iOS 7 Programming Cookbook

2.2. Detecting and Reacting to Collisions Between UI Components

Problem

You want to specify collision boundaries between your UI components on the screen so that they will not overlap one another.

Solution

Instantiate an object of type UICollisionBehavior and attach it to your animator object. Set the translatesReferenceBoundsIntoBoundary property of your collision behavior to YES and ensure that your animator is initialized with your superview as its reference value. This will ensure that the subviews that are the targets of your collision behavior (as will be discussed soon) will not go outside the boundaries of your superview.

Discussion

A collision behavior of type UICollisionBehavior takes in objects that conform to the UIDynamicItem protocol. All views of type UIView already conform to this protocol, so all you have to do is instantiate your views and add them to the collision behavior. A collision behavior requires you to define the boundaries that the items in the animator will not be able to go past. For instance, if you define a line that runs from the bottom-left edge to the bottom-right edge of your reference view (the bottommost horizontal line of your reference view), and add a gravity behavior to your view as well, those views will be pulled down by gravity to the bottom of the view but will not be able to go further because they will collide with the bottom edge of the view, defined by the collision behavior.

If you want your reference view’s boundaries to be considered ...