欲了解更多信息 – Documentation
iOS 8 makes dealing with screen size and orientation much more
versatile. It is easier than ever to create a single interface for
your app that works well on both iPad and iPhone,adjusting to
orientation changes and different screen sizes as needed. Design apps
with a common interface and then customize them for different size
classes. Adapt your user interface to the strengths of each form
factor. You no longer need to create a specific iPad storyboard;
instead target the appropriate size classes and tune your interface
for the best experience.
There are two types of size classes in iOS 8: regular and compact. A
regular size class denotes either a large amount of screen space,such
as on an iPad,or a commonly adopted paradigm that provides the
illusion of a large amount of screen space,such as scrolling on an
iPhone. Every device is defined by a size class,both vertically and
horizontally. iPad size classes shows the native size classes for the
iPad. With the amount of screen space available,the iPad has a
regular size class in the vertical and horizontal directions in both
portrait and landscape orientations.
编辑：它仅支持iOS 8(向后兼容仅适用于iOS 7)及更高版本.