利用模态弹出VC实现类似AlertView的透明黑色背景

• iOS 7：利用AppDelegate

AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
    
UIViewController *vc = [[UIViewController alloc] init];
appDelegate.window.rootViewController.modalPresentationStyle = UIModalPresentationCurrentContext;
[appDelegate.window.rootViewController presentViewController:vc animated:YES completion:^{
    vc.view.backgroundColor=[UIColor colorWithRed:0 green:0 blue:0 alpha:0.8];
    appDelegate.window.rootViewController.modalPresentationStyle = UIModalPresentationFullScreen;
}];

• iOS 8：利用最新的modalpresentationstyle:UIModalPresentationOverCurrentContext

TestController *vc = [[TestController alloc] init];
vc.modalPresentationStyle = UIModalPresentationOverCurrentContext;
[self presentViewController:vc animated:NO completion:^{
        vc.view.backgroundColor = [UIColor colorWithRed:0 green:0 blue:0 alpha:0.8];
}];