问题：

在主线程之外的线程更改界面，会报错：This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes. This will cause an exception in a future release.

解决方法：

把更改界面的语句写在这里面：

dispatch_async(dispatch_get_main_queue(), {

　　 // code here

})

只能在主线程更改界面