GalleryItemCell.swift

//
//  GalleryItemCell.swift
//  BrunchApp
//
//  Created by Liam Nichols on 30/04/2017.
//  Copyright © 2017 Bottomless Brunches. All rights reserved.
//

import UIKit

class GalleryItemCell: UICollectionViewCell {

    @IBOutlet fileprivate var activityIndicatorView: UIActivityIndicatorView!
    @IBOutlet fileprivate var scrollView: UIScrollView!
              fileprivate let imageView = UIImageView()
    private var imageLoadOperation: WebImageOperation?

    override func awakeFromNib() {
        super.awakeFromNib()
        scrollView.addSubview(imageView)
    }

    override func prepareForReuse() {
        super.prepareForReuse()
        image = nil
    }

    func handleDoubleTap() {

        UIView.animate(withDuration: 0.3) { [weak self] in
            guard let scrollView = self?.scrollView else { return }

            // If we have zoomed out fully, zoom in by 0.5x otherwise revert to zoomed out state
            if scrollView.zoomScale == scrollView.minimumZoomScale {
                scrollView.zoomScale += 0.5
            } else {
                scrollView.zoomScale = scrollView.minimumZoomScale
            }
        }
    }

    var imageState: GalleryTransitionController.SourceContext? {
        guard let image = image else { return nil }
        return GalleryTransitionController.SourceContext(
            view: contentView,
            rect: contentView.convert(imageView.frame, from: scrollView),
            image: image)
    }

    var image: Image? {
        didSet {
            // Cancel any existing image loads and reset the state
            imageLoadOperation?.cancel()
            imageLoadOperation = nil
            configureEmptyState(showActivity: false)

            // Make sure we have an item that we want to load
            guard let item = image else { return }

            // If we have the source image cached then just use that and be dome
            if let image = WebImageManager.shared.cachedImage(from: item.sourceURL) {
                configurePopulatedState(with: image)
                return
            }

            // Load the thumnail as a placeholder or just show blank depending on what we have
            if let thumbnail = WebImageManager.shared.cachedImage(from: item.thumbnailURL) {
                configurePopulatedState(with: thumbnail)
            } else {
                configureEmptyState(showActivity: true)
            }

            // Load the full resolution image
            imageLoadOperation = WebImageManager.shared.loadImage(from: item.sourceURL) { [weak self] image, _ in
                guard let `self` = self else { return }

                if let image = image {
                    self.configurePopulatedState(with: image)

                } else if self.activityIndicatorView.isAnimating {
                    self.configureEmptyState(showActivity: false)
                }
            }
        }
    }

    private func configureEmptyState(showActivity: Bool = true) {

        // Show or hide the activity indicator
        if showActivity {
            activityIndicatorView.startAnimating()
        } else {
            activityIndicatorView.stopAnimating()
        }

        // Clear the scroll view
        imageView.image = nil
        scrollView.contentSize = .zero
        scrollView.zoomScale = 1.0
        scrollView.minimumZoomScale = 1.0
        scrollView.maximumZoomScale = 1.0
    }

    private func configurePopulatedState(with image: UIImage) {

        // Stop the activity and unhide
        activityIndicatorView.stopAnimating()

        // Configure the image view
        imageView.image = image

        // Reset the content size
        scrollView.zoomScale = 1.0
        scrollView.contentSize = image.size
        imageView.frame = CGRect(origin: .zero, size: image.size)

        // Work out the smallest scale
        let smallestScale = min(contentView.bounds.width / image.size.width,
                                contentView.bounds.height / image.size.height)

        // Ensure all the zoom and positioning is correct
        scrollView.maximumZoomScale = 2.0
        scrollView.minimumZoomScale = smallestScale
        scrollView.zoomScale = smallestScale
    }
}

// MARK: - UIScrollViewDelegate
extension GalleryItemCell: UIScrollViewDelegate {

    func viewForZooming(in scrollView: UIScrollView) -> UIView? {
        return imageView
    }

    func scrollViewDidZoom(_ scrollView: UIScrollView) {

        // Work out the offsets
        let offsetX = max((scrollView.bounds.width - scrollView.contentSize.width) * 0.5, 0.0)
        let offsetY = max((scrollView.bounds.height - scrollView.contentSize.height) * 0.5, 0.0)

        // Update the center
        imageView.center = CGPoint(x: scrollView.contentSize.width * 0.5 + offsetX,
                                   y: scrollView.contentSize.height * 0.5 + offsetY)
    }
}