sizeClass

Size class is a design concept used in user interface development to categorize screen space into a small set of buckets so UIs can adapt across devices and orientations. In iOS, a size class is defined along two axes: horizontal and vertical. Each axis can be compact or regular, forming a size-class pair such as (compact, regular) or (regular, regular). There is also an unspecified or any category used in certain tools to indicate applicability across multiple classes.

Developers access size class information through trait collections as part of a view's environment. At design time, Interface Builder lets you vary layouts by size class, while at runtime you can query the current trait collection and adjust constraints, visibility, font sizes, or control layouts accordingly. Tools like Auto Layout and SwiftUI encourage building interfaces that reflow automatically when the size class changes, rather than building separate screens for each device.

Common usage patterns include simplifying navigation for compact width (for example, stacking content or collapsing sidebars) and expanding multi-panel layouts in regular width. On iPhone in portrait, the horizontal size class is typically compact and the vertical is regular; on iPad, in landscape you may see both axes as regular. However, exact pixel dimensions are not encoded by size classes, so developers may need additional breakpoints or runtime checks for edge cases.

Limitations: size classes are coarse, and newer layouts may require more granular adjustments. They remain a foundational tool for creating responsive, platform-adaptive interfaces.