【React Native】错误提示： 在React开发中，我们可能经常会遇到这个一个警告： Can't perform a React state update on an unmounted component.

　　
　　在React开发中，我们可能经常会遇到这个一个警告：

Can't perform a React state update on an unmounted component. 
This is a no-op, but it indicates a memory leak in your application.
 To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method”

　　不能在组件销毁后设置state，防止出现内存泄漏的情况。