React的Transaction浅析

1.两个示例

• 示例1

let SonClass = React.createClass({
  render: function(){
    console.log("render", this.props.num);
    return null;
  },
  componentDidMount: function(){
    console.log('componentDidMount', this.props.num);
  }
});

let FatherClass = React.createClass({
  render:function(){

    return (
      <div>
        <SonClass num="one" />
        <SonClass num="two" />      
      </div>
    )
  }
});

ReactDOM.render( <FatherClass /> ,
    document.getElementById("test")
);

输出为
render *2
componentDidMount *2

• 示例2

let React = require('react');
let ReactDOM = require('react-dom');

let Hello = React.createClass({
    getInitialState: function() {
        return {
            clicked: 0
        };
    },

    handleClick: function() {
        this.setState({
            clicked:this.state.clicked + 1
        });

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

            
    },

    render: function() {
        return <button onClick = {
            this.handleClick
        } > {
            this.state.clicked
        } </button>;
    }
});

ReactDOM.render( <Hello /> ,
    document.getElementById("test")
);

点击后this.state.clicked递增1，而不是递增2。

2.解释

首先介绍React的Transaction。
其源码在React/lib/Transaction.js。
Transaction就是给需要执行的方法fn用wrapper封装了 initialize 和 close 方法。且支持多次封装。再通过 Transaction 提供的 perform 方法执行。 perform执行前，调用所有initialize 方法。perform 方法执行后，调用所有close方法。

Transaction的use case是

1. Preserving the input selection ranges before/after reconciliation.
   Restoring selection even in the event of an unexpected error.
2. Deactivating events while rearranging the DOM, preventing blurs/focuses,
   while guaranteeing that afterwards, the event system is reactivated.
3. Flushing a queue of collected DOM mutations to the main UI thread after a
   reconciliation takes place in a worker thread.
4. Invoking any collected componentDidUpdate callbacks after rendering new
   content.
5. (Future use case): Wrapping particular flushes of the ReactWorker queue
   to preserve the scrollTop (an automatic scroll aware DOM).
6. (Future use case): Layout calculations before and after DOM updates.

示例一，对应的是第4点use case。整个生命周期就是一个Transaction，在Transaction执行期间，componentDidUpdate方法被推入一个队列中。DOM reconciliation后，再调用队列中的所有componentDidUpdate。

示例二，对应的是第3点use case。react的事件回调也是一个Transaction。handleClick里面的this.setState不会马上生效，而是先通过 ReactUpdates.batchedUpdate 方法存入临时队列。所以每次setState时，拿到的this.state.clicked都是初始值。直到transaction 完成，通过ReactUpdates.flushBatchedUpdates方法进行UI更新。
更详细的流程参考此图

3.参考文章（强烈推荐去看）

http://undefinedblog.com/what-happened-after-set-state/
http://zhuanlan.zhihu.com/purerender/20328570