如何获取当前视图中成为第一响应者的组件,并取消注册

转：http://www.cnblogs.com/Piosa/archive/2011/10/12/2208323.html

UIWindow *keyWindow = [[UIApplication sharedApplication] keyWindow]; 
UIView *firstResponder = [keyWindow performSelector:@selector(firstResponder)]; 
[firstResponder resignFirstResponder];