ios实现tableView顶部弹簧图片效果

import UIKit

class ViewController: UIViewController,UITableViewDataSource,UITableViewDelegate {

 lazy var myTableView : UITableView! = {
  var tableView = UITableView.init(frame: self.view.frame,style:UITableViewStyle.plain)
  tableView.delegate = self
  tableView.dataSource = self
  tableView.register(UITableViewCell.classForCoder(), forCellReuseIdentifier: "mycell")
  return tableView
 }()
 
 var headerImageView:UIImageView?
 var headerView:UIView?
 var headerViewHeight:CGFloat = 0.0
 
 
 override func viewDidLoad() {
  super.viewDidLoad()
  // Do any additional setup after loading the view, typically from a nib.
  setupUI()
 }

 func setupUI(){
  headerView = UIView.init(frame: CGRect(x:0,y:0,width:self.view.frame.width,height:300))
  headerViewHeight = headerView!.frame.height;
  self.view.addSubview(headerView!)
  headerImageView = UIImageView.init(frame: headerView!.frame)
  headerImageView?.image = UIImage.init(named: "bg-mine")
  headerView?.addSubview(headerImageView!)
  myTableView.tableHeaderView = headerView
  self.view.addSubview(myTableView)
  
 }
 

 func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
  return 2
 }
 func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
  let cell:UITableViewCell = tableView.dequeueReusableCell(withIdentifier: "mycell", for: indexPath)
  cell.textLabel?.text = "测试"
  return cell
 }
 func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
  return 50
 }
 
 func scrollViewDidScroll(_ scrollView: UIScrollView) {
  let contentOffSetY = scrollView.contentOffset.y
  if contentOffSetY < 0 {
   var rect = headerView?.frame
   rect?.size.height = headerViewHeight - contentOffSetY
   let headerViewWidth = headerView?.frame.size.width
   rect?.size.width = headerViewWidth!*(headerViewHeight-contentOffSetY)/headerViewHeight
   rect?.origin.x = -((rect?.size.width)! - headerViewWidth!)/2
   rect?.origin.y = contentOffSetY
   headerView?.frame = rect!
   headerImageView?.frame = rect!
  } 
 }
}