怎么判断iphone4、iphone4s、iphone5、iPad、iPad retina

if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)     {        CGFloat scale = [[UIScreen mainScreen] scale];           if (scale > 1.0)            {                //iPad retina screen           }             else           {                //iPad screen           }    }     else    {         if ([UIScreen instancesRespondToSelector:@selector(scale)])          {               CGFloat scale = [[UIScreen mainScreen] scale];               if (scale > 1.0)                {                    if([[ UIScreen mainScreen ] bounds ].size.height == 568)                    {                        //iphone 5                    }                    else                    {                        //iphone retina screen                    }               }               else               {                    //iphone screen               }         }    }