In Swift’s UIKit Dynamics, how can I define a circle boundary to contain a UIView? - swift

Question

I have researched a LOT, but the only examples I can find anywhere are for the purpose of defining the bounds of a UIView so that they collide/bounce off each other on the OUTSIDE of the objects.
Example: A ball hits another ball and they bounce away from each other.
But what I want to do is create a circular view to CONTAIN other UIViews, such that the containing boundary is a circle, not the default square. Is there a way to achieve this?

Answer 1

Yes, that's totally possible. The key to achieving collision within a circle is to
Set the boundary for the collision behaviour to be a circle path (custom UIBezierPath) and
Set the animator’s referenceView to be the circle view.
Output:
Storyboard setup:
Below is the code of the view controller for the above Storyboard. The magic happens in the simulateGravityAndCollision method:
Full Xcode project
class ViewController: UIViewController {
#IBOutlet weak var redCircle: UIView!
#IBOutlet weak var whiteSquare: UIView!
var animator:UIDynamicAnimator!
override func viewDidLoad() {
super.viewDidLoad()
self.redCircle.setCornerRadius(self.redCircle.bounds.width / 2)
}
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 0.5) { [unowned self] in
self.simulateGravityAndCollision()
}
}
func simulateGravityAndCollision() {
//The dynamic animation happens only within the reference view, i.e., our red circle view
animator = UIDynamicAnimator.init(referenceView: self.redCircle)
//Only the inside white square will be affected by gravity
let gravityBehaviour = UIGravityBehavior.init(items: [self.whiteSquare])
//We also apply collision only to the white square
let collisionBehaviour = UICollisionBehavior.init(items:[self.whiteSquare])
//This is where we create the circle boundary from the redCircle view's bounds
collisionBehaviour.addBoundary(withIdentifier: "CircleBoundary" as NSCopying, for: UIBezierPath.init(ovalIn: self.redCircle.bounds))
animator.addBehavior(gravityBehaviour)
animator.addBehavior(collisionBehaviour)
}
}
extension UIView {
open override func awakeFromNib() {
super.awakeFromNib()
self.layer.allowsEdgeAntialiasing = true
}
func setCornerRadius(_ amount:CGFloat) {
self.layer.cornerRadius = amount
self.layer.masksToBounds = true
self.clipsToBounds = true
}
}