React source reading--Events

react event source codes reading(v16.13.0)

According to the official documentation, the event in react is implemented via a SyntheticEvent, writes a wrapper itself, and uses the eventpool for performance. Let’s see how it works in source codes, this article’s code in my github’s repo event branch。There are serveral types build cmd, please use webpack one.

First of all, we need a demo to see how event is running, the demo in SRC/index. The JSX file (don’t care the other files, I put all react relevant source files here, a bit of a mess 🐶)

import React from './react-dev/react';
import { render } from './react-dev/react-dom';
class App extends React.Component {
  constructor(props) {
    this.state = {count: 0};
    this.handleClick = this.handleClick.bind(this);

  handleClick() {
    this.setState((state) => {
        return {count: state.count + 1};

  render() {
    return [
        <button key="1" onClick={this.handleClick}>Update counter</button>,
        <span key="2">{this.state.count}</span>

render(<App />, document.getElementById('app'));

handleClick is the core of this article. First of all, JSX files need to be compiled into js, which we can compile from the command line of Babel Run in the project root directory:

./node_modules/.bin/babel src/index.jsx

which will generate a index.compiled.js file, this file also in repo’s src/index.compiled.js,main part like this:

var App = /*#__PURE__*/ function (_React$Component) {
  _inherits(App, _React$Component);

  function App(props) {
    var _this;

    _classCallCheck(this, App);

    _this = _possibleConstructorReturn(this, _getPrototypeOf(App).call(this, props));
    _this.state = {
      count: 0
    _this.handleClick = _this.handleClick.bind(_assertThisInitialized(_this));
    return _this;

  _createClass(App, [{
    key: "handleClick",
    value: function handleClick() {
      this.setState(function (state) {
        return {
          count: state.count + 1
  }, {
    key: "render",
    value: function render() {
      return [_react["default"].createElement("button", {
        key: "1",
        onClick: this.handleClick
      }, "Update counter"), _react["default"].createElement("span", {
        key: "2"
      }, this.state.count)];

  return App;

This time we are looking at the event, how do React knows which one is an event, it’s actually what we wrote ‘onClick’, how is this event bound? By debugging the breakpoint in the chrome browser, you can get a chain of calls like this. This is one of the steps of building the page when react is first time rendered: call chain

In the addEventBubbleListener function code, you can see that called on an element of native addEventListener method to bind event, which will be passed through the browser when you click on the event to trigger the function. The function called by browser is an function wrapped the real function by React. In many articles said that react event is binding on the document, how do we know? That’s the function, if you print element, you’ll notice that it’s the root element HTML.

When click event, the dispatchDiscreteEvent function called, in this function to initialize SyntheticEvent, and through the getPooled to get SyntheticEvent instance, which can be confirmed by this demo, as for how to use the pool, when I look at the second click event is directly from eventPool gained, so I think that is the cache of the event object, don’t recreate every time.