ElasticTransition.swift

public class ElasticTransition : NSObject, ElasticMotionStateMachineDelegate {
  init(presentedViewController: UIViewController, presentingViewWidth: Float) {
    self.stateMachine = ElasticMotionStateMachine(ElasticMotionDirection.Right, threshold: threshold, vibrationSec: 2.0)
    self.stateMachine.delegate = self
  }
  
  func elasticMotionStateMachine(stateMachine: ElasticMotionStateMachine, didChangeState state: ElasticMotionState, deltaPoint: CGPoint) {
    if stateMachine.direction == ElasticMotionDirection.Right {
      let fullOpenedWidth = CGFloat(presentingViewWidth)
      switch state {
      case .MayOpen:
          let newOriginX = presentedViewController.view.frame.origin.x + deltaPoint.x
          if newOriginX >= 0 && newOriginX < CGFloat(threshold) {
              presentedViewController.view.center = CGPointMake(presentedViewController.view.center.x + deltaPoint.x, presentedViewController.view.center.y)
          }
      case .MayClose:
          let newOriginX = presentedViewController.view.frame.origin.x + deltaPoint.x
          if newOriginX >= 0 && newOriginX < fullOpenedWidth {
              presentedViewController.view.center = CGPointMake(presentedViewController.view.center.x + deltaPoint.x, presentedViewController.view.center.y)
          }
      case .WillClose:
          presentedViewController.view.center = CGPointMake(presentedViewController.view.frame.width / 2 + CGFloat(threshold), presentedViewController.view.center.y)
      case .Closed:
          presentedViewController.view.center = CGPointMake(presentedViewController.view.frame.width / 2, presentedViewController.view.center.y)
      case .WillOpen:
          presentedViewController.view.center = CGPointMake(presentedViewController.view.frame.width / 2 + CGFloat(threshold), presentedViewController.view.center.y)
      case .Opened:
          presentedViewController.view.center = CGPointMake(presentedViewController.view.frame.width / 2 + fullOpenedWidth, presentedViewController.view.center.y)
      }
    }
    // TODO: handle other directions
}