[under MIT License] Show image with using focus point.

FocusableImageView.swift

import SIMDKit
import UIKit

public final class FocusableImageView: UIImageView {
	public var isFocusEnabled: Bool = true {
		didSet { layer.setNeedsLayout() }
	}
	
	public var focus: CGPoint = .zero {
		didSet { layer.setNeedsLayout() }
	}
	
	public override var image: UIImage? {
		didSet { layer.setNeedsLayout() }
	}
	
	public override init(frame: CGRect) {
		super.init(frame: frame)
		setup()
	}
	
	required init?(coder aDecoder: NSCoder) {
		super.init(coder: aDecoder)
		setup()
	}
	
	private func setup() { }
	
	public override func layoutSublayers(of layer: CALayer) {
		guard isFocusEnabled else {
			layer.contentsRect = CGRect(x: 0.0, y: 0.0, width: 1.0, height: 1.0)
			return
		}
		
		guard let image = image else { return }
		
		let imageSize = image.size
		let containerSize = bounds.size
		let ratio = imageSize / containerSize
		if ratio.width > ratio.height {
			let factorX = FocusableImageView.calcShift(containerRatioOtherAxis: ratio.height,
													   container: containerSize.width,
													   image: imageSize.width,
													   focus: focus.x)
			layer.contentsRect = CGRect(x: factorX.point, y: 0.0, width: factorX.length, height: 1.0)
		} else {
			let factorY = FocusableImageView.calcShift(containerRatioOtherAxis: ratio.width,
													   container: containerSize.height,
													   image: imageSize.height,
													   focus: -focus.y)
			layer.contentsRect = CGRect(x: 0.0, y: factorY.point, width: 1.0, height: factorY.length)
		}
	}
	
	private static func calcShift(containerRatioOtherAxis: CGFloat, container: CGFloat, image: CGFloat, focus: CGFloat) -> (point: CGFloat, length: CGFloat) {
		let focusFactor = 0.5 * (focus + 1.0)
		let lengthFactor = (container * containerRatioOtherAxis) / image
		let beginFocusFactor = min(max(0.0, focusFactor - 0.5 * (1.0 - abs(focus)) * lengthFactor), 1.0 - lengthFactor)
		return (beginFocusFactor, lengthFactor)
	}
}