override func viewDidLoad() {
    super.viewDidLoad()
    
        self.view.backgroundColor = UIColor.white

        // custom configuration
        let waterDropsView = WaterDropsView(frame: self.view.frame,
                                          direction: .up,
                                          dropNum: 10,
                                          color: UIColor.red.withAlphaComponent(0.7),
                                          minDropSize: 10,
                                          maxDropSize: 20,
                                          minLength: 50,
                                          maxLength: 100,
                                          minDuration: 4,
                                          maxDuration: 8)
        
        // add animation 
        waterDropsView.addAnimation()
        self.view.addSubview(waterDropsView)
}