react.js,_componentWillMount和DidMount具体有什么用,难道只提供一个渲染前和渲染后内调用方法的环境，react.js

componentWillMount和DidMount具体有什么用,难道只提供一个渲染前和渲染后内调用方法的环境

!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>react.js</title>
    <script src="js/react.js"></script>
    <script src="js/react-dom.js"></script>    
      <script src="js/browser.min.js"></script>
      
     
    </head>
    <body>
        <div id="container">
            
        </div>
        <script type="text/babel">
            var Timer = React.createClass({displayName: "Timer",
  getInitialState: function() {
    return {secondsElapsed: 0};
  },
  tick: function() {
    this.setState({secondsElapsed: this.state.secondsElapsed + 1});
  },
  componentDidMount: function() {
    this.interval = setInterval(this.tick, 1000);
  },
  componentWillUnmount: function() {
    clearInterval(this.interval);
  },
  render: function() {
    return (
      React.createElement("div", null, "Seconds Elapsed: ", this.state.secondsElapsed)
    );
  }
});

ReactDOM.render(React.createElement(Timer, null),
 document.getElementById('container'));
        
    
        </script>
    </body>
</html>

基本是这意思。componentWillMount用的较少，因为它必须得是同步的；componentDidMount用的较多，基本上调ajax，改DOM的事情都放这里