1）componentWillMount is executed before rendering, on both the server and the client side.
2）componentDidMount is executed after the first render only on the client side. This is where AJAX requests and DOM or state updates should occur. This method is also used for integration with other JavaScript frameworks and any functions with delayed execution such as setTimeout or setInterval. We are using it to update the state so we can trigger the other lifecycle methods.
3）componentWillReceiveProps is invoked as soon as the props are updated before another render is called. We triggered it from setNewNumber when we updated the state.
4）shouldComponentUpdate should return true or false value. This will determine if the component will be updated or not. This is set to true by default. If you are sure that the component doesn't need to render after state or props are updated, you can return false value.
5）componentWillUpdate is called just before rendering.
6）componentDidUpdate is called just after rendering.
7）componentWillUnmount is called before the component is unmounted from the dom.
其中componentDidUpdate(prevProps, prevState)
在render()之后调用
里面的参数是更新前的props和state
遇到的问题是，得到的prevProps和当前更新后的props是一样的
研究后发现props里面是一个数组的引用，所以始终是同个引用
如果props里是个整数之类的，就没问题的