I'm a newbie of iOS development and I'm confused regarding universal app.
We can use the same code, same xib file for iPhone 4(retina) and previous iPhones, but why we should write two different xibs for iPhone and iPad? What's the difference?
For iPhone and retina iPhone, we use "point" instead of pixel for the coordinate. Why we can't use the similar method for iPhone and iPad?
For some simple apps, it is possible to design your iPhone UI and reuse the same xib file for the iPad. Just select your Target in XCode and copy the Main Interface text from iPhone / iPod Deployment Info to iPad Deployment Info. If you're using a Main Storyboard, copy that too. However, the iPad does not simply scale everything up from the 320*480 / 640*960 iPhone screen to the 768*1024 / 1536*2048 iPad screen. @elgarva correctly says that this would look terrible. Instead, the iPad version makes use of your autosizing masks to resize or reposition each view.
If all of your views can be considered to be left-middle-right or top-middle-bottom, this may work. If you have anything more complicated, you'll need to design a separate iPad interface.
Duplicating your iPhone UI is not just discouraged for aesthetic reasons - iPhones often end up containing a deep and confusing navigation tree for tasks that the iPad can fit on a single screen.
The main reason, is that if you just scale the elements on the UI to fit the larger screen, it wouldn't look nice... and you don't need to do anything for it to work, it automatically does it for you if your app is iPhone only and installed on an iPad (if the user chooses to).
Having a different XIB lets you rearrange your app, and think it so that you can take advantage of the larger screen. You can probably show more information on one iPad view than on 3 different screens on the iPhone... so, your iPhone app could show basic info and expand it when the user taps on it, while your iPad version could show all the information on load, plus extra graphics that look nice but aren't needed, and wouldn't make sense on the iPhone screen.
PS: If you're starting a new app, I strongly suggest you using the storyboard if your app won't have a lot of views... it's really easy to get started and it lets you see your app flow at a glance.
The ratina display just doubles the resolution of original iPhone. If you don't provide separate graphics for retina display, then system just doubles the resolution of resources.
The points are related to physical size of screen, which is similar in old and new iPhones.
For iPads, the screen size changes. This means that its dimension in points will be different from that of iPhone.
duplicating the xib file and renaming that as filename~ipad.xib is working great for me in ios6.1
