The Core iOS Developer’s Cookbook, Fifth Edition

CGRects and Centers

As you’ve seen, UIView instances use a CGRect structure, composed of an origin and a size, to define their frame. A CGRect structure contains no references to a center point. At the same time, UIViews depend on their center property to update a view’s position when you move a view to a new point. Unfortunately, Core Graphics doesn’t use centers as a primary rectangle concept. As far as centers are concerned, the built-in utilities in Core Graphics are limited to recovering a rectangle’s midpoint along the x- or y-axis.

You can bridge this gap by constructing functions that coordinate between the origin-based CGRect struct and center-based UIView objects. Such a function retrieves the center from a rectangle by building a point ...