ios实践之tableView顶部“弹簧”图片

大家可能注意到有些tableView的顶部图片，会随着你拉伸而跟着拉伸变大。
一种思路是将图片放置tableView的tableHeaderView上当tablview下移改变图片的frame达到效果。当然这个效果特别简单，只是单纯试试简书。

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!
        }  
    }
}