在当前controller present A Controller，透明显示
A*a = [[A alloc]init];
self.definesPresentationContext=YES;
a.view.backgroundColor= [UIColor colorWithRed:0 green:0 blue:0 alpha:0];
catalogViewController.modalPresentationStyle=UIModalPresentationOverCurrentContext;
[selfpresentViewController:catalogViewControlleranimated:YEScompletion:nil];
