/*

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

    See LICENSE.txt for this sample’s licensing information

    Abstract:

    Contains the `ImageFile` class, which represents an image at a given URL.

*/

import Cocoa

/**

    `ImageFile` represents an image on disk. It can create thumbnail and full image representations.

*/

class ImageFile {

    // MARK: Notifications

    static let thumbnailDidLoadNotification = "ImageFileThumbnailDidLoadNotification"

    static let imageDidLoadNotification = "ImageFileImageDidLoadNotification"

    // MARK: Properties

    /// The date the ImageFile was last updated from its URL source.

    fileprivate(set) var dateLastUpdated = Date()

    /// The url the receiver references.

    fileprivate(set) var URL: Foundation.URL

    /// The filename of the receiver, with the extension.

    var fileName: String? {

        return URL.lastPathComponent

    }

    /// The filename of the receiver, without the extension. Suitable for presentation names.

    var fileNameExcludingExtension: String? {

        let lastPathComponent = URL.lastPathComponent

        return (lastPathComponent as NSString).deletingPathExtension

    }

    /// The operation queue all `ImageFile`s use to load their thumbnails.

    fileprivate static var thumbnailLoadingOperationQueue: OperationQueue = {

        let queue = OperationQueue()

        queue.name = "ImageFile Thumbnail Loading Queue"

        return queue

    }()

    /// The operation queue all `ImageFile`s use to load their images.

    fileprivate static var imageLoadingOperationQueue: OperationQueue = {

        let queue = OperationQueue()

        queue.name = "ImageFile Image Loading Queue"

        return queue

    }()

    /// The image source representing the image at the set `url`.

    fileprivate lazy var imageSource: CGImageSource? = {

        let imageSource = CGImageSourceCreateWithURL(self.URL.absoluteURL as CFURL, nil)

        if let imageSource = imageSource {

            // Verify the image source has a valid uniform type identifier.

            guard CGImageSourceGetType(imageSource) != nil else { return nil }

        }

        return imageSource

    }()

    /// The private cache for the loaded thumbnail.

    fileprivate var _thumbnail: NSImage?

    /// The cache for the loaded image.

    fileprivate var _image: NSImage?

    // MARK: Initializer

    init(URL: Foundation.URL) {

        self.URL = URL

    }

    // MARK: Image Loading

    /**

        Fetch the thumbnail with a callback closure. If the thumbnail has already

        been loaded, the completionHandler is invoked immediately with the cached

        thumbnail.

    */

    func fetchThumbnailWithCompletionHandler(_ completionHandler: @escaping (NSImage) -> Void) {

        if let thumbnail = _thumbnail {

            // If the thumbnail is already loaded, invoke the `completionHandler` with it immediately.

            completionHandler(thumbnail)

        }

        else {

            // Otherwise, load the thumbnail and callback once it has loaded.

            ImageFile.thumbnailLoadingOperationQueue.addOperation {

                guard let imageSource = self.imageSource else { return }

                /*

                    Create a thumbnail from the image source. If the file doesn't 

                    come with a thumbnail, ask CG to create one, with a maximum 

                    size of 160 pts.

                */

                let thumbnailOptions = [

                    String(kCGImageSourceCreateThumbnailFromImageIfAbsent): true,

                    String(kCGImageSourceThumbnailMaxPixelSize): 160

                ] as [String : Any]

                guard let thumbnailRef = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, thumbnailOptions as CFDictionary?) else { return }

                let thumbnailImage = NSImage(cgImage: thumbnailRef, size: NSSize.zero)

                /*

                    Update the `_thumbnail` cache on the main thread and send 

                    notification that it has loaded.

                */

                OperationQueue.main.addOperation {

                    self._thumbnail = thumbnailImage

                    NotificationCenter.default.post(name: Notification.Name(rawValue: ImageFile.thumbnailDidLoadNotification), object: self)

                    completionHandler(thumbnailImage)

                }

            }

        }

    }

    /**

        Fetch the image with a callback closure. If the image has already been

        loaded, the `completionHandler` is invoked immediately with the cached image.

    */

    func fetchImageWithCompletionHandler(_ completionHandler: @escaping (NSImage) -> Void) {

        if let image = _image {

            // If the image is already loaded, invoke the completionHandler with it immediately.

            completionHandler(image)

        }

        else {

            // Otherwise, load the image and callback once it has loaded.

            ImageFile.thumbnailLoadingOperationQueue.addOperation() {

                guard let imageSource = self.imageSource else { return }

                // Create a full image from the image source.

                guard let imageRef = CGImageSourceCreateImageAtIndex(imageSource, 0, nil) else { return }

                let image = NSImage(cgImage: imageRef, size: NSZeroSize)

                // Update the `_image` cache on the main thread and send notification that it has loaded.

                OperationQueue.main.addOperation {

                    self._image = image

                    NotificationCenter.default.post(name: Notification.Name(rawValue: ImageFile.imageDidLoadNotification), object: self)

                    completionHandler(image)

                }

            }

        }

    }

}

// `ImagesFile`s are equivalent if their URLs are equivalent.

extension ImageFile: Hashable {

    var hashValue: Int {

        return URL.hashValue

    }

}

extension ImageFile: Equatable { }

func ==(lhs: ImageFile, rhs: ImageFile) -> Bool {

    return lhs.URL == rhs.URL

}