React异步请求数据出现setState(...): Can only update a mounted or mounting component...

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
}