UIViewController present出来一个透明的UIViewController  

其实很简单，代码如下

MyViewController *checkOut = [[MyViewController alloc]  init];
    checkOut.providesPresentationContextTransitionStyle = YES;
    checkOut.definesPresentationContext = YES;
    checkOut.modalPresentationStyle = UIModalPresentationOverCurrentContext;
    [self presentViewController:checkOut animated:YES completion:nil];

参考

​​http://stackoverflow.com/questions/21760698/ios-modalview-with-background-transparent​​