when you back to a keepAlive Page, the keepAlive page's activated will be called before the returning page's componentWillUnmount gets called.

recore的时序问题，和keepAlive页面相关。
当从其它页面back回到keepAlive页面时，keepAlive页面的activated会先得到调用，然后才是返回前页面的componentWillUnmount得到调用。