三种获取当前window的方法
in 知识 on Object-c
UIWindow *window = [[[UIApplication sharedApplication] delegate] window];
UIWindow *window = [[UIApplication sharedApplication].delegate window]
UIWindow * window = [[UIApplication sharedApplication].keyWindow ];
in 知识 on Object-c
UIWindow *window = [[[UIApplication sharedApplication] delegate] window];
UIWindow *window = [[UIApplication sharedApplication].delegate window]
UIWindow * window = [[UIApplication sharedApplication].keyWindow ];