React的Transaction淺析

1.兩個示例

• 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")
);

輸出爲
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")
);

點擊後this.state.clicked遞增1，而不是遞增2。

2.解釋

首先介紹React的Transaction。
其源碼在React/lib/Transaction.js。
Transaction就是給須要執行的方法fn用wrapper封裝了 initialize 和 close 方法。且支持屢次封裝。再經過 Transaction 提供的 perform 方法執行。 perform執行前，調用全部initialize 方法。perform 方法執行後，調用全部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，對應的是第4點use case。整個生命週期就是一個Transaction，在Transaction執行期間，componentDidUpdate方法被推入一個隊列中。DOM reconciliation後，再調用隊列中的全部componentDidUpdate。

示例二，對應的是第3點use case。react的事件回調也是一個Transaction。handleClick裏面的this.setState不會立刻生效，而是先經過 ReactUpdates.batchedUpdate 方法存入臨時隊列。因此每次setState時，拿到的this.state.clicked都是初始值。直到transaction 完成，經過ReactUpdates.flushBatchedUpdates方法進行UI更新。
更詳細的流程參考此圖

3.參考文章（強烈推薦去看）

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