詳解react setState

更新時間：2021年04月10日 11:56:29 作者：一個前端王

這篇文章主要介紹了react setState的相關(guān)資料，幫助大家更好的理解和學(xué)習(xí)使用react，感興趣的朋友可以了解下

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

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;

點擊一次，最終this.state.count的打印結(jié)果是1，頁面展示的是2。通過現(xiàn)象看，三次setState只是最后一次setState生效了，前兩次都setState無效果。因為假如把第一次setState改為+3，count打印結(jié)果為1，展示結(jié)果為2，沒有發(fā)生變化。而且沒有同步獲得count的結(jié)果。

此時，我們可以調(diào)整代碼，通過setState的第二個參數(shù)，來獲得更新后的state：

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，點擊一次，三個setState的回調(diào)函數(shù)中，打印結(jié)果分別是。

1
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，但是它們第二個參數(shù)中還是會打印出2。

此時將setState的第一個參數(shù)換成函數(shù)，通過函數(shù)的第一個參數(shù)可以獲得更新前的state。

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，打印出的結(jié)果為1，但是頁面展示出來的count為4?？梢园l(fā)現(xiàn)，如果setState以傳參的方式去更新state，幾次setState并不會只更新最后一次，而是幾次更新state都會生效。

接下來看下第二個函數(shù)中打印的count是多少：

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，點擊一次，三個setState的回調(diào)函數(shù)中，打印結(jié)果如下，可想而知，頁面的展示結(jié)果也為4

1
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。

因為，可以得知，在自定義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

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，打印出的結(jié)果如下：

1: 2
2: 3
3: 4
4

將setState第一個參數(shù)換為函數(shù)：

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，在setTimeout中，多次setState都會生效，而且在每一個setState的第二個參數(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。

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

如下代碼均來自react17.0.2版本

目錄 ./packages/react/src/ReactBaseClasses.js

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可以接收兩個參數(shù)，第一個參數(shù)可以是object，function，和null，undefined，就不會拋出錯誤。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實例。

enqueueSetState: function(
  publicInstance,
  partialState,
  callback,
  callerName,
) {
  warnNoop(publicInstance, 'setState');
}

接著看warnNoop方法：

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)前有這個屬性則返回；如果當(dāng)前沒這個屬性或者這個屬性值為false，則設(shè)置這個屬性的值為true。

再去看另外一個目錄：

目錄 ./react-reconciler/src/ReactFiberClassComponent.new.js和ReactFiberClassComponent.old.js

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 這個函數(shù)

目錄 ./react-reconciler/src/ReactUpdateQueue.new.js和ReactUpdateQueue.old.js

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)這個方法是將此次更新的update加入到更新隊列中，而在這個版本中并沒有發(fā)現(xiàn)isBatchingUpdates這個屬性的出現(xiàn)。貌似React Fiber改動還挺大，暫時先寫到這里，如果有新的發(fā)現(xiàn)會補充到這里。