presentModalViewController显示半透明的一个view
UIViewController* transparentView = [[UIViewController alloc] init];
UIViewController* controller = self.view.window.rootViewController;
transparentView.view.backgroundColor = [UIColor blackColor];
transparentView.view.alpha = 0.5f;
controller.modalPresentationStyle = UIModalPresentationCurrentContext;
[controller presentModalViewController:transparentView animated:YES];
其要点就是使用iOS特有的rootViewController来显示Modal View