An Illustrated History of iOS
The world, once more, is flat: iOS 7 marks the most extensive design change in the history of the platform. With a flat, simplistic, and colorful redesign, Jony Ive took command of the design on iOS, too.
Adaptivity and Layout - Visual Design - iOS - Human Interface Guidelines - Apple Developer
People generally want to be able to use their favorite apps on all of their devices and in any context. In an iOS app, you can configure interface elements and layouts to automatically change shape and size on different devices, during multitasking on iPad, in split view, when the screen rotates, and more.