class ViewController: UIViewController {

    var navView: UIView!
    var searchBar: UISearchBar!
    var leftConstraint: NSLayoutConstraint!
    var widthConstraint: NSLayoutConstraint!

    override func viewDidLoad() {
        super.viewDidLoad()

        // Background view.
        navView = UIView()
        navView.backgroundColor = UIColor.greenColor()
        navView.translatesAutoresizingMaskIntoConstraints = false
        navigationItem.titleView = navView
        navView.superview!.addConstraint(NSLayoutConstraint(item: navView, attribute: .Top, relatedBy: .Equal, toItem: navView.superview!, attribute: .Top, multiplier: 1, constant: 0))
        navView.addConstraint(NSLayoutConstraint(item: navView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 0, constant: UIScreen.mainScreen().bounds.width))
        navView.addConstraint(NSLayoutConstraint(item: navView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 0, constant: navigationController!.navigationBar.bounds.height))

        // Search button.
        navigationItem.rightBarButtonItem = UIBarButtonItem(barButtonSystemItem: .Search, target: self, action: #selector(open))

        // Search bar.
        searchBar = UISearchBar()
        searchBar.translatesAutoresizingMaskIntoConstraints = false
        navView.addSubview(searchBar)

        navView.addConstraint(NSLayoutConstraint(item: searchBar, attribute: .CenterY, relatedBy: .Equal, toItem: navView, attribute: .CenterY, multiplier: 1, constant: 0))
        leftConstraint = NSLayoutConstraint(item: searchBar, attribute: .Left, relatedBy: .Equal, toItem: navigationItem.titleView, attribute: .LeftMargin, multiplier: 1, constant: 0)
        leftConstraint.priority = UILayoutPriorityDefaultLow // Starts out as low priority so it has no effect.
        navView.addConstraint(leftConstraint)

        navView.addConstraint(NSLayoutConstraint(item: navigationItem.titleView!, attribute: .Right, relatedBy: .Equal, toItem: searchBar, attribute: .Right, multiplier: 1, constant: 45))
        widthConstraint = NSLayoutConstraint(item: searchBar, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 0, constant: 10)
        widthConstraint.priority = UILayoutPriorityDefaultHigh  // Starts out as high priority so it does have effect.
        navView.addConstraint(widthConstraint)

        // Remove search bar border.
        searchBar.layer.borderColor = UIColor.greenColor().CGColor
        searchBar.layer.borderWidth = 1

        // Match background color.
        searchBar.barTintColor = UIColor.greenColor()

        // Search bar starts invisible.
        searchBar.alpha = 0
    }

    func open() {

        // Switch the priorities to make search bar expand.
        leftConstraint.priority = UILayoutPriorityDefaultHigh
        widthConstraint.priority = UILayoutPriorityDefaultLow

        // Animate change to visible.
        UIView.animateWithDuration(2, animations: {
            self.searchBar.alpha = 1
            self.searchBar.layoutIfNeeded()
        })
    }
}