space-ios/iOSClient/Viewer/NCViewerImage/NCViewerImageZoom.swift

import UIKit

protocol NCViewerImageZoomDelegate: class {
    func viewerImageZoom(_ viewerImageZoom: NCViewerImageZoom, scrollViewDidScroll scrollView: UIScrollView)
}

class NCViewerImageZoom: UIViewController {
    
    @IBOutlet weak var imageViewBottomConstraint: NSLayoutConstraint!
    @IBOutlet weak var imageViewLeadingConstraint: NSLayoutConstraint!
    @IBOutlet weak var imageViewTopConstraint: NSLayoutConstraint!
    @IBOutlet weak var imageViewTrailingConstraint: NSLayoutConstraint!
    @IBOutlet weak var scrollView: UIScrollView!
    @IBOutlet weak var imageView: UIImageView!
    
    weak var delegate: NCViewerImageZoomDelegate?
    
    var image: UIImage!
    var index: Int = 0

    var doubleTapGestureRecognizer: UITapGestureRecognizer!
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        self.doubleTapGestureRecognizer = UITapGestureRecognizer(target: self, action: #selector(didDoubleTapWith(gestureRecognizer:)))
        self.doubleTapGestureRecognizer.numberOfTapsRequired = 2
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.scrollView.delegate = self
        if #available(iOS 11, *) {
            self.scrollView.contentInsetAdjustmentBehavior = .never
        }
        self.imageView.image = self.image
        self.imageView.frame = CGRect(x: self.imageView.frame.origin.x,
                                      y: self.imageView.frame.origin.y,
                                      width: self.image.size.width,
                                      height: self.image.size.height)
        self.view.addGestureRecognizer(self.doubleTapGestureRecognizer)        
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        updateZoomScaleForSize(view.bounds.size)
        updateConstraintsForSize(view.bounds.size)
    }

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)
        updateZoomScaleForSize(view.bounds.size)
        updateConstraintsForSize(view.bounds.size)
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
    }
    
    @objc func didDoubleTapWith(gestureRecognizer: UITapGestureRecognizer) {
        let pointInView = gestureRecognizer.location(in: self.imageView)
        var newZoomScale = self.scrollView.maximumZoomScale
        
        if self.scrollView.zoomScale >= newZoomScale || abs(self.scrollView.zoomScale - newZoomScale) <= 0.01 {
            newZoomScale = self.scrollView.minimumZoomScale
        }
        
        let width = self.scrollView.bounds.width / newZoomScale
        let height = self.scrollView.bounds.height / newZoomScale
        let originX = pointInView.x - (width / 2.0)
        let originY = pointInView.y - (height / 2.0)
        
        let rectToZoomTo = CGRect(x: originX, y: originY, width: width, height: height)
        self.scrollView.zoom(to: rectToZoomTo, animated: true)
    }
    
    fileprivate func updateZoomScaleForSize(_ size: CGSize) {
        
        let widthScale = size.width / imageView.bounds.width
        let heightScale = size.height / imageView.bounds.height
        let minScale = min(widthScale, heightScale)
        scrollView.minimumZoomScale = minScale
        
        scrollView.zoomScale = minScale
        scrollView.maximumZoomScale = minScale * 4
    }
    
    fileprivate func updateConstraintsForSize(_ size: CGSize) {
        let yOffset = max(0, (size.height - imageView.frame.height) / 2)
        imageViewTopConstraint.constant = yOffset
        imageViewBottomConstraint.constant = yOffset
        
        let xOffset = max(0, (size.width - imageView.frame.width) / 2)
        imageViewLeadingConstraint.constant = xOffset
        imageViewTrailingConstraint.constant = xOffset

        let contentHeight = yOffset * 2 + self.imageView.frame.height
        view.layoutIfNeeded()
        self.scrollView.contentSize = CGSize(width: self.scrollView.contentSize.width, height: contentHeight)
    }
}

extension NCViewerImageZoom: UIScrollViewDelegate {
    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return imageView
    }
    
    func scrollViewDidZoom(_ scrollView: UIScrollView) {
        updateConstraintsForSize(self.view.bounds.size)
    }
    
    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        self.delegate?.viewerImageZoom(self, scrollViewDidScroll: scrollView)
    }
}