/ performSyncWorkOnRoot会调用该方法
function workLoopSync() {
  while (workInProgress !== null) {
    performUnitOfWork(workInProgress);
  }
}

// performConcurrentWorkOnRoot会调用该方法
function workLoopConcurrent() {
  while (workInProgress !== null && !shouldYield()) {
    performUnitOfWork(workInProgress);
  }
}

“递”阶段

“归”阶段

举个 🌰

function App() {
  return (
    <div>
      i am
      <span>1874</span>
    </div>
  );
}

ReactDOM.render(<App />, document.getElementById("root"));

render阶段

1. rootFiber beginWork2. App Fiber beginWork3. div Fiber beginWork4. "i am" Fiber beginWork5. "i am" Fiber completeWork6. span Fiber beginWork7. span Fiber completeWork8. div Fiber completeWork9. App Fiber completeWork10. rootFiber completeWork

注意 之所以没有 “1874” Fiber 的 beginWork/completeWork，是因为作为一种性能优化手段，针对只有单一文本子节点的Fiber，React会特殊处理。

beginWork

流程概述

// 从传参看方法执行
function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes
): Fiber | null {
  // ...省略函数体
}

• current：当前组件对应的Fiber节点在上一次更新时的Fiber节点，即workInProgress.alternate

• workInProgress：当前组件对应的Fiber节点

• renderLanes：优先级相关

• update时：如果current存在，在满足一定条件时可以复用current节点，这样就能克隆current.child作为workInProgress.child，而不需要新建workInProgress.child。

• mount时：除fiberRootNode以外，current === null。会根据fiber.tag不同，创建不同类型的子Fiber节点

function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes
): Fiber | null {
  // update时：如果current存在可能存在优化路径，可以复用current（即上一次更新的Fiber节点）
  if (current !== null) {
    // ...省略

    // 复用current
    return bailoutOnAlreadyFinishedWork(current, workInProgress, renderLanes);
  } else {
    didReceiveUpdate = false;
  }

  // mount时：根据tag不同，创建不同的子Fiber节点
  switch (workInProgress.tag) {
    case IndeterminateComponent:
    // ...省略
    case LazyComponent:
    // ...省略
    case FunctionComponent:
    // ...省略
    case ClassComponent:
    // ...省略
    case HostRoot:
    // ...省略
    case HostComponent:
    // ...省略
    case HostText:
    // ...省略
    // ...省略其他类型
  }
}

update 时

• oldProps === newProps && workInProgress.type === current.type，即props与fiber.type不变

• !includesSomeLane(renderLanes, updateLanes)，即当前Fiber节点优先级不够。

mount 时

可以从这里看到tag对应的组件类型

// mount时：根据tag不同，创建不同的Fiber节点
switch (workInProgress.tag) {
  case IndeterminateComponent:
  // ...省略
  case LazyComponent:
  // ...省略
  case FunctionComponent:
  // ...省略
  case ClassComponent:
  // ...省略
  case HostRoot:
  // ...省略
  case HostComponent:
  // ...省略
  case HostText:
  // ...省略
  // ...省略其他类型
}

reconcileChildren

• 对于mount的组件，他会创建新的子Fiber节点

• 对于update的组件，他会将当前组件与该组件在上次更新时对应的Fiber节点比较（也就是俗称的Diff算法），将比较的结果生成新Fiber节点

export function reconcileChildren(
  current: Fiber | null,
  workInProgress: Fiber,
  nextChildren: any,
  renderLanes: Lanes
) {
  if (current === null) {
    // 对于mount的组件
    workInProgress.child = mountChildFibers(
      workInProgress,
      null,
      nextChildren,
      renderLanes
    );
  } else {
    // 对于update的组件
    workInProgress.child = reconcileChildFibers(
      workInProgress,
      current.child,
      nextChildren,
      renderLanes
    );
  }
}

注意 值得一提的是，mountChildFibers与reconcileChildFibers这两个方法的逻辑基本一致。唯一的区别是：reconcileChildFibers会为生成的Fiber节点带上effectTag属性，而mountChildFibers不会。

effectTag

你可以从这里看到effectTag对应的DOM操作

// 通过二进制表示effectTag，可以方便的使用位操作为fiber.effectTag赋值多个effect。

// DOM需要插入到页面中
export const Placement = /*                */ 0b00000000000010;
// DOM需要更新
export const Update = /*                   */ 0b00000000000100;
// DOM需要插入到页面中并更新
export const PlacementAndUpdate = /*       */ 0b00000000000110;
// DOM需要删除
export const Deletion = /*                 */ 0b00000000001000;

1. fiber.stateNode存在，即Fiber节点中保存了对应的DOM节点

2. (fiber.effectTag & Placement) !== 0，即Fiber节点存在Placement effectTag

completeWork

function completeWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
): Fiber | null {
  const newProps = workInProgress.pendingProps;

  switch (workInProgress.tag) {
    case IndeterminateComponent:
    case LazyComponent:
    case SimpleMemoComponent:
    case FunctionComponent:
    case ForwardRef:
    case Fragment:
    case Mode:
    case Profiler:
    case ContextConsumer:
    case MemoComponent:
      return null;
    case ClassComponent: {
      // ...省略
      return null;
    }
    case HostRoot: {
      // ...省略
      updateHostContainer(workInProgress);
      return null;
    }
    case HostComponent: {
      // ...省略
      return null;
    }
  // ...省略

处理 HostComponent

case HostComponent: {
  popHostContext(workInProgress);
  const rootContainerInstance = getRootHostContainer();
  const type = workInProgress.type;

  if (current !== null && workInProgress.stateNode != null) {
    // update的情况
    // ...省略
  } else {
    // mount的情况
    // ...省略
  }
  return null;
}

update 时

• onClick、onChange等回调函数的注册

• 处理style prop

• 处理DANGEROUSLY_SET_INNER_HTML prop

• 处理children prop

if (current !== null && workInProgress.stateNode != null) {
  // update的情况
  updateHostComponent(
    current,
    workInProgress,
    type,
    newProps,
    rootContainerInstance
  );
}

updateHostComponent = function(
    current: Fiber,
    workInProgress: Fiber,
    type: Type,
    newProps: Props,
    rootContainerInstance: Container,
  ) {
    // If we have an alternate, that means this is an update and we need to
    // schedule a side-effect to do the updates.
    const oldProps = current.memoizedProps;
    if (oldProps === newProps) {
      // In mutation mode, this is sufficient for a bailout because
      // we won't touch this node even if children changed.
      return;
    }

    // If we get updated because one of our children updated, we don't
    // have newProps so we'll have to reuse them.
    // TODO: Split the update API as separate for the props vs. children.
    // Even better would be if children weren't special cased at all tho.
    const instance: Instance = workInProgress.stateNode;
    const currentHostContext = getHostContext();
    // TODO: Experiencing an error where oldProps is null. Suggests a host
    // component is hitting the resume path. Figure out why. Possibly
    // related to `hidden`.
    const updatePayload = prepareUpdate(
      instance,
      type,
      oldProps,
      newProps,
      rootContainerInstance,
      currentHostContext,
    );
    // TODO: Type this specific to this type of component.
    workInProgress.updateQueue = (updatePayload: any);
    // If the update payload indicates that there is a change or if there
    // is a new ref we mark this as an update. All the work is done in commitWork.
    if (updatePayload) {
      markUpdate(workInProgress);
    }
  };

workInProgress.updateQueue = (updatePayload: any);

mount 时

• 为Fiber节点生成对应的DOM节点

• 将子孙DOM节点插入刚生成的DOM节点中

• 与update逻辑中的updateHostComponent类似的处理props的过程

// mount的情况

// ...省略服务端渲染相关逻辑

const currentHostContext = getHostContext();
// 为fiber创建对应DOM节点
const instance = createInstance(
  type,
  newProps,
  rootContainerInstance,
  currentHostContext,
  workInProgress
);
// 将子孙DOM节点插入刚生成的DOM节点中
appendAllChildren(instance, workInProgress, false, false);
// DOM节点赋值给fiber.stateNode
workInProgress.stateNode = instance;

// 与update逻辑中的updateHostComponent类似的处理props的过程
if (
  finalizeInitialChildren(
    instance,
    type,
    newProps,
    rootContainerInstance,
    currentHostContext
  )
) {
  markUpdate(workInProgress);
}

effectList

                       nextEffect         nextEffect
rootFiber.firstEffect -----------> fiber -----------> fiber

你可以在这里看到这段代码逻辑。

流程结尾

commitRoot(root);

代码见这里。