The Component Lifecycle
Each component has several "lifecycle methods" that you can override to run code at particular times in the process. Methods prefixed with **will
** are called right before something happens, and methods prefixed with **did
** are called right after something happens.
Mounting
These methods are called when an instance of a component is being created and inserted into the DOM:
constructor()
componentWillMount()
render()
componentDidMount()
mount 就是第一次让组件出现在页面中的过程。这个过程的关键就是 render 方法。React 会将 render 的返回值（一般是虚拟 DOM，也可以是 DOM 或者 null）插入到页面中。
这个过程会暴露几个钩子（hook）方便你往里面加代码：
constructor()
componentWillMount()
render()
componentDidMount()
Updating
mount 之后，如果数据有任何变动，就会来到 update 过程，这个过程有 5 个钩子：
1.componentWillReceiveProps(nextProps) - 我要读取 props 啦！
- shouldComponentUpdate(nextProps, nextState) - 请问要不要更新组件？true / false
- componentWillUpdate() - 我要更新组件啦！
- render() - 更新！
- componentDidUpdate() - 更新完毕啦！
An update can be caused by changes to props or state. These methods are called when a component is being re-rendered:
componentWillReceiveProps()
shouldComponentUpdate()
componentWillUpdate()
render()
componentDidUpdate()
Unmounting
当一个组件将要从页面中移除时，会进入 unmount 过程，这个过程就一个钩子：
componentWillUnmount() - 我要死啦！
你可以在这个组件死之前做一些清理工作。
This method is called when a component is being removed from the DOM:
componentWillUnmount()
