swift中tableView导航栏渐变和下拉变大效果

1.这个是纯代码
//
// ViewController.swift
// 导航栏渐变
//
// Created by ty on 16/1/3.
// Copyright © 2016年 ty. All rights reserved.
//

import UIKit

// 顶部图片的高度
private let topImageHeight: CGFloat = 200
// 顶部图片
private var topImag: UIImageView?
// 自定义导航栏
private var customNavc: UIView?
// 自定义返回按钮
private var customBackBtn: UIButton?
// 当导航栏透明的时候 加载在view上的按钮
private var viewBackBtn: UIButton?
// 自定义导航栏标题
private var customTitleLabel: UILabel?
// // 当导航栏透明的时候 加载在view上的标题
private var viewTitleLabel: UILabel?

class ViewController: UIViewController , UITableViewDelegate, UITableViewDataSource{

override func viewDidLoad() {    super.viewDidLoad()    self.automaticallyAdjustsScrollViewInsets = false    // 顶部图片    let topImage = UIImageView(frame: CGRect(x: 0, y:-topImageHeight, width: view.bounds.width, height: topImageHeight))    topImage.image = UIImage(named: "ceshi.jpg")//这个是tableView的图片    topImage.contentMode = .scaleAspectFill    topImage.clipsToBounds = true    topImag = topImage    // tableView    let tableView = UITableView(frame: CGRect(x: 0, y: 0, width: view.bounds.width, height: view.bounds.height))    tableView.delegate = self    tableView.dataSource = self;    view.addSubview(tableView)    tableView.contentInset = UIEdgeInsetsMake(topImageHeight, 0, 0, 0)    tableView.addSubview(topImage)    // 自定义导航栏    let backView = UIView(frame: CGRect(x: 0, y: 0, width: view.bounds.width, height: 64))    view.addSubview(backView)    backView.backgroundColor = UIColor.white    backView.alpha = 0.0    customNavc = backView    // 自定义返回按钮    let backBtn = UIButton(frame: CGRect(x: 0, y: 20, width: 40, height: 44))    backBtn.setImage(UIImage(named: "back_0"), for: UIControlState())//这个是导航栏渐变返回图片    backView.addSubview(backBtn)    customBackBtn = backBtn    // 返回按钮    let btn = UIButton(frame: CGRect(x: 0, y: 20, width: 40, height: 44))    btn.setImage(UIImage(named: "back_0"), for: UIControlState())    view.addSubview(btn)    viewBackBtn = btn    // 自定义标题    let titleLabel = UILabel(frame: CGRect(x: 0, y: 0, width: 80, height: 44))    titleLabel.center = CGPoint(x: view.frame.width / 2, y: 20 + 22)    titleLabel.text = "标题"    titleLabel.textColor = UIColor.white    customTitleLabel = titleLabel    customNavc?.addSubview(titleLabel)    // 标题    let viewTitleLabe = UILabel(frame: CGRect(x: 0, y: 0, width: 80, height: 44))    viewTitleLabe.center = CGPoint(x: view.frame.width / 2, y: 20 + 22)    viewTitleLabe.text = "标题"    viewTitleLabe.textColor = UIColor.black    viewTitleLabel = viewTitleLabe    view?.addSubview(titleLabel)}func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {    return 30}func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {    let cellID = "cellID"    var cell = tableView.dequeueReusableCell(withIdentifier: cellID)    if cell == nil {        cell = UITableViewCell(style: .default, reuseIdentifier: cellID)    }    cell!.textLabel!.text = "cell\(indexPath.row)"    return cell!}func scrollViewDidScroll(_ scrollView: UIScrollView) {    let offY = scrollView.contentOffset.y    // 根据偏移量改变alpha的值    customNavc?.alpha = (offY + 64) / (topImageHeight - 64) + 1    // 设置图片的高度 和 Y 值    if offY < -topImageHeight {        topImag?.frame.origin.y = offY        topImag?.frame.size.height = -offY    }    // 改变导航栏（自定义View）返回按钮的图片 和 标题颜色    if offY >= -64 {        customBackBtn?.setImage(UIImage(named: "back_1"), for: UIControlState())        viewBackBtn?.isHidden = true        customTitleLabel?.textColor = UIColor.black    } else {        customBackBtn?.setImage(UIImage(named: "back_0"), for: UIControlState())        viewBackBtn?.isHidden = false        customTitleLabel?.textColor = UIColor.white    }}override func didReceiveMemoryWarning() {    super.didReceiveMemoryWarning()    // Dispose of any resources that can be recreated.}

}