问题
My question is simple: when an iPhone app also supports retina display, it does not need an additional xib file. (Fonts and images are auto-scaled, you just need to prepare double-resolution images.) I want that retina view also applies to iPad and hence there's no additional xib files. (Scale a bit and leave a bit margin, maybe.) Yes, I just want it look bigger, but not in the low-resolution version scaled up from 320x480.
回答1:
The iPhone, even with a retina display, is not an iPad. You can update your targets and xcode will convert automatically your views to use the entire screen of the iPad, but it won't make the application conforms to 1. Apple guidelines 2. Users expectations of an universal app.
But, as I said, if you do update your targets, your app might look relatively good (just programatically use UI_USER_INTERFACE_IDIOM to use the @2x.png version of your images).
Edit: I misunderstood your question, and now the corrected answer: There's nothing you can do. The iPad will launch the app as an iPhone app (the small non retina display, pixelated if double sized) if it is defined as such in your plist, and iTunes Connect will sell your app as universal if it isn't defined as an iPhone in the info.plist. You basically have little choice here but to port the app or to more or less forget about iPad users. And Apple certainly wanted things to be that way...
来源：https://stackoverflow.com/questions/6259921/how-to-make-an-iphone-and-iphone4-retina-compatible-app-done-in-cocoa-easily-a