ScollingImagerViewer.swift

import Foundation
import UIKit

class ScrollingImageCell: UICollectionViewCell {
	var imageView: UIImageView!
	var scrollView: UIScrollView!
	var doubleTapGesture: UITapGestureRecognizer!
	var image: UIImage? {
		didSet{
//			setNeedsLayout()
			guard let image = image else { return }
			if image.size.height > image.size.width {
    imageView.contentMode = .scaleAspectFill  // Portrait
			} else {
    imageView.contentMode = .scaleAspectFit  // Landscape
			}
		}
	}
	var topInset: CGFloat = 0 {
		didSet {
			centerIfNeeded()
		}
	}
	
	required init?(coder aDecoder: NSCoder) {
		super.init(coder: aDecoder)
		scrollView = UIScrollView(frame: bounds)
		imageView = UIImageView(frame: bounds)
		imageView.contentMode = .scaleAspectFit
		scrollView.addSubview(imageView)
		scrollView.maximumZoomScale = 3
		scrollView.delegate = self
		scrollView.contentMode = .center
		scrollView.showsHorizontalScrollIndicator = false
		scrollView.showsVerticalScrollIndicator = false
		scrollView.zoomScale = 1
		if #available(iOS 9.0, *) {
			scrollView.panGestureRecognizer.allowedTouchTypes = [ NSNumber(value:UITouchType.indirect.rawValue) ]
		}
		addSubview(scrollView)
		
		doubleTapGesture = UITapGestureRecognizer(target: self, action: #selector(doubleTap(gr:)))
		doubleTapGesture.numberOfTapsRequired = 2
		addGestureRecognizer(doubleTapGesture)
	}
	
	func zoomRectForScale(scale: CGFloat, center: CGPoint) -> CGRect {
		var zoomRect = CGRect.zero
		zoomRect.size.height = imageView.frame.size.height / scale
		zoomRect.size.width  = imageView.frame.size.width  / scale
		let newCenter = imageView.convert(center, from: scrollView)
		zoomRect.origin.x = newCenter.x - (zoomRect.size.width / 2.0)
		zoomRect.origin.y = newCenter.y - (zoomRect.size.height / 2.0)
		return zoomRect
	}
	
	@objc func doubleTap(gr: UITapGestureRecognizer) {
		if scrollView.zoomScale == 1 {
			scrollView.zoom(to: zoomRectForScale(scale: scrollView.maximumZoomScale, center: gr.location(in: gr.view)), animated: true)
		} else {
			scrollView.setZoomScale(1, animated: true)
		}
	}
	
	override func layoutSubviews() {
		super.layoutSubviews()
		scrollView.frame = bounds
		let size: CGSize
		if let image = imageView.image {
			let containerSize = CGSize(width: bounds.width, height: bounds.height - topInset)
			if containerSize.width / containerSize.height < image.size.width / image.size.height {
				size = CGSize(width: containerSize.width, height: containerSize.width * image.size.height / image.size.width )
			} else {
				size = CGSize(width: containerSize.height * image.size.width / image.size.height, height: containerSize.height )
			}
		} else {
			size = CGSize(width: bounds.width, height: bounds.width)
		}
		imageView.frame = CGRect(origin: .zero, size: size)
		scrollView.contentSize = size
		centerIfNeeded()
	}
	
	//	override func prepareForReuse() {
	//		super.prepareForReuse()
	//		scrollView.setZoomScale(1, animated: false)
	//	}
	
	
	//
	func centerIfNeeded() {
		var inset = UIEdgeInsets(top: topInset, left: 0, bottom: 0, right: 0)
		if scrollView.contentSize.height < scrollView.bounds.height - topInset {
			let insetV = (scrollView.bounds.height - topInset - scrollView.contentSize.height)/2
			inset.top += insetV
			inset.bottom = insetV
		}
		if scrollView.contentSize.width < scrollView.bounds.width {
			let insetV = (scrollView.bounds.width - scrollView.contentSize.width)/2
			inset.left = insetV
			inset.right = insetV
		}
		scrollView.contentInset = inset
		
	}
}

extension ScrollingImageCell: UIScrollViewDelegate {
	func viewForZooming(in scrollView: UIScrollView) -> UIView? {
		return imageView
	}
	
	func scrollViewDidZoom(_ scrollView: UIScrollView) {
		centerIfNeeded()
	}
}