ReactDom.render

//packages/react-dom/src/client/ReactDOMLegacy.js
render(
    element: React$Element<any>,
    container: DOMContainer,
    callback: ?Function,
  ) {
    // 调用legacyRenderSubtreeIntoContainer
    return legacyRenderSubtreeIntoContainer(
      null,
      element,
      container,
      false,
      callback,
    );
  }

function legacyRenderSubtreeIntoContainer(
  parentComponent: ?React$Component<any, any>,
  children: ReactNodeList,
  container: DOMContainer,
  forceHydrate: boolean,
  callback: ?Function,
  ){
      let root = container._reactRootContainer
      let fiberRoot
      // 判断root是否存在
      if(!root) {
        // 首次渲染不存在，则生成root，赋值fiberRoot
        root = container_reactRootContainer = legacyCreateRootFromDOMContainer(container,forceHydrate)
        fiberRoot = root._internalRoot
        /**fiberRoot :{tag: 0
        current: FiberNode {tag: 3, key: null, elementType: null, type: null, stateNode: FiberRootNode, …}
        containerInfo: div#root
        finishedExpirationTime: 0
        finishedWork: null} **/
        // 字面理解不要批量更新，首次渲染需要立即执行内部函数，即updateContainer
        unbatchedUpdates(() => {
          // 生成expirationTime, 调度  ps: 注释1
          updateContainer(children, fiberRoot, parentComponent, callback);
        });  
      }else {
          fiberRoot = root._internalRoot
          updateContainer(children, fiberRoot, parentComponent, callback);
      }
      return getPublicRootInstance(fiberRoot)
  }

// 注释1 生成expirationTime, 并 scheduleWork调度
function updateContainer(
  element: ReactNodeList,
  container: fiberRoot,
  parentComponent: ?React$Component<any, any>,
  callback: ?Function,
  ): ExpirationTime {
      const current = container.current; // 当前Root Fiber
      const currentTime = requestCurrentTimeForUpdate(); //当前时间
      // 生成expirationTime 注释2
      const expirationTime = computeExpirationForFiber(
          currentTime,
          current,
          suspenseConfig,
      );
      // 创建更新
      const update = createUpdate(expirationTime, suspenseConfig)
      enqueueUpdate(current, update); // 加入队列
      // 注释 3
      scheduleWork(current, expirationTime) //调度root fiber
      return expirationTime
  }

// 注释2 计算expirationTime
function computeExpirationForFiber(
    currentTime,
    fiber,
    suspenseConfig 
    ): ExpirationTime {
        const priorityLevel = getCurrentPriorityLevel();
        let expirationTime;
        if(suspenseConfig !== null) {
            //有挂起任务的情况
        }else{
            switch(priorityLevel) {
                case ImmediatePriority:
                    expirationTime = Sync // 同步
                    break;
                case NormalPriority:
                case LowPriority:
                    expirationTime = computeAsyncExpiration(currentTime) //异步
            }
        }
        return expirationTime;
    }

// 注释 3 scheduleWork调度
function scheduleUpdateOnFiber(
    fiber,
    expirationTime 
    ) {
        if(expirationTime === Sync) {
            //注释4
            performSyncWorkOnRoot(root)
        }else {
            
        }
    }

// 注释 4
function performSyncWorkOnRoot(root) {
    if(workInProgress !== null) {
        do{
            try {
                // 注释 5
                workLoopSync()
            }
        }while(true)
    }
    
    if(workInProgress !==null){
        
    }else{
        // 已经没有当前工作的单元任务，提交 注释7
        commitRoot(root)
    }
}
    
// 注释 5
    function workLoopSync() {
        // 向下遍历，构建fiber tree
        while (workInProgress !== null) {
            // 注释 6
            workInProgress = performUnitOfWork(workInProgress);
        }
    }
    
// 注释 6  构建fiber tree, 并返回下一个fiber node
    function performUnitOfWork(unitOfWork) {
        const current = unitOfWork.alternate
        startWorkTimer(unitOfWork)
        let next
        // 向下构建fiber tree，更加tag 进行updatexx,如果nextProps中有children ,惊喜reconcileChildren
        next = beginWork(current, unitOfWork, renderExpirationTime)
        
        if(next === null){
            // 向上构建，设置firstEffect／nextEffect,并设置stateNode，真实dom
            next = completeUnitOfWork(unitOfWork)
        }
        return next
    }   
    
 // 注释7 commitRoot(root)
    function commitRootImpl(root, renderPriorityLevel){
        const finishedWork = root.finishedWork
        /** finishedWork:
        { tag: 3
            key: null
            elementType: null
            type: null
            stateNode: FiberRootNode {tag: 0, current: FiberNode, containerInfo: div#root, pendingChildren: null, pingCache: null, …}
            return: null
            child: FiberNode {tag: 1, key: null, stateNode: Demo, elementType: ƒ, type: ƒ, …}
            sibling: null
            index: 0
            ref: null
            pendingProps: null
            memoizedProps: null
            updateQueue: {baseState: {…}, firstUpdate: null, lastUpdate: null, firstCapturedUpdate: null, lastCapturedUpdate: null, …}
            memoizedState: {element: {…}}
            mode: 8
            effectTag: 0
            nextEffect: null
            firstEffect: FiberNode {tag: 1, key: null, stateNode: Demo, elementType: ƒ, type: ƒ, …}
            lastEffect: FiberNode {tag: 1, key: null, stateNode: Demo, elementType: ƒ, type: ƒ, …}
            expirationTime: 0
            childExpirationTime: 0
            alternate: FiberNode {tag: 3, key: null, elementType: null, type: null, stateNode: FiberRootNode, …}
        }
        **/
        
        firstEffect = finishedWork.firstEffect;
        nextEffect = firstEffect
        // 根据fiberRoot上 nextEffect,进行更新
        do{
            commitBeforMutationEffects()
        }while(nextEffect !== null)
            
        do{ 
            // 根据nextEffect.effectTag 做响应的dom操作 commitPlacement/commitWork/commitDeletion
            commitMutationEffects()
        }while(nextEffect !== null)
        do{
            // commitLifeCycles 执行后续的生命周期
            commitLayoutEffects(root, expirationTime)
        }while(nextEffect !== null)    
            
        onCommitRoot(finishedWork.stateNode, expirationTime)
        flushSyncCallbackQueue()
        return null;
    }