Zoom into and page through an array of images

FullscreenPhotoViewController.swift

import UIKit

class FullscreenPhotoViewController: UIViewController, UIScrollViewDelegate {

    // MARK: - IBOutlets

    @IBOutlet weak var scrollView: UIScrollView!

    // MARK: - IBActions

    @IBAction func closeButtonTapped(sender: AnyObject) {
        dismissViewControllerAnimated(true, completion: nil)
    }

    // MARK: - Private Properties

    private let imageViewTag = 1

    // MARK: Public Properties

    var images: [UIImage]?
    var currentImageIndex: Int?

    // MARK: UIViewController

    override func viewDidLayoutSubviews() {
        setupNestedImageScrollViews()
    }

    override func viewDidAppear(animated: Bool) {
        Analytics.service.pageView(.FullScreenPhoto)

    }

    // MARK: UIScrollViewDelegate

    func viewForZoomingInScrollView(scrollView: UIScrollView) -> UIView? {
        return scrollView.viewWithTag(imageViewTag)
    }

    func scrollViewDidZoom(scrollView: UIScrollView) {
        centerImageIfSmallerThanView(scrollView)
    }

    func scrollViewDidScroll(scrollView: UIScrollView) {
        if scrollView == self.scrollView {
            currentImageIndex = Int(round(scrollView.contentOffset.x / scrollView.bounds.width))
        }
    }

    // MARK: Private Functions

    private func setupNestedImageScrollViews() {
        for view in scrollView.subviews {
            view.removeFromSuperview()
        }
        let previousImageIndex = currentImageIndex
        scrollView.bounds = view.bounds
        scrollView.contentOffset = CGPointZero
        if let images = images {
            var imageScrollViewFrame = scrollView.bounds
            for image in images {
                let imageView = UIImageView(image: image)
                imageView.tag = imageViewTag
                let imageScrollView = UIScrollView(frame: imageScrollViewFrame)
                imageScrollView.addSubview(imageView)
                imageScrollView.contentSize = imageView.bounds.size
                imageScrollView.delegate = self
                scrollView.addSubview(imageScrollView)
                zoomImageToFit(imageScrollView)
                centerImageIfSmallerThanView(imageScrollView)
                imageScrollViewFrame.origin.x += imageScrollViewFrame.size.width
            }
            scrollView.contentSize = CGSizeMake(CGFloat(images.count) * scrollView.bounds.width, scrollView.bounds.height)
            currentImageIndex = previousImageIndex
            setOffsetToCurrentImageIndex()
        }
    }

    private func centerImageIfSmallerThanView(imageScrollView: UIScrollView) {
        guard let imageView = imageScrollView.viewWithTag(imageViewTag) as? UIImageView,
            image = imageView.image else { return }

        var imageViewFrame = imageView.frame
        let yOffset = max(0, (view.bounds.size.height - (image.size.height * imageScrollView.zoomScale)) / 2)
        imageViewFrame.origin.y = yOffset

        let xOffset = max(0, (view.bounds.size.width - (image.size.width * imageScrollView.zoomScale)) / 2)
        imageViewFrame.origin.x = xOffset

        imageView.frame = imageViewFrame
    }

    private func zoomImageToFit(imageScrollView: UIScrollView) {
        guard let imageView = imageScrollView.viewWithTag(imageViewTag) as? UIImageView,
            image = imageView.image else { return }

        let scrollViewFrame = imageScrollView.frame
        let scaleWidth = scrollViewFrame.size.width / image.size.width
        let scaleHeight = scrollViewFrame.size.height / image.size.height
        let minScale = min(scaleHeight, scaleWidth)

        imageScrollView.minimumZoomScale = minScale
        imageScrollView.maximumZoomScale = 1
        imageScrollView.zoomScale = minScale
    }

    private func setOffsetToCurrentImageIndex() {
        guard let images = images, index = currentImageIndex else { return }
        if images.count > index {
            scrollView.contentOffset = CGPointMake(scrollView.bounds.width * CGFloat(index), 0)
        }
    }

}