Warning:
setState(...): Can only update a mounted or mounting component.
This usually means you called setState() on an unmounted component.
This is a no-op.
Please check the code for the xxx component.
大概意思就是我们可能对一个没有装载的组件执行了setState()操作，在React的官网里有一个解决这个办法的方案，isMounted
原因：
Such situations most commonly occur due to callbacks,
when a component is waiting for some data and gets unmounted before the data arrives.
Ideally, any callbacks should be canceled in componentWillUnmount, prior to unmounting.
大概意思是这种情况会出现在callback中，我们的异步请求返回数据之前，组件可能就已经被卸载了，等数据回来再使用setState就会报出上面的警告，所以我们应该手动在componentWillUnmount里去取消callback在它被unmounting之前。
解决办法：
Just set a _isMounted property to true in componentDidMount
and set it to false in componentWillUnmount, and use this variable to check your component's status.
很好理解，我们使用一个标志位_isMounted，在componentDidMount里设置为true,在componentWillUnmount里设置为false，仅当_isMounted为true即还未被卸载，才执行setState()。
我的代码：
componentWillMount() {
this._isMounted = true
fetch('网络请求').then (status, response)=>{
if (this._isMounted) {
this.setState({
activityTipsImg: response.data.tips_url,
activityTipsContent: response.data.tips_content
})
}
});
}
componentWillUnmount() {
this._isMounted = false
}
