React Fiber源码分析 第二篇（同步模式）

function scheduleRootUpdate(current$$1, element, expirationTime, callback) {
var update = createUpdate(expirationTime);
  update.payload = { element: element };

  callback = callback === undefined ? null : callback;
  if (callback !== null) {
    update.callback = callback;
  }
  enqueueUpdate(current$$1, update);

  scheduleWork(current$$1, expirationTime);
  return expirationTime;
}

function createUpdate(expirationTime) {
  return {
    // 优先级
    expirationTime: expirationTime,
    // 更新类型
    tag: UpdateState,
    // 更新的对象
    payload: null,
    callback: null,
    // 指向下一个更新
    next: null,
    // 指向下一个更新effect
    nextEffect: null
  };
}

function scheduleWork(fiber, expirationTime) {
  // 更新优先级
  var root = scheduleWorkToRoot(fiber, expirationTime);
  ...if (!isWorking && nextRenderExpirationTime !== NoWork && expirationTime < nextRenderExpirationTime) {
    // This is an interruption. (Used for performance tracking.) 如果这是一个打断原有更新的任务， 先把现有任务记录
    interruptedBy = fiber;
    resetStack();
  }
  // 设置下一个操作时间nextExpirationTimeToWorkOn
  markPendingPriorityLevel(root, expirationTime);
  if (
  // If we're in the render phase, we don't need to schedule this root
  // for an update, because we'll do it before we exit...
  !isWorking || isCommitting$1 ||
  // ...unless this is a different root than the one we're rendering.
  nextRoot !== root) {
    var rootExpirationTime = root.expirationTime;
    requestWork(root, rootExpirationTime);
  }
  ...
}

function scheduleWorkToRoot(fiber, expirationTime) {
  // Update the source fiber's expiration time
  if (fiber.expirationTime === NoWork || fiber.expirationTime > expirationTime) {
    fiber.expirationTime = expirationTime;
  }
  var alternate = fiber.alternate;
  if (alternate !== null && (alternate.expirationTime === NoWork || alternate.expirationTime > expirationTime)) {
    alternate.expirationTime = expirationTime;
  }
  // 如果是HostRoot 即直接返回
  var node = fiber.return;
  if (node === null && fiber.tag === HostRoot) {
    return fiber.stateNode;
  }
  // 若子fiber中有更新， 即更新其childrenExpirationTime
  while (node !== null) {
    ...
  }
  return null;
}

function requestWork(root, expirationTime) {
  addRootToSchedule(root, expirationTime);
  if (isRendering) {
    // rendering状态，直接返回
    return;
  }

  if (isBatchingUpdates) {
    // isBatchingUpdates， 直接返回。 react的state更新是会合并的
    ...return;
  }

  // TODO: Get rid of Sync and use current time?
  if (expirationTime === Sync) {
    // 执行同步
    performSyncWork();
  } else {
    // 异步， 暂不分析
    scheduleCallbackWithExpirationTime(root, expirationTime);
  }
}

function addRootToSchedule(root, expirationTime) {
   // root尚未开始过任务 将root加到schedule
  if (root.nextScheduledRoot === null) {
    ...
  } else {
    // root已经开始执行过任务， 更新root的expirationTime
    var remainingExpirationTime = root.expirationTime;
    if (remainingExpirationTime === NoWork || expirationTime < remainingExpirationTime) {
      root.expirationTime = expirationTime;
    }
  }
}

function performWork(minExpirationTime, dl) {
  deadline = dl;
  // 找出优先级最高的root
  findHighestPriorityRoot();

  if (deadline !== null) {
    // ...异步
  } else {
    // 循环执行root任务
    while (nextFlushedRoot !== null && nextFlushedExpirationTime !== NoWork && (minExpirationTime === NoWork || minExpirationTime >= nextFlushedExpirationTime)) {
      performWorkOnRoot(nextFlushedRoot, nextFlushedExpirationTime, true);
      findHighestPriorityRoot();
    }
  }
  ...
  // If there's work left over, schedule a new callback.
  if (nextFlushedExpirationTime !== NoWork) {
    scheduleCallbackWithExpirationTime(nextFlushedRoot, nextFlushedExpirationTime);
  }
　...
}

function findHighestPriorityRoot() {
  var highestPriorityWork = NoWork;
  var highestPriorityRoot = null;
  if (lastScheduledRoot !== null) {
    var previousScheduledRoot = lastScheduledRoot;
    var root = firstScheduledRoot;
    while (root !== null) {
      var remainingExpirationTime = root.expirationTime;
      if (remainingExpirationTime === NoWork) {
         // 判断是否还有任务并移除
      } else {
         // 找出最高的优先级root和其对应的优先级
      }
    }
  }
  // 赋值
  nextFlushedRoot = highestPriorityRoot;
  nextFlushedExpirationTime = highestPriorityWork;
}

    while (nextFlushedRoot !== null && nextFlushedExpirationTime !== NoWork && (minExpirationTime === NoWork || minExpirationTime >= nextFlushedExpirationTime)) {
      performWorkOnRoot(nextFlushedRoot, nextFlushedExpirationTime, true);
      findHighestPriorityRoot();
    }

function performWorkOnRoot(root, expirationTime, isExpired) {
  // 将rendering状态设为true
  isRendering = true;

  // Check if this is async work or sync/expired work.
  if (deadline === null || isExpired) {
    // Flush work without yielding.
    // 同步
    var finishedWork = root.finishedWork;
    if (finishedWork !== null) {
      // This root is already complete. We can commit it.
      completeRoot(root, finishedWork, expirationTime);
    } else {
      root.finishedWork = null;
      // If this root previously suspended, clear its existing timeout, since
      // we're about to try rendering again.
      var timeoutHandle = root.timeoutHandle;
      if (enableSuspense && timeoutHandle !== noTimeout) {
        root.timeoutHandle = noTimeout;
        // $FlowFixMe Complains noTimeout is not a TimeoutID, despite the check above
        cancelTimeout(timeoutHandle);
      }
      var isYieldy = false;
      renderRoot(root, isYieldy, isExpired);
      finishedWork = root.finishedWork;
      if (finishedWork !== null) {
        // We've completed the root. Commit it.
        completeRoot(root, finishedWork, expirationTime);
      }
    }
  } else {
    // Flush async work.异步操作
    ......
    }
  }

  isRendering = false;
}

 if (expirationTime !== nextRenderExpirationTime || root !== nextRoot || nextUnitOfWork === null) {
    // 判断是否是一个从新开始的root
    resetStack();
    nextRoot = root;
    nextRenderExpirationTime = expirationTime;
    nextUnitOfWork = createWorkInProgress(nextRoot.current, null, nextRenderExpirationTime);
    root.pendingCommitExpirationTime = NoWork;
    ....
    ....
  }

接着执行wookLoop（isYield）函数， 该函数通过循环执行， 遍历每一个nextUniOfWork,

function workLoop(isYieldy) {
  if (!isYieldy) {
    // Flush work without yielding
    while (nextUnitOfWork !== null) {
      nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
    }
  } else {
    // Flush asynchronous work until the deadline runs out of time.
    while (nextUnitOfWork !== null && !shouldYield()) {
      nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
    }
  }
}

next = beginWork(current$$1, workInProgress, nextRenderExpirationTime);

isWorking = false;
if (nextUnitOfWork !== null) {
 onYield(root);
 return;
}

重置nextRoot等

nextRoot = null;
interruptedBy = null;

赋值finishWork

var rootWorkInProgress = root.current.alternate;
onComplete(root, rootWorkInProgress, expirationTime);
function onComplete(root, finishedWork, expirationTime) {
  root.pendingCommitExpirationTime = expirationTime;
  root.finishedWork = finishedWork;
}

  if (deadline !== null) {
    callbackExpirationTime = NoWork;
    callbackID = null;
  }
  // If there's work left over, schedule a new callback.
  if (nextFlushedExpirationTime !== NoWork) {
    scheduleCallbackWithExpirationTime(nextFlushedRoot, nextFlushedExpirationTime);
  }

  // Clean-up.
  deadline = null;
  deadlineDidExpire = false;