setState是同步還是異步
自定義合成事件和react鉤子函數(shù)中異步更新state
以在自定義click事件中的setState為例
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import React, { Component } from 'react';class Test extends Component { constructor(props) { super(props); this.state = { count: 1 }; } handleClick = () => { this.setState({ count: this.state.count + 1 }); this.setState({ count: this.state.count + 1 }); this.setState({ count: this.state.count + 1 }); console.log(this.state.count); } render() { return ( <div style={{ width: '100px', height: '100px', backgroundColor: "yellow" }}> {this.state.count} </div> ) }}export default Test; |
點(diǎn)擊一次，最終this.state.count的打印結(jié)果是1，頁(yè)面展示的是2。通過(guò)現(xiàn)象看，三次setState只是最后一次setState生效了，前兩次都setState無(wú)效果。因?yàn)榧偃绨训谝淮蝧etState改為+3，count打印結(jié)果為1，展示結(jié)果為2，沒(méi)有發(fā)生變化。而且沒(méi)有同步獲得count的結(jié)果。
此時(shí)，我們可以調(diào)整代碼，通過(guò)setState的第二個(gè)參數(shù)，來(lái)獲得更新后的state：
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import React, { Component } from 'react';class Test extends Component { constructor(props) { super(props); this.state = { count: 1 }; } handleClick = () => { this.setState({ count: this.state.count + 3 }, () => { console.log('1', this.state.count) }); this.setState({ count: this.state.count + 1 }, () => { console.log('2', this.state.count); }); this.setState({ count: this.state.count + 1 }, () => { console.log('3', this.state.count); }); console.log(this.state.count); } render() { return ( <div style={{ width: '100px', height: '100px', backgroundColor: "yellow" }}> {this.state.count} </div> ) }}export default Test; |
此時(shí)，點(diǎn)擊一次，三個(gè)setState的回調(diào)函數(shù)中，打印結(jié)果分別是。
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1: 2
2: 2
3: 2
首先，最后一行直接打印1。然后，在setState的回調(diào)中，打印出的結(jié)果都是最新更新的2。雖然前兩次setState未生效，但是它們第二個(gè)參數(shù)中還是會(huì)打印出2。
此時(shí)將setState的第一個(gè)參數(shù)換成函數(shù)，通過(guò)函數(shù)的第一個(gè)參數(shù)可以獲得更新前的state。
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import React, { Component } from 'react';class Test extends Component { constructor(props) { super(props); this.state = { count: 1 }; } handleClick = () => { this.setState((prevState, props) => { return { count: prevState.count + 1 } }); this.setState((prevState, props) => { return { count: prevState.count + 1 } }); this.setState((prevState, props) => { return { count: prevState.count + 1 } }); console.log(this.state.count); } render() { return ( <div style={{ width: '100px', height: '100px', backgroundColor: "yellow" }}> {this.state.count} </div> ) }}export default Test; |
此時(shí)，打印出的結(jié)果為1，但是頁(yè)面展示出來(lái)的count為4。可以發(fā)現(xiàn)，如果setState以傳參的方式去更新state，幾次setState并不會(huì)只更新最后一次，而是幾次更新state都會(huì)生效。
接下來(lái)看下第二個(gè)函數(shù)中打印的count是多少：
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import React, { Component } from 'react';class Test extends Component { constructor(props) { super(props); this.state = { count: 1 }; } handleClick = () => { this.setState((prevState, props) => { return { count: prevState.count + 1 } }, () => { console.log('1', this.state.count); }); this.setState((prevState, props) => { return { count: prevState.count + 1 } }, () => { console.log('2', this.state.count); }); this.setState((prevState, props) => { return { count: prevState.count + 1 } }, () => { console.log('3', this.state.count); }); console.log(this.state.count); } render() { return ( <div style={{ width: '100px', height: '100px', backgroundColor: "yellow" }}> {this.state.count} </div> ) }}export default Test; |
此時(shí)，點(diǎn)擊一次，三個(gè)setState的回調(diào)函數(shù)中，打印結(jié)果如下，可想而知，頁(yè)面的展示結(jié)果也為4
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1: 4
2: 4
3: 4
將上邊代碼放入如componentDidMount中，輸出結(jié)果跟上邊一致。
因?yàn)椋梢缘弥谧远x合成事件和鉤子函數(shù)中，state的更新是異步的。
原生事件和setTimeout中同步更新state
以在setTimeout中setState為例
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import React, { Component } from 'react';class Test extends Component { constructor(props) { super(props); this.state = { count: 1 }; } componentDidMount() { setTimeout(() => { this.setState({ count: this.state.count + 1 }, () => { console.log('1:', this.state.count); }); this.setState({ count: this.state.count + 1 }, () => { console.log('2:', this.state.count); }); this.setState({ count: this.state.count + 1 }, () => { console.log('3:', this.state.count); }); console.log(this.state.count); }, 0); } render() { return ( <div style={{ width: '100px', height: '100px', backgroundColor: "yellow" }}> {this.state.count} </div> ) }}export default Test; |
此時(shí)，打印出的結(jié)果如下：
1: 2
2: 3
3: 4
4
將setState第一個(gè)參數(shù)換為函數(shù)：
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componentDidMount() { setTimeout(() => { this.setState((prevState, props) => { return { count: prevState.count + 1 } }, () => { console.log('1', this.state.count); }); this.setState((prevState, props) => { return { count: prevState.count + 1 } }, () => { console.log('2', this.state.count); }); this.setState((prevState, props) => { return { count: prevState.count + 1 } }, () => { console.log('3', this.state.count); }); console.log(this.state.count); }, 0);} |
打印出的結(jié)果和上邊一致。
是不是有一種state完全可控的感覺(jué)，在setTimeout中，多次setState都會(huì)生效，而且在每一個(gè)setState的第二個(gè)參數(shù)中都可以得到更新后的state。
同樣地，在原生事件中輸出地結(jié)果和setTimeout中一致，也是同步的。
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import React, { Component } from 'react';class Test extends Component { constructor(props) { super(props); this.state = { count: 1 }; } componentDidMount() { document.body.addEventListener('click', this.handleClick, false); } componentWillUnmount() { document.body.removeEventListener('click', this.handleClick, false); } handleClick = () => { this.setState((prevState, props) => { return { count: prevState.count + 1 } }, () => { console.log('1', this.state.count); }); this.setState((prevState, props) => { return { count: prevState.count + 1 } }, () => { console.log('2', this.state.count); }); this.setState((prevState, props) => { return { count: prevState.count + 1 } }, () => { console.log('3', this.state.count); }); console.log(this.state.count); } render() { return ( <div style={{ width: '100px', height: '100px', backgroundColor: "yellow" }} > {this.state.count} </div> ) }}export default Test; |
setState相關(guān)源碼
如下代碼均來(lái)自react17.0.2版本
目錄 ./packages/react/src/ReactBaseClasses.js
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function Component(props, context, updater) { this.props = props; this.context = context; // If a component has string refs, we will assign a different object later. this.refs = emptyObject; // We initialize the default updater but the real one gets injected by the // renderer. this.updater = updater || ReactNoopUpdateQueue;}Component.prototype.isReactComponent = {};Component.prototype.setState = function(partialState, callback) { invariant( typeof partialState === 'object' || typeof partialState === 'function' || partialState == null, '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');}; |
setState可以接收兩個(gè)參數(shù)，第一個(gè)參數(shù)可以是object，function，和null，undefined，就不會(huì)拋出錯(cuò)誤。執(zhí)行下邊的this.updater.enqueueSetState方法。全局查找enqueueSetState，找到兩組目錄下有這個(gè)變量。
首先是第一組目錄：
目錄 ./packages/react/src/ReactNoopUpdateQueue.js 第100行enqueueSetState方法，參數(shù)分別為this，初始化state，回調(diào)，和字符串setState，this是指當(dāng)前React實(shí)例。
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enqueueSetState: function( publicInstance, partialState, callback, callerName,) { warnNoop(publicInstance, 'setState');} |
接著看warnNoop方法：
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const didWarnStateUpdateForUnmountedComponent = {};function warnNoop(publicInstance, callerName) { if (__DEV__) { const constructor = publicInstance.constructor; const componentName = (constructor && (constructor.displayName || constructor.name)) || 'ReactClass'; const warningKey = `${componentName}.${callerName}`; if (didWarnStateUpdateForUnmountedComponent[warningKey]) { return; } console.error( "Can't call %s on a component that is not yet mounted. " + 'This is a no-op, but it might indicate a bug in your application. ' + 'Instead, assign to `this.state` directly or define a `state = {};` ' + 'class property with the desired state in the %s component.', callerName, componentName, ); didWarnStateUpdateForUnmountedComponent[warningKey] = true; }} |
這段代碼相當(dāng)于給didWarnStateUpdateForUnmountedComponent對(duì)象中加入屬性，屬性的key為React 當(dāng)前要setState的組件.setState，如果當(dāng)前有這個(gè)屬性則返回；如果當(dāng)前沒(méi)這個(gè)屬性或者這個(gè)屬性值為false，則設(shè)置這個(gè)屬性的值為true。
再去看另外一個(gè)目錄：
目錄 ./react-reconciler/src/ReactFiberClassComponent.new.js和ReactFiberClassComponent.old.js
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const classComponentUpdater = { enqueueSetState(inst, payload, callback) { const fiber = getInstance(inst); const eventTime = requestEventTime(); const lane = requestUpdateLane(fiber); const update = createUpdate(eventTime, lane); update.payload = payload; if (callback !== undefined && callback !== null) { if (__DEV__) { warnOnInvalidCallback(callback, 'setState'); } update.callback = callback; } enqueueUpdate(fiber, update, lane); const root = scheduleUpdateOnFiber(fiber, lane, eventTime); if (root !== null) { entangleTransitions(root, fiber, lane); } if (__DEV__) { if (enableDebugTracing) { if (fiber.mode & DebugTracingMode) { const name = getComponentNameFromFiber(fiber) || 'Unknown'; logStateUpdateScheduled(name, lane, payload); } } } if (enableSchedulingProfiler) { markStateUpdateScheduled(fiber, lane); } }} |
其中主要看 enqueueUpdate 這個(gè)函數(shù)
目錄 ./react-reconciler/src/ReactUpdateQueue.new.js和ReactUpdateQueue.old.js
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export function enqueueUpdate<State>( fiber: Fiber, update: Update<State>, lane: Lane,) { const updateQueue = fiber.updateQueue; if (updateQueue === null) { // Only occurs if the fiber has been unmounted. return; } const sharedQueue: SharedQueue<State> = (updateQueue: any).shared; if (isInterleavedUpdate(fiber, lane)) { const interleaved = sharedQueue.interleaved; if (interleaved === null) { // This is the first update. Create a circular list. update.next = update; // At the end of the current render, this queue's interleaved updates will // be transfered to the pending queue. pushInterleavedQueue(sharedQueue); } else { update.next = interleaved.next; interleaved.next = update; } sharedQueue.interleaved = update; } else { const pending = sharedQueue.pending; if (pending === null) { // This is the first update. Create a circular list. update.next = update; } else { update.next = pending.next; pending.next = update; } sharedQueue.pending = update; } if (__DEV__) { if ( currentlyProcessingQueue === sharedQueue && !didWarnUpdateInsideUpdate ) { console.error( 'An update (setState, replaceState, or forceUpdate) was scheduled ' + 'from inside an update function. Update functions should be pure, ' + 'with zero side-effects. Consider using componentDidUpdate or a ' + 'callback.', ); didWarnUpdateInsideUpdate = true; } }} |
看到這里，發(fā)現(xiàn)這個(gè)方法是將此次更新的update加入到更新隊(duì)列中，而在這個(gè)版本中并沒(méi)有發(fā)現(xiàn)isBatchingUpdates這個(gè)屬性的出現(xiàn)。貌似React Fiber改動(dòng)還挺大，暫時(shí)先寫到這里，如果有新的發(fā)現(xiàn)會(huì)補(bǔ)充到這里。
總結(jié)
- 自定義合成事件和react鉤子函數(shù)中異步更新state
- 原生事件和setTimeout中同步更新state
以上就是詳解react setState的詳細(xì)內(nèi)容，更多關(guān)于react setState的資料請(qǐng)關(guān)注服務(wù)器之家其它相關(guān)文章！
原文鏈接：https://juejin.cn/post/6948979480358551583
