Obtaining the device's screen dimensions

Before iPhone 4, developers were lucky to have to work with just one resolution—320 x 480 pixels. When iPhone 4 arrived, it came with a retina screen. This effectively doubled the resolution, while allowing developers to lay out apps with non-retina dimensions.

So, for example, if you wanted to position something in the middle the screen, you would usually specify 160 pixels. On an iPhone 4, this would actually be 320 pixels. iOS would take care of the positioning based on whether you were using a retina or non-retina device.

When iPhone 5 was released, the effective non-retina resolution changed to 320 x 568 pixels. Again, this was manageable because of the way iOS handled the screen densities, but also because it effectively made the screen taller.

All this changed yet again with the release of iPhone 6 and 6 Plus. Apple introduced new non-retina resolutions of 375 x 667 pixels for iPhone 6 and 540 x 960 pixels for iPhone 6 Plus. Suddenly...