/*

    Copyright (C) 2016 Apple Inc. All Rights Reserved.

    See LICENSE.txt for this sample’s licensing information

    Abstract:

                PhotoCollectionViewCell is a UICollectionViewCell subclass that shows a Photo.

*/

import UIKit

/// The KVO context used for all `PhotoCollectionViewCell` instances.

private var photoCollectionViewCellObservationContext = 0

class PhotoCollectionViewCell: UICollectionViewCell {

    // MARK: Properties

    @IBOutlet var imageView: UIImageView!

    @IBOutlet var progressView: UIProgressView!

    fileprivate let fractionCompletedKeyPath = "photoImport.progress.fractionCompleted"

    fileprivate let imageKeyPath = "image"

    var photo: Photo? {

        willSet {

            if let formerPhoto = photo {

                formerPhoto.removeObserver(self, forKeyPath: fractionCompletedKeyPath, context: &photoCollectionViewCellObservationContext)

                formerPhoto.removeObserver(self, forKeyPath: imageKeyPath, context: &photoCollectionViewCellObservationContext)

            }

        }

        didSet {

            if let newPhoto = photo {

                newPhoto.addObserver(self, forKeyPath: fractionCompletedKeyPath, options: [], context: &photoCollectionViewCellObservationContext)

                newPhoto.addObserver(self, forKeyPath: imageKeyPath, options: [], context: &photoCollectionViewCellObservationContext)

            }

            updateImageView()

            updateProgressView()

        }

    }

    fileprivate func updateProgressView() {

        if let photoImport = photo?.photoImport {

            let fraction = Float(photoImport.progress.fractionCompleted)

            progressView.progress = fraction

            progressView.isHidden = false

        }

        else {

            progressView.isHidden = true

        }

    }

    fileprivate func updateImageView() {

        UIView.transition(with: imageView, duration: 0.5, options: .transitionCrossDissolve, animations: {

            self.imageView.image = self.photo?.image

        }, completion: nil)

    }

    // MARK: Key-Value Observing

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey: Any]?, context: UnsafeMutableRawPointer?) {

        guard context == &photoCollectionViewCellObservationContext else {

            super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)

            return

        }

        OperationQueue.main.addOperation {

            if keyPath == self.fractionCompletedKeyPath {

                self.updateProgressView()

            }

            else if keyPath == self.imageKeyPath {

                self.updateImageView()

            }

        }

    }

}