iOS presentViewController背景色透明

ShowResultsViewController *vc = [ShowResultsViewController new];
vc.passDict = dict0;
vc.view.backgroundColor=[UIColor colorWithWhite:0 alpha:0.4];
//关键语句，必须有
vc.modalPresentationStyle = UIModalPresentationOverFullScreen;
[self presentViewController:vc animated:YES completion:^(void){
vc.view.superview.backgroundColor = [UIColor clearColor];
}];