UITableView stretchy header

ViewController.swift

import UIKit

class ViewController: UITableViewController {
    
    
    private let kTableViewHeaderHeight: CGFloat = 300.0
    
    var headerView: UIView!
    
    
    override var prefersStatusBarHidden: Bool {
        return true
    }
    

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        
        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "Cell")
        
        headerView = tableView.tableHeaderView
        tableView.tableHeaderView = nil
        tableView.backgroundColor = UIColor.blackTheme
        tableView.addSubview(headerView)
        
        tableView.contentInset = UIEdgeInsets(top: kTableViewHeaderHeight,
                                              left: 0,
                                              bottom: 0,
                                              right: 0)
        tableView.contentOffset = CGPoint(x: 0,
                                          y: -kTableViewHeaderHeight)
        
        updateHeaderView()
    }
    
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        let navButtonLeft = UIBarButtonItem(barButtonSystemItem: .done,
                                        target: self,
                                        action: nil)
        let navButtonRight = UIBarButtonItem(barButtonSystemItem: .action,
                                            target: self,
                                            action: nil)
        
        navigationItem.leftBarButtonItem = navButtonLeft
        navigationItem.rightBarButtonItem = navButtonRight
        
        let alphaImage = getImageFrom(color: UIColor.white.withAlphaComponent(0.0))
        
        navigationController!.navigationBar.isTranslucent = true
        navigationController!.navigationBar.barStyle = .blackTranslucent
        navigationController!.navigationBar.tintColor = .white
        navigationController!.navigationBar.setBackgroundImage(alphaImage!,
                                                               for: .default)
        navigationController!.navigationBar.shadowImage = UIImage()
    }
    
    
    private func updateHeaderView() {
        var headerRect = CGRect(x: 0,
                                y: -kTableViewHeaderHeight,
                                width: tableView.bounds.width,
                                height: kTableViewHeaderHeight)
        
        if tableView.contentOffset.y < -kTableViewHeaderHeight {
            headerRect.origin.y = tableView.contentOffset.y
            headerRect.size.height = -tableView.contentOffset.y
        }
        
        headerView.frame = headerRect
    }


}


extension ViewController {
    
    override func scrollViewDidScroll(_ scrollView: UIScrollView) {
        self.updateHeaderView()
        
        let offset = tableView.contentOffset.y + kTableViewHeaderHeight
        
        if offset >= 100 && offset < kTableViewHeaderHeight {
            let alpha = (offset * 1.0) / kTableViewHeaderHeight
            let alphaImage = getImageFrom(color: UIColor.black.withAlphaComponent(alpha))!
            navigationController!.navigationBar.setBackgroundImage(alphaImage, for: .compact)
            navigationController!.navigationBar.setBackgroundImage(alphaImage, for: .default)
            self.title = "Spider-Man"
        } else if offset >= kTableViewHeaderHeight {
            let alphaImage = getImageFrom(color: UIColor.black)!
            navigationController!.navigationBar.setBackgroundImage(alphaImage, for: .compact)
            navigationController!.navigationBar.setBackgroundImage(alphaImage, for: .default)
            self.title = "Spider-Man"
        } else {
            let alphaImage = getImageFrom(color: UIColor.clear)!
            navigationController!.navigationBar.setBackgroundImage(alphaImage, for: .compact)
            navigationController!.navigationBar.setBackgroundImage(alphaImage, for: .default)
            self.title = nil
        }
    }
    
}


extension ViewController {
    
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return 50
    }
    
    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath)
        
        cell.backgroundColor = .clear
        cell.textLabel?.text = "Cell \(indexPath.row+1)"
        cell.textLabel?.textColor = .white
        
        return cell
    }
    
}


extension ViewController {
    
    func getImageFrom(color: UIColor) -> UIImage? {
        if let navigationBar = self.navigationController?.navigationBar {
            let gradient = CAGradientLayer()
            var bounds = navigationBar.bounds
            bounds.size.height += UIApplication.shared.statusBarFrame.size.height
            gradient.frame = bounds
            gradient.colors = [color.cgColor, color.cgColor]
            gradient.startPoint = CGPoint(x: 0, y: 0)
            gradient.endPoint = CGPoint(x: 1, y: 0)
            
            var gradientImage:UIImage?
            UIGraphicsBeginImageContext(gradient.frame.size)
            if let context = UIGraphicsGetCurrentContext() {
                gradient.render(in: context)
                gradientImage = UIGraphicsGetImageFromCurrentImageContext()?.resizableImage(withCapInsets: UIEdgeInsets.zero, resizingMode: .stretch)
            }
            UIGraphicsEndImageContext()
            return gradientImage
        }
        
        return nil
    }
    
}