Shows how to configure UIDynamics to dismiss the modal.

DynamicModalBehaviour_dismiss_cancel.swift

let pushBehaviour = UIPushBehavior(items: [mainModalView], mode: .instantaneous)
pushBehaviour.pushDirection = velocity.vector * Constants.pushForceRatio
pushBehaviour.setTargetOffsetFromCenter(offset, for: mainModalView)
animator.addBehavior(pushBehaviour)

animator.addBehavior(gravityBehaviour)

animator.removeBehaviors(itemsAttachmentBehaviours)

// Handy extension
public extension CGPoint {
    var vector: CGVector {
        return CGVector(dx: x, dy: y)
    }
}