详解react setState

setState是同步还是异步

自定义合成事件和react钩子函数中异步更新state

以在自定义click事件中的setState为例

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;

点击一次，最终this.state.count的打印结果是1，页面展示的是2。通过现象看，三次setState只是最后一次setState生效了，前两次都setState无效果。因为假如把第一次setState改为+3，count打印结果为1，展示结果为2，没有发生变化。而且没有同步获得count的结果。

此时，我们可以调整代码，通过setState的第二个参数，来获得更新后的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;

此时，点击一次，三个setState的回调函数中，打印结果分别是。

1
1: 2
2: 2
3: 2

首先，最后一行直接打印1。然后，在setState的回调中，打印出的结果都是最新更新的2。虽然前两次setState未生效，但是它们第二个参数中还是会打印出2。

此时将setState的第一个参数换成函数，通过函数的第一个参数可以获得更新前的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;

此时，打印出的结果为1，但是页面展示出来的count为4。可以发现，如果setState以传参的方式去更新state，几次setState并不会只更新最后一次，而是几次更新state都会生效。

接下来看下第二个函数中打印的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;

此时，点击一次，三个setState的回调函数中，打印结果如下，可想而知，页面的展示结果也为4

1
1: 4
2: 4
3: 4

将上边代码放入如componentDidMount中，输出结果跟上边一致。

因为，可以得知，在自定义合成事件和钩子函数中，state的更新是异步的。

原生事件和setTimeout中同步更新state

以在setTimeout中setState为例

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;

此时，打印出的结果如下：

1: 2
2: 3
3: 4
4

将setState第一个参数换为函数：

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);
}

打印出的结果和上边一致。

是不是有一种state完全可控的感觉，在setTimeout中，多次setState都会生效，而且在每一个setState的第二个参数中都可以得到更新后的state。

同样地，在原生事件中输出地结果和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相关源码

如下代码均来自react17.0.2版本

目录 ./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可以接收两个参数，第一个参数可以是object，function，和null，undefined，就不会抛出错误。执行下边的this.updater.enqueueSetState方法。全局查找enqueueSetState，找到两组目录下有这个变量。

首先是第一组目录：

目录 ./packages/react/src/ReactNoopUpdateQueue.js 第100行enqueueSetState方法，参数分别为this，初始化state，回调，和字符串setState，this是指当前React实例。

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

接着看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;
  }
}

这段代码相当于给didWarnStateUpdateForUnmountedComponent对象中加入属性，属性的key为React 当前要setState的组件.setState，如果当前有这个属性则返回；如果当前没这个属性或者这个属性值为false，则设置这个属性的值为true。

再去看另外一个目录：

目录 ./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 这个函数

目录 ./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;
    }
  }
}

看到这里，发现这个方法是将此次更新的update加入到更新队列中，而在这个版本中并没有发现isBatchingUpdates这个属性的出现。貌似React Fiber改动还挺大，暂时先写到这里，如果有新的发现会补充到这里。

总结

• 自定义合成事件和react钩子函数中异步更新state
• 原生事件和setTimeout中同步更新state

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