面试官：react中的setState是同步的还是异步的_2023-02-19

export default class App extends React.Component {
  state = {
    num: 0,
  };
  updateNum = () => {
    console.log("before", this.state.num);

    this.setState({ num: this.state.num + 1 });
    this.setState({ num: this.state.num + 1 });
    console.log("after", this.state.num);

  };
  render() {
    const { num } = this.state;
    console.log("render", num);
    return <button onClick={this.updateNum}>hello {num}</button>;
  }
}

//打印结果
//render     0
//before     0
//after     0
//render     1

export default class App extends React.Component {
  state = {
    num: 0,
  };
  updateNum = () => {
    console.log("before", this.state.num);

    setTimeout(() => {
        this.setState({ num: this.state.num + 1 });
        this.setState({ num: this.state.num + 1 });
        console.log("after", this.state.num);
    }, 0);
  };
  render() {
    const { num } = this.state;
    console.log("render", num);
    return <button onClick={this.updateNum}>hello {num}</button>;
  }
}

//打印结果
//render     0
//before     0
//render     1
//render     2
//after     2

import { unstable_batchedUpdates } from "react-dom";

export default class App extends React.Component {
  state = {
    num: 0,
  };
  updateNum = () => {
    console.log("before", this.state.num);

    setTimeout(() => {
      unstable_batchedUpdates(()=>{
        this.setState({ num: this.state.num + 1 });
        this.setState({ num: this.state.num + 1 });
        console.log("after", this.state.num);
      })
    }, 0);
  };
  render() {
    const { num } = this.state;
    console.log("render", num);
    return <button onClick={this.updateNum}>hello {num}</button>;
  }
}

//打印结果
//render     0
//before     0
//after     0
//render     1

import React from "react";
import ReactDOM from "react-dom";

export default class App extends React.Component {
  state = {
    num: 0,
  };
  updateNum = () => {
    console.log("before", this.state.num);

    setTimeout(() => {
        this.setState({ num: this.state.num + 1 });
        this.setState({ num: this.state.num + 1 });
        console.log("after", this.state.num);
    }, 0);
  };
  render() {
    const { num } = this.state;
    console.log("render", num);
    return <button onClick={this.updateNum}>hello {num}</button>;
  }
}

ReactDOM.unstable_createRoot(rootEl).render(<App />);

//打印结果
//render     0
//before     0
//after     0
//render     1

onClick() {
  this.setState({ count: this.state.count + 1 });
  this.setState({ count: this.state.count + 1 });
}

在之前的react版本中如果脱离当前的上下文就不会被合并，例如把多次更新放在setTimeout中，原因是处于同一个context的多次setState的executionContext都会包含BatchedContext，包含BatchedContext的setState会合并，当executionContext等于NoContext，就会同步执行SyncCallbackQueue中的任务，所以setTimeout中的多次setState不会合并，而且会同步执行。

onClick() {
 setTimeout(() => {
    this.setState({ count: this.state.count + 1 });
    this.setState({ count: this.state.count + 1 });
  });
}

export function batchedUpdates<A, R>(fn: A => R, a: A): R {
  const prevExecutionContext = executionContext;
  executionContext |= BatchedContext;
  try {
    return fn(a);
  } finally {
    executionContext = prevExecutionContext;
    if (executionContext === NoContext) {
      resetRenderTimer();
       //executionContext为NoContext就同步执行SyncCallbackQueue中的任务
      flushSyncCallbackQueue();
    }
  }
}

在Concurrent mode下，上面的例子也会合并为一次更新，根本原因在如下一段简化的源码，如果多次setState，会比较这几次setState回调的优先级，如果优先级一致，则先return掉，不会进行后面的render阶段

function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
  const existingCallbackNode = root.callbackNode;//之前已经调用过的setState的回调
  //...
    if (existingCallbackNode !== null) {
    const existingCallbackPriority = root.callbackPriority;
    //新的setState的回调和之前setState的回调优先级相等 则进入batchedUpdate的逻辑
    if (existingCallbackPriority === newCallbackPriority) {
      return;
    }
    cancelCallback(existingCallbackNode);
  }
    //调度render阶段的起点
    newCallbackNode = scheduleCallback(
    schedulerPriorityLevel,
    performConcurrentWorkOnRoot.bind(null, root),
  );
    //...
}

那为什么在Concurrent mode下，在setTimeout回调多次setState优先级一致呢，因为在获取Lane的函数requestUpdateLane，只有第一次setState满足currentEventWipLanes === NoLanes，所以他们的currentEventWipLanes参数相同，而在findUpdateLane中schedulerLanePriority参数也相同（调度的优先级相同），所以返回的lane相同。

export function requestUpdateLane(fiber: Fiber): Lane {
    //...
  if (currentEventWipLanes === NoLanes) {//第一次setState满足currentEventWipLanes === NoLanes
    currentEventWipLanes = workInProgressRootIncludedLanes;
  }
  //...
  //在setTimeout中schedulerLanePriority, currentEventWipLanes都相同，所以返回的lane也相同
  lane = findUpdateLane(schedulerLanePriority, currentEventWipLanes);
  //...

  return lane;
}