UINavigationBar transparency by ScrollView

UINavigationBar+.swift

/**
 * 可渐变的 UINavigationBar
 * 从 objective-c 版本 https://github.com/ltebean/LTNavigationBar 翻译而来
 */
extension UINavigationBar {
    @nonobjc static var overlay:UIView?
    
    // 设置 overlay 背景色
    func lt_setBackgroundColor(backgroundColor: UIColor) {
        if UINavigationBar.overlay == nil {
            self.setBackgroundImage(UIImage(), forBarMetrics: UIBarMetrics.Default)
            UINavigationBar.overlay = UIView(frame: CGRectMake(0, -20, CGRectGetWidth(self.bounds), CGRectGetHeight(self.bounds) + 20))
            UINavigationBar.overlay!.userInteractionEnabled = false
            // let mask = UIViewAutoresizing.FlexibleWidth | UIViewAutoresizing.FlexibleHeight
            UINavigationBar.overlay!.autoresizingMask = UIViewAutoresizing.FlexibleWidth
            UINavigationBar.overlay!.autoresizingMask = UIViewAutoresizing.FlexibleHeight
            self.insertSubview(UINavigationBar.overlay!, atIndex: 0)
        }
        UINavigationBar.overlay?.backgroundColor = backgroundColor
    }
    
    // 设置 UINavigationItem alpha 通道
    func lt_setElementsAlpha(alpha: CGFloat) {
        self.valueForKey("_leftViews")?.enumerateObjectsUsingBlock({ (view:AnyObject, p: UnsafeMutablePointer<ObjCBool>) in
            (view as? UIView)?.alpha = alpha
        })
        
        self.valueForKey("_rightViews")?.enumerateObjectsUsingBlock({ (view:AnyObject, p: UnsafeMutablePointer<ObjCBool>) in
            (view as? UIView)?.alpha = alpha
        })
        
        let titleView = self.valueForKey("_titleView") as? UIView
        titleView?.alpha = alpha
        
        // when viewController first load, the titleView maybe nil
        for v in self.subviews {
            if v.isKindOfClass(UINavigationItem) {
                (v as UIView).alpha = alpha
                break
            }
        }
    }
    
    // 设置 UINavigationBar偏移量
    func lt_setTranslationY(translationY: CGFloat) {
        self.transform = CGAffineTransformMakeTranslation(0, translationY);
    }
    
    // 重置
    func lt_reset() {
        self.setBackgroundImage(nil, forBarMetrics: UIBarMetrics.Default)
        UINavigationBar.overlay?.removeFromSuperview()
        UINavigationBar.overlay = nil;
    }
    
}

UIViewController+.swift

// tableView 交互
extension UIViewController: UIScrollViewDelegate {
    @nonobjc static let NAVBAR_CHANGE_POINT: CGFloat = CGFloat(50)
    
    // 监听滚动
    func scrollViewDidScroll(scrollView: UIScrollView) {
        let offsetY = scrollView.contentOffset.y
        let alpha: CGFloat
        if (offsetY > DetailViewController.NAVBAR_CHANGE_POINT) {
            alpha = min(1, 1 - ((DetailViewController.NAVBAR_CHANGE_POINT + 64 - offsetY) / 64))
        } else {
            alpha = 0
        }
        let color = UIColor(red:0/255.0, green:175/255.0, blue:240/255.0, alpha:1).colorWithAlphaComponent(alpha)
        self.navigationController?.navigationBar.lt_setBackgroundColor(color)
    }
    
    override func viewWillAppear(animated: Bool) {
        super.viewWillDisappear(animated)
        UIApplication.sharedApplication().statusBarStyle = UIStatusBarStyle.Default
        
        tableView.delegate = self
        self.scrollViewDidScroll(tableView)
        self.navigationController?.navigationBar.shadowImage = UIImage()
    }
    
    override func viewWillDisappear(animated: Bool) {
        super.viewWillDisappear(animated)
        UIApplication.sharedApplication().statusBarStyle = UIStatusBarStyle.LightContent
        
        tableView.delegate = nil
        self.navigationController?.navigationBar.lt_reset()
    }
    
}