iOS模态出一个半透明的ViewController

UIViewController * vc = [[UIViewController alloc]init];    vc.definesPResentationContext = YES;     vc.view.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:.4];    vc.modalPresentationStyle = UIModalPresentationOverCurrentContext;    [self presentViewController:vc animated:YES completion:nil];