## react源码解析12.状态更新流程 #### 视频课程（高效学习）：[进入课程](https://xiaochen1024.com/series/60b1b600712e370039088e24/60b1b636712e370039088e25) #### 课程目录： [1.开篇介绍和面试题](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b311cf10a4003b634719) [2.react的设计理念](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b31ccf10a4003b63471a) [3.react源码架构](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b328cf10a4003b63471b) [4.源码目录结构和调试](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b32ecf10a4003b63471c) [5.jsx&核心api](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b334cf10a4003b63471d) [6.legacy和concurrent模式入口函数](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b33acf10a4003b63471e) [7.Fiber架构](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b340cf10a4003b63471f) [8.render阶段](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b348cf10a4003b634720) [9.diff算法](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b354cf10a4003b634721) [10.commit阶段](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b360cf10a4003b634722) [11.生命周期](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b367cf10a4003b634723) [12.状态更新流程](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b36ecf10a4003b634724) [13.hooks源码](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b374cf10a4003b634725) [14.手写hooks](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b37acf10a4003b634726) [15.scheduler&Lane](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b556cf10a4003b634727) [16.concurrent模式](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b55ccf10a4003b634728) [17.context](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b564cf10a4003b634729) [18事件系统](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b56ccf10a4003b63472a) [19.手写迷你版react](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b57bcf10a4003b63472b) [20.总结&第一章的面试题解答](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b581cf10a4003b63472c) [21.demo](https://xiaochen1024.com/courseware/60b1b2f6cf10a4003b634718/60b1b587cf10a4003b63472d) #### 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 ```js //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'); }; ``` ```js //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 ```js //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; } ```  2. this.forceUpdate和this.setState一样，只是会让tag赋值ForceUpdate ```js //ReactBaseClasses.js Component.prototype.forceUpdate = function(callback) { this.updater.enqueueForceUpdate(this, callback, 'forceUpdate'); }; ``` ```js //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 ```js //ReactFiberClassComponent.old.js const shouldUpdate = checkHasForceUpdateAfterProcessing() || checkShouldComponentUpdate( workInProgress, ctor, oldProps, newProps, oldState, newState, nextContext, ); ``` **状态更新整体流程**  #### 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 ```js //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; } ``` **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 ```js //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） ```js //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  #### 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 ```js //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  #### 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  - 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  看demo_8的优先级 现在来看下计算状态的函数 ```js //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; } //... } ```
