More fun, by adding UIDynamicItemBehavior

FallingSquareViewController.swift

    lazy var dynamicItemBehavior:UIDynamicItemBehavior = {
        let lazyBehavior = UIDynamicItemBehavior()
        // Let's make our square elastic
        // 0 = no elacticity, 1.0 = max elacticity
        lazyBehavior.elasticity = 0.8

        // Other configurations
        // lazyBehavior.allowsRotation
        // lazyBehavior.density
        // lazyBehavior.friction
        // lazyBehavior.resistance
        
        return lazyBehavior
        }()
        
    func animateSquare(){
        animator.addBehavior(gravity)
        animator.addBehavior(collider)
        // Add the dynamicItemBehavior to the animator
        animator.addBehavior(dynamicItemBehavior)

        collider.addItem(squareView)
        gravity.addItem(squareView)
        // And add the squareView to the dynamicItemBehavior
        dynamicItemBehavior.addItem(squareView)
    }