UIScrollView+headerLayoutGuide

UIScrollView+headerLayoutGuide.swift

extension UIScrollView {
    fileprivate var layoutGuideID: String { #function }
    
    public var headerLayoutGuide: UILayoutGuide {
        if let layoutGuide = addedHeaderLayoutGuide() {
            return layoutGuide
        }
        // workaround: UIScrollView can't add new layoutGuide directly.
        let anchorView = UIView()
        addSubview(anchorView)
        
        let newLayoutGuide = UILayoutGuide()
        newLayoutGuide.identifier = layoutGuideID
        anchorView.addLayoutGuide(newLayoutGuide)
        
        let topConstraint = newLayoutGuide.topAnchor.constraint(
            equalTo: frameLayoutGuide.topAnchor
        )
        topConstraint.priority = .defaultLow
        NSLayoutConstraint.activate([
            topConstraint,
            newLayoutGuide.leadingAnchor.constraint(
                equalTo: leadingAnchor
            ),
            newLayoutGuide.trailingAnchor.constraint(
                equalTo: trailingAnchor
            ),
            newLayoutGuide.bottomAnchor.constraint(
                equalTo: topAnchor
            ),
        ])
        return newLayoutGuide
    }
    
    fileprivate func addedHeaderLayoutGuide() -> UILayoutGuide? {
        layoutGuides.first(where: { $0.identifier == layoutGuideID })
    }
}