const deletions = workInProgress.deletions;
    if (deletions === null) {
      workInProgress.deletions = [currentFallbackChildFragment];
      workInProgress.flags |= ChildDeletion;
    } else {
      deletions.push(currentFallbackChildFragment);
    }
  }
  workInProgress.child = primaryChildFragment;
  return primaryChildFragment;
}

function updateSuspenseFallbackChildren(
  current,
  workInProgress,
  primaryChildren,
  fallbackChildren,
  renderLanes,
) {
  const progressedPrimaryFragment: Fiber = (workInProgress.child: any);
    primaryChildFragment = progressedPrimaryFragment;
    primaryChildFragment.childLanes = NoLanes;
    primaryChildFragment.pendingProps = primaryChildProps;
    if (enableProfilerTimer && workInProgress.mode & ProfileMode) {
      primaryChildFragment.actualDuration = 0;
      primaryChildFragment.actualStartTime = -1;
      primaryChildFragment.selfBaseDuration =
        currentPrimaryChildFragment.selfBaseDuration;
      primaryChildFragment.treeBaseDuration =
        currentPrimaryChildFragment.treeBaseDuration;
    }
    // 清除 deletions，確保 fallback 可以展示
    workInProgress.deletions = null;
    let fallbackChildFragment;
    if (currentFallbackChildFragment !== null) {
    fallbackChildFragment = createWorkInProgress(
      currentFallbackChildFragment,
      fallbackChildren,
    );
  } else {
    fallbackChildFragment = createFiberFromFragment(
      fallbackChildren,
      mode,
      renderLanes,
      null,
    );
    fallbackChildFragment.flags |= Placement;
  }
  fallbackChildFragment.return = workInProgress;
  primaryChildFragment.return = workInProgress;
  primaryChildFragment.sibling = fallbackChildFragment;
  workInProgress.child = primaryChildFragment; // 同樣的操作，workInProgress.child 為 primaryChildFragment
  return fallbackChildFragment; // 但是返回 fallbackChildFragment
  }

// 模擬請求 promise
function mockApi(){
  return delay(5000).then(() => "data fetched")
}
// 處理請求狀態(tài)變更
function fetchData(){
  let status = "uninit"
  let data = null
  let promise = null
  return () => {
    switch(status){
      // 初始狀態(tài)，發(fā)出請求并拋出 promise
      case "uninit": {
        const p = mockApi()
          .then(x => {
            status = "resolved"
            data = x
          })
          status = "loading"
          promise = p
        throw promise
      };
      // 加載狀態(tài)，直接拋出 promise
      case "loading": throw promise;
      // 如果加載完成直接返回?cái)?shù)據(jù)
      case "resolved": return data;
      default: break;
    }
  }
}
const reader = fetchData()
function TestDataLoad(){
  const data = reader()
  return (
    <p>{data}</p>
  )
}
function App() {
  const [count, setCount] = useState(1)
  useEffect(() => {
    setInterval(() => setCount(c => c > 100 ? c: c + 1), 1000)
  }, [])
  return (
     <>
        <Suspense fallback={"loading"}>
          <TestDataLoad/>
        </Suspense>
        <p>count: {count}</p>
     </>
  )
}