模态出页面，半透明效果

/这是要模态出的页面/
QuitViewController *quitVC = [[QuitViewController alloc]init];
self.definesPresentationContext = YES;
quitVC.modalPresentationStyle = UIModalPresentationCustom;
// UIModalPresentationCustom修改不同，可以实现不同效果
quitVC.view.backgroundColor = [UIColor colorWithWhite:0.f alpha:0.7];
[self presentViewController:quitVC animated:YES completion:^{

}];