from https://github.com/jongwonwoo/elastic-motion/blob/master/ElasticMotion/ElasticMotionStateMachine.swift

ElasticStateMachine.swift

class ElasticMotionStateMachine {
  func keepMoving(currentPoint: CGPoint) {
    if self.state == .Closed || self.state == .WillClose || self.state == .Opened || self.state == .WillOpen {
        self.beginPoint = currentPoint
        self.totalMovingPoint = CGPointZero
    }
    
    let delta = self.deltaPointFromCurrentPoint(currentPoint)
    self.addMovingPoint(delta)
    
    self.changeState()
  }
    
  private func changeState() {
    switch self.state {
    case .Closed, .Opened:
        self.moveNextState()
    case .MayOpen, .MayClose:
        if self.isOverThreshold() {
            self.moveNextState()
            
            let time = dispatch_time(DISPATCH_TIME_NOW, Int64(self.vibrationSec * Double(NSEC_PER_SEC)))
            dispatch_after(time, dispatch_get_main_queue(), moveNextState)
        } else {
            self.stayCurrentState()
        }
    default:
        break
    }
  }
}