React渲染机制超详细讲解

function App(){
  const [count, setCount] = useState(0)
  useEffect(() => {
    setCount(1)
  }, [])
  const handleClick = () => setCount(count => count++)
  return (
    <div>
        勇敢牛牛,        <span>不怕困难</span>
        <span onClick={handleClick}>{count}</span>
    </div>
  )
}
ReactDom.render(<App />, document.querySelector('#root'))

React.createElement("App", null)
or
jsx("App", null)

if (!root) {
    // Initial mount
    root = container._reactRootContainer = legacyCreateRootFromDOMContainer(container, forceHydrate);
    fiberRoot = root._internalRoot;
}

var update = {
    eventTime: eventTime,
    lane: lane,
    tag: UpdateState,
    payload: null,
    callback: element,
    next: null
  };

var sharedQueue = updateQueue.shared;
  var pending = sharedQueue.pending;
  if (pending === null) {   
  // mount时只有一个update，直接闭环
    update.next = update;
  } else {   
  // update时，将最新的update的next指向上一次的update， 上一次的update的next又指向最新的update形成闭环
    update.next = pending.next;
    pending.next = update;
  }
  // pending指向最新的update， 这样我们遍历update链表时， pending.next会指向第一个插入的update。
  sharedQueue.pending = update;   

function workLoopSync() {
  while (workInProgress !== null) {
    performUnitOfWork(workInProgress);
  }
}

if (flags > PerformedWork) {
  if (returnFiber.lastEffect !== null) {
    returnFiber.lastEffect.nextEffect = completedWork;
  } else {
    returnFiber.firstEffect = completedWork;
  }
  returnFiber.lastEffect = completedWork;
}

var finishedWork = root.current.alternate;  // workInProgress fiber树
root.finishedWork = finishedWork;  // 这里的root是fiberRoot
root.finishedLanes = lanes;
commitRoot(root);

var firstEffect;
// 判断当前rootFiber树是否有副作用
if (finishedWork.flags > PerformedWork) {
    // 下面代码的目的还是为了将这个effectList链表形成闭环
    if (finishedWork.lastEffect !== null) {
      finishedWork.lastEffect.nextEffect = finishedWork;
      firstEffect = finishedWork.firstEffect;
    } else {
      firstEffect = finishedWork;
    }
} else {
// 这个rootFiber树没有副作用
firstEffect = finishedWork.firstEffect;
}

do {  // mutation之前
  invokeGuardedCallback(null, commitBeforeMutationEffects, null);
} while (nextEffect !== null);

function commitBeforeMutationEffects() {
  while (nextEffect !== null) {
    var current = nextEffect.alternate;
    // 处理DOM节点渲染/删除后的 autoFocus、blur 逻辑；
    if (!shouldFireAfterActiveInstanceBlur && focusedInstanceHandle !== null){...}
    var flags = nextEffect.flags;
    // 调用getSnapshotBeforeUpdate，fiberRoot和ClassComponent会走这里
    if ((flags & Snapshot) !== NoFlags) {...}
    // 调度useEffect（异步）
    if ((flags & Passive) !== NoFlags) {
      // rootDoesHavePassiveEffects变量表示当前是否有副作用
      if (!rootDoesHavePassiveEffects) {
        rootDoesHavePassiveEffects = true;
        // 创建任务并加入任务队列，会在layout阶段之后触发
        scheduleCallback(NormalPriority$1, function () {
          flushPassiveEffects();
          return null;
        });
      }
    }
    // 继续遍历下一个effect
    nextEffect = nextEffect.nextEffect;
    }
}

var newTask = {
    id: taskIdCounter++,
    callback: callback,  //上面代码传入的回调函数
    priorityLevel: priorityLevel,
    startTime: startTime,
    expirationTime: expirationTime,
    sortIndex: -1
};

do {    // mutation  dom渲染
  invokeGuardedCallback(null, commitMutationEffects, null, root, renderPriorityLevel);
} while (nextEffect !== null);

function commitMutationEffects(root, renderPriorityLevel) {
  // TODO: Should probably move the bulk of this function to commitWork.
  while (nextEffect !== null) {     // 遍历EffectList
    setCurrentFiber(nextEffect);
    // 根据flags分别处理
    var flags = nextEffect.flags;
    // 根据 ContentReset flags重置文字节点
    if (flags & ContentReset) {...}
    // 更新ref
    if (flags & Ref) {...}
    var primaryFlags = flags & (Placement | Update | Deletion | Hydrating);
    switch (primaryFlags) {
      case Placement:   // 插入dom
        {...}
      case PlacementAndUpdate:    //插入dom并更新dom
        {
          // Placement
          commitPlacement(nextEffect);
          nextEffect.flags &= ~Placement; // Update
          var _current = nextEffect.alternate;
          commitWork(_current, nextEffect);
          break;
        }
      case Hydrating:     //SSR
        {...}
      case HydratingAndUpdate:      // SSR
        {...}
      case Update:      // 更新dom
        {...}
      case Deletion:    // 删除dom
        {...}
    }
    resetCurrentFiber();
    nextEffect = nextEffect.nextEffect;
  }
}

function appendChildToContainer(container, child) {
  var parentNode;
  if (container.nodeType === COMMENT_NODE) {
    parentNode = container.parentNode;
    parentNode.insertBefore(child, container);
  } else {
    parentNode = container;
    parentNode.appendChild(child);    // 直接将整个dom作为子节点插入到root中
  }
}

root.current = finishedWork  // 将`workInProgress`fiber树变成`current`树

do {   // 调用生命周期和hook相关操作， 赋值ref
   invokeGuardedCallback(null, commitLayoutEffects, null, root, lanes);
} while (nextEffect !== null);

function commitLayoutEffects(root, committedLanes) {
  while (nextEffect !== null) {
    setCurrentFiber(nextEffect);
    var flags = nextEffect.flags;
    // 调用生命周期或钩子函数
    if (flags & (Update | Callback)) {
      var current = nextEffect.alternate;
      commitLifeCycles(root, current, nextEffect);
    }
    {
      // 获取dom实例，更新ref
      if (flags & Ref) {
        commitAttachRef(nextEffect);
      }
    }
    resetCurrentFiber();
    nextEffect = nextEffect.nextEffect;
  }
}

// 创建任务并加入任务队列，会在layout阶段之后触发
scheduleCallback(NormalPriority$1, function () {
  flushPassiveEffects();
  return null;
});