import Foundation

final class ConcealingTitleView: UIView {
	private let label = UILabel()
	private var contentOffset: CGFloat = 0 {
		didSet {
			label.frame.origin.y = titleVerticalPositionAdjusted(by: contentOffset)
		}
	}
	var text: String = "" {
		didSet {
			label.text = text
			label.textColor = UIColor.white
			label.font = UIFont.systemFont(ofSize: 17.0, weight: .semibold)
			setNeedsLayout()
		}
	}

	// MARK: Initializers

	override init(frame: CGRect) {
		super.init(frame: frame)

		addSubview(label)
		clipsToBounds = true
		isUserInteractionEnabled = false
	}

	required init?(coder aDecoder: NSCoder) {
		fatalError("init(coder:) has not been implemented")
	}

	// MARK: View lifecycle

	override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
		super.traitCollectionDidChange(previousTraitCollection)
		sizeToFit()
	}

	override func sizeThatFits(_ size: CGSize) -> CGSize {
		if #available(iOS 11.0, *) {
			return super.sizeThatFits(size)
		} else {
			let height = (typedSuperview() as? UINavigationBar)?.bounds.height ?? 0.0
			let labelSize = label.sizeThatFits(CGSize(width: size.width, height: height))
			return CGSize(width: min(labelSize.width, size.width), height: height)
		}
	}

	private func layoutSubviews_10() {
		guard let navBar = typedSuperview() as? UINavigationBar else { return }
		let center = convert(navBar.center, from: navBar)
		let size = label.sizeThatFits(bounds.size)
		let x = max(bounds.minX, center.x - size.width * 0.5)
		let y: CGFloat

		if contentOffset == 0 {
			y = bounds.maxY
		} else {
			y = titleVerticalPositionAdjusted(by: contentOffset)
		}

		label.frame = CGRect(x: x, y: y, width: min(size.width, bounds.width), height: size.height)
	}

	private func layoutSubviews_11() {
		let size = label.sizeThatFits(bounds.size)
		let x: CGFloat
		let y: CGFloat

		x = bounds.midX - size.width * 0.5

		if contentOffset == 0 {
			y = bounds.maxY
		} else {
			y = titleVerticalPositionAdjusted(by: contentOffset)
		}

		label.frame = CGRect(x: x, y: y, width: size.width, height: size.height)
	}

	override func layoutSubviews() {
		super.layoutSubviews()

		if #available(iOS 11.0, *) {
			layoutSubviews_11()
		} else {
			layoutSubviews_10()
		}
	}

	// MARK:

	/// Using the UIScrollViewDelegate it moves the inner UILabel to move up and down following the scroll offset.
	///
	/// - Parameters:
	///   - scrollView: The scroll-view object in which the scrolling occurred.
	///   - threshold: The minimum distance that must be scrolled before the title view will begin scrolling up into view
	func scrollViewDidScroll(_ scrollView: UIScrollView, threshold: CGFloat = 0) {
		contentOffset = scrollView.contentOffset.y - threshold
	}

	// MARK: Private

	private func titleVerticalPositionAdjusted(by yOffset: CGFloat) -> CGFloat {
		let midY = bounds.midY - label.bounds.height * 0.5
		return max(bounds.maxY - yOffset, midY).rounded()
	}
}

private extension UIView {
	func typedSuperview<T: UIView>() -> T? {
		var parent = superview

		while parent != nil {
			if let view = parent as? T {
				return view
			} else {
				parent = parent?.superview
			}
		}

		return nil
	}
}