ViewController.swift

class ViewController: UIViewController {

    // MARK: Properties
    
    @IBOutlet weak var scrollView: UIScrollView!
    @IBOutlet weak var searchView: SearchView!
    let goal: CGFloat = 50
    var observeScrollViewContentOffset: NSKeyValueObservation?
    
    // MARK: Lifecycle

    override func viewDidLoad() {
        super.viewDidLoad()
        
        observeScrollViewContentOffset = scrollView.observe(\.contentOffset, options: .new) { [weak self] (scrollView, change) in
            guard let `self` = self, let newValue = change.newValue?.y else { return }
            
            // Remove negative value 
            var progress = -newValue/self.goal
            
            // Be sure that the progress is always between 0 and 1
            progress = min(max(0, progress), 1)

            self.searchView.update(progress: progress)
        }
    }
}