React createClass 和 Component 有什么区别?

以前是这样写的

let app = React.createClass({
    getInitialState: function(){
        // some thing
    }
})

然后也看到用 React.Component，于是

class TodoApp extends React.Component{
    getInitialState(){
         // some thing
    }
}

但是压根没有执行 getInitialState，控制台提示

Warning: getInitialState was defined on TodoApp, a plain JavaScript class. This is only supported for classes created using React.createClass. Did you mean to define a state property instead?

问：

• createÇlass Component 区别，什么时候用什么？

• 生命周期的componentDidMount 的 component 又啥联系？

• 用Component形式又怎么初始化state?

---

官网上有提及两者：

The API (via 'extends React.Component') is similar to React.createClass with the exception of getInitialState. Instead of providing a separate getInitialState method, you set up your own state property in the constructor.

链接描述

也就是说 通过es6类的继承实现时 state的初始化要在constructor中声明

class exampleComponent extends React.Component {
    constructor(props) {
        super(props);
        this.state = {example: 'example'}
    }

}

---

React在ES6的实现中去掉了getInitialState这个hook函数，规定state在constructor中实现，如下：

Class App extends React.Component {

constructor(props) {
    super(props);
    this.state = {};
}    
...

}

Babel的Blog上还有一种实现方法，即直接使用赋值语句：

Class App extends React.Component {

constructor(props) {
    super(props);
}

state = {}    
...

}