react源码解析12.状态更新流程

setState&forceUpdate

在react中触发状态更新的几种方式:

  • ReactDOM.render
  • this.setState
  • this.forceUpdate
  • useState
  • useReducer

我们重点看下重点看下this.setState和this.forceUpdate,hook在第13章讲

  1. this.setState内调用this.updater.enqueueSetState,主要是将update加入updateQueue中
//ReactBaseClasses.js
Component.prototype.setState = function (partialState, callback) {
  if (!(typeof partialState === 'object' || typeof partialState === 'function' || partialState == null)) {
    {
      throw Error( "setState(...): takes an object of state variables to update or a function which returns an object of state variables." );
    }
  }
  this.updater.enqueueSetState(this, partialState, callback, 'setState');
};
//ReactFiberClassComponent.old.js
enqueueSetState(inst, payload, callback) {
  const fiber = getInstance(inst);//fiber实例

  const eventTime = requestEventTime();
  const suspenseConfig = requestCurrentSuspenseConfig();

  const lane = requestUpdateLane(fiber, suspenseConfig);//优先级

  const update = createUpdate(eventTime, lane, suspenseConfig);//创建update

  update.payload = payload;

  if (callback !== undefined && callback !== null) {  //赋值回调
    update.callback = callback;
  }

  enqueueUpdate(fiber, update);//update加入updateQueue
  scheduleUpdateOnFiber(fiber, lane, eventTime);//调度update
}

enqueueUpdate用来将update加入updateQueue队列

//ReactUpdateQueue.old.js
export function enqueueUpdate(fiber: Fiber, update: Update) {
  const updateQueue = fiber.updateQueue;
  if (updateQueue === null) {
    return;
  }

  const sharedQueue: SharedQueue = (updateQueue: any).shared;
  const pending = sharedQueue.pending;
  if (pending === null) {
    update.next = update;//与自己形成环状链表
  } else {
    update.next = pending.next;//加入链表的结尾
    pending.next = update;
  }
  sharedQueue.pending = update;
}

react源码12.6

  1. this.forceUpdate和this.setState一样,只是会让tag赋值ForceUpdate
//ReactBaseClasses.js
Component.prototype.forceUpdate = function(callback) {
  this.updater.enqueueForceUpdate(this, callback, 'forceUpdate');
};
//ReactFiberClassComponent.old.js
enqueueForceUpdate(inst, callback) {
    const fiber = getInstance(inst);
    const eventTime = requestEventTime();
    const suspenseConfig = requestCurrentSuspenseConfig();
    const lane = requestUpdateLane(fiber, suspenseConfig);

    const update = createUpdate(eventTime, lane, suspenseConfig);

    //tag赋值ForceUpdate
    update.tag = ForceUpdate;

    if (callback !== undefined && callback !== null) {
      update.callback = callback;
    }

    enqueueUpdate(fiber, update);
    scheduleUpdateOnFiber(fiber, lane, eventTime);

  },
};

如果标记ForceUpdate,render阶段组件更新会根据checkHasForceUpdateAfterProcessing,和checkShouldComponentUpdate来判断,如果Update的tag是ForceUpdate,则checkHasForceUpdateAfterProcessing为true,当组件是PureComponent时,checkShouldComponentUpdate会浅比较state和props,所以当使用this.forceUpdate一定会更新

//ReactFiberClassComponent.old.js
const shouldUpdate =
  checkHasForceUpdateAfterProcessing() ||
  checkShouldComponentUpdate(
    workInProgress,
    ctor,
    oldProps,
    newProps,
    oldState,
    newState,
    nextContext,
  );

状态更新整体流程

react源码12.1

Update&updateQueue

HostRoot或者ClassComponent触发更新后,会在函数createUpdate中创建update,并在后面的render阶段的beginWork中计算Update。FunctionComponent对应的Update在第11章讲,它和HostRoot或者ClassComponent的Update结构有些不一样

//ReactUpdateQueue.old.js
export function createUpdate(eventTime: number, lane: Lane): Update<*> {//创建update
  const update: Update<*> = {
    eventTime,
    lane,

    tag: UpdateState,
    payload: null,
    callback: null,

    next: null,
  };
  return update;
}

相关参考视频讲解:进入学习

我们主要关注这些参数:

  • lane:优先级(第12章讲)
  • tag:更新的类型,例如UpdateState、ReplaceState
  • payload:ClassComponent的payload是setState第一个参数,HostRoot的payload是ReactDOM.render的第一个参数
  • callback:setState的第二个参数
  • next:连接下一个Update形成一个链表,例如同时触发多个setState时会形成多个Update,然后用next 连接

对于HostRoot或者ClassComponent会在mount的时候使用initializeUpdateQueue创建updateQueue,然后将updateQueue挂载到fiber节点上

//ReactUpdateQueue.old.js
export function initializeUpdateQueue(fiber: Fiber): void {
  const queue: UpdateQueue = {
    baseState: fiber.memoizedState,
    firstBaseUpdate: null,
    lastBaseUpdate: null,
  shared: {
      pending: null,
    },
    effects: null,
  };
fiber.updateQueue = queue;
}
  • baseState:初始state,后面会基于这个state,根据Update计算新的state
  • firstBaseUpdate、lastBaseUpdate:Update形成的链表的头和尾
  • shared.pending:新产生的update会以单向环状链表保存在shared.pending上,计算state的时候会剪开这个环状链表,并且链接在lastBaseUpdate后
  • effects:calback不为null的update

从触发更新的fiber节点向上遍历到rootFiber

在markUpdateLaneFromFiberToRoot函数中会从触发更新的节点开始向上遍历到rootFiber,遍历的过程会处理节点的优先级(第15章讲)

//ReactFiberWorkLoop.old.js
function markUpdateLaneFromFiberToRoot(
    sourceFiber: Fiber,
    lane: Lane,
  ): FiberRoot | null {
    sourceFiber.lanes = mergeLanes(sourceFiber.lanes, lane);
    let alternate = sourceFiber.alternate;
    if (alternate !== null) {
      alternate.lanes = mergeLanes(alternate.lanes, lane);
    }
    let node = sourceFiber;
    let parent = sourceFiber.return;
    while (parent !== null) {//从触发更新的节点开始向上遍历到rootFiber
      parent.childLanes = mergeLanes(parent.childLanes, lane);//合并childLanes优先级
      alternate = parent.alternate;
      if (alternate !== null) {
        alternate.childLanes = mergeLanes(alternate.childLanes, lane);
      } else {
      }
      node = parent;
      parent = parent.return;
    }
    if (node.tag === HostRoot) {
      const root: FiberRoot = node.stateNode;
      return root;
    } else {
      return null;
    }
  }

例如B节点触发更新,B节点被被标记为normal的update,也就是图中的u1,然后向上遍历到根节点,在根节点上打上一个normal的update,如果此时B节点又触发了一个userBlocking的Update,同样会向上遍历到根节点,在根节点上打上一个userBlocking的update。

如果当前根节点更新的优先级是normal,u1、u2都参与状态的计算,如果当前根节点更新的优先级是userBlocking,则只有u2参与计算

react源码12.5

调度

在ensureRootIsScheduled中,scheduleCallback会以一个优先级调度render阶段的开始函数performSyncWorkOnRoot或者performConcurrentWorkOnRoot

//ReactFiberWorkLoop.old.js
if (newCallbackPriority === SyncLanePriority) {
  // 任务已经过期,需要同步执行render阶段
  newCallbackNode = scheduleSyncCallback(
    performSyncWorkOnRoot.bind(null, root)
  );
} else {
  // 根据任务优先级异步执行render阶段
  var schedulerPriorityLevel = lanePriorityToSchedulerPriority(
    newCallbackPriority
  );
  newCallbackNode = scheduleCallback(
    schedulerPriorityLevel,
    performConcurrentWorkOnRoot.bind(null, root)
  );
}

状态更新

classComponent状态计算发生在processUpdateQueue函数中,涉及很多链表操作,看图更加直白

  • 初始时fiber.updateQueue单链表上有firstBaseUpdate(update1)和lastBaseUpdate(update2),以next连接
  • fiber.updateQueue.shared环状链表上有update3和update4,以next连接互相连接
  • 计算state时,先将fiber.updateQueue.shared环状链表‘剪开’,形成单链表,连接在fiber.updateQueue后面形成baseUpdate
  • 然后遍历按这条链表,根据baseState计算出memoizedState

    react源码12.2

带优先级的状态更新

类似git提交,这里的c3意味着高优先级的任务,比如用户出发的事件,数据请求,同步执行的代码等。

  • 通过ReactDOM.render创建的应用没有优先级的概念,类比git提交,相当于先commit,然后提交c3
    react源码12.3
  • 在concurrent模式下,类似git rebase,先暂存之前的代码,在master上开发,然后rebase到之前的分支上

    优先级是由Scheduler来调度的,这里我们只关心状态计算时的优先级排序,也就是在函数processUpdateQueue中发生的计算,例如初始时有c1-c4四个update,其中c1和c3为高优先级

    1. 在第一次render的时候,低优先级的update会跳过,所以只有c1和c3加入状态的计算
    2. 在第二次render的时候,会以第一次中跳过的update(c2)之前的update(c1)作为baseState,跳过的update和之后的update(c2,c3,c4)作为baseUpdate重新计算

    在在concurrent模式下,componentWillMount可能会执行多次,变现和之前的版本不一致

    注意,fiber.updateQueue.shared会同时存在于workInprogress Fiber和current Fiber,目的是为了防止高优先级打断正在进行的计算而导致状态丢失,这段代码也是发生在processUpdateQueue中

react源码12.4

看demo_8的优先级

现在来看下计算状态的函数

//ReactUpdateQueue.old.js
export function processUpdateQueue(
  workInProgress: Fiber,
  props: any,
  instance: any,
  renderLanes: Lanes,
): void {
  const queue: UpdateQueue = (workInProgress.updateQueue: any);
  hasForceUpdate = false;

  let firstBaseUpdate = queue.firstBaseUpdate;//updateQueue的第一个Update
  let lastBaseUpdate = queue.lastBaseUpdate;//updateQueue的最后一个Update
  let pendingQueue = queue.shared.pending;//未计算的pendingQueue

  if (pendingQueue !== null) {
    queue.shared.pending = null;
    const lastPendingUpdate = pendingQueue;//未计算的ppendingQueue的最后一个update
    const firstPendingUpdate = lastPendingUpdate.next;//未计算的pendingQueue的第一个update
    lastPendingUpdate.next = null;//剪开环状链表
    if (lastBaseUpdate === null) {//将pendingQueue加入到updateQueue
      firstBaseUpdate = firstPendingUpdate;
    } else {
      lastBaseUpdate.next = firstPendingUpdate;
    }
    lastBaseUpdate = lastPendingUpdate;

    const current = workInProgress.alternate;//current上做同样的操作
    if (current !== null) {
      const currentQueue: UpdateQueue = (current.updateQueue: any);
      const currentLastBaseUpdate = currentQueue.lastBaseUpdate;
      if (currentLastBaseUpdate !== lastBaseUpdate) {
        if (currentLastBaseUpdate === null) {
          currentQueue.firstBaseUpdate = firstPendingUpdate;
        } else {
          currentLastBaseUpdate.next = firstPendingUpdate;
        }
        currentQueue.lastBaseUpdate = lastPendingUpdate;
      }
    }
  }

  if (firstBaseUpdate !== null) {
    let newState = queue.baseState;

    let newLanes = NoLanes;

    let newBaseState = null;
    let newFirstBaseUpdate = null;
    let newLastBaseUpdate = null;

    let update = firstBaseUpdate;
    do {
      const updateLane = update.lane;
      const updateEventTime = update.eventTime;
      if (!isSubsetOfLanes(renderLanes, updateLane)) {//判断优先级是够足够
        const clone: Update = {//优先级不够 跳过当前update
          eventTime: updateEventTime,
          lane: updateLane,

          tag: update.tag,
          payload: update.payload,
          callback: update.callback,

          next: null,
        };
        if (newLastBaseUpdate === null) {//保存跳过的update
          newFirstBaseUpdate = newLastBaseUpdate = clone;
          newBaseState = newState;
        } else {
          newLastBaseUpdate = newLastBaseUpdate.next = clone;
        }
        newLanes = mergeLanes(newLanes, updateLane);
      } else {
        //直到newLastBaseUpdate为null才不会计算,防止updateQueue没计算完
        if (newLastBaseUpdate !== null) {
          const clone: Update = {
            eventTime: updateEventTime,
            lane: NoLane,

            tag: update.tag,
            payload: update.payload,
            callback: update.callback,

            next: null,
          };
          newLastBaseUpdate = newLastBaseUpdate.next = clone;
        }

        newState = getStateFromUpdate(//根据updateQueue计算state
          workInProgress,
          queue,
          update,
          newState,
          props,
          instance,
        );
        const callback = update.callback;
        if (callback !== null) {
          workInProgress.flags |= Callback;//Callback flag
          const effects = queue.effects;
          if (effects === null) {
            queue.effects = [update];
          } else {
            effects.push(update);
          }
        }
      }
      update = update.next;//下一个update
      if (update === null) {//重置updateQueue
        pendingQueue = queue.shared.pending;
        if (pendingQueue === null) {
          break;
        } else {
          const lastPendingUpdate = pendingQueue;

          const firstPendingUpdate = ((lastPendingUpdate.next: any): Update);
          lastPendingUpdate.next = null;
          update = firstPendingUpdate;
          queue.lastBaseUpdate = lastPendingUpdate;
          queue.shared.pending = null;
        }
      }
    } while (true);

    if (newLastBaseUpdate === null) {
      newBaseState = newState;
    }

    queue.baseState = ((newBaseState: any): State);//新的state
    queue.firstBaseUpdate = newFirstBaseUpdate;//新的第一个update
    queue.lastBaseUpdate = newLastBaseUpdate;//新的最后一个update

    markSkippedUpdateLanes(newLanes);
    workInProgress.lanes = newLanes;
    workInProgress.memoizedState = newState;
  }

    //...
}

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