When testing on my iPad 3,
[[UIScreen mainScreen] scale] == 1.0
My understanding is that this should be 2.0.
I am using the iOS 5.1 SDK with Xcode 4.2 on Snow Leopard, which I understand is not an "officially supported" configuration. However, I can't imagine why that would affect what is happening purely on the device. (I can imagine all sorts of ways it would break the simulator.)
Before XCode version 4.3, [UIScreen scale] will return 1.0 for an iPad 3. Updating to 4.3 will enable the correct value of 2.0 being returned.
That's because the screen is not scaled. You can get the ratio like this:
[[CCDirector sharedDirector] winSize].width/[[CCDirector sharedDirector] winSizeInPixels].width
If this is equal to 1 then you are on a non-retina device. Otherwise , if 2 , on a retina display. Also , don't forget to enable retina support.
Regards,
George
来源：https://stackoverflow.com/questions/11117041/why-is-uiscale-mainscreen-scale-1-on-an-ipad-3-with-xcode-4-2