Snapshotting a view that has not been rendered results in an empty snapshot. Ensure your view has been rendered at least once before snapshotting or snapshot after screen updates.

妹的，实在是令人烦躁，在ios7的时候没有问题的。

UIImagePickerController *photoVC=[[UIImagePickerController alloc]init];    photoVC.delegate=self;    photoVC.mediaTypes=@[(NSString *)kUTTypeImage];    photoVC.sourceType=UIImagePickerControllerSourceTypeCamera;    photoVC.allowsEditing=YES;    [self presentViewController:photoVC animated:YES completion:NULL];

查了很多资料，原来不止我一个人遇到。。。

http://stackoverflow.com/questions/25884801/ios-8-snapshotting-a-view-that-has-not-been-rendered-results-in-an-empty-snapsho

反正对程序无影响，就继续用，真心不知道该如何完美解决。