I program for both iPhone and iPad. When doing iPhone stuff, I prefer the retina iPhone simulator. When doing iPad stuff, I prefer the non-retina simulator. Swapping between them is a two-step process for me. I have to switch xCode from the "iPhone simulator" target to the "iPad simulator" target. The problem is that when I do this, the simulator switches from retina iPhone to retina iPad. I therefore also have to go into the simulator app and change the hardware to non-retina iPad.
Is there a way to accomplish that in a single step?
Yes! and its super easy. In Xcode 4+ click the dropdown with the name of your target in it in the top menu bar next to the play button. Then click edit scheme.
Click the run action and then click the Options tab. You will see you can specify the type of screen you want to use when you click the play button (build and run) in Xcode.
Then when you switch device target in Xcode it will automatically switch the simulator to the right device and screen type for you.
Also if you're running less than iOS 5.1 the simulator will switch back to non-retina. Set SDK to 5.1+
来源：https://stackoverflow.com/questions/12897963/ios-simulator-swap-from-retina-iphone-to-non-retina-ipad-in-a-single-step