vds_ios/VDS/Managers/ImageManager.swift

//
//  ImageManager.swift
//  VDS
//
//  Created by Matt Bruce on 1/26/23.
//

import Foundation
import UIKit.UIImage
import Combine

// Declares in-memory image cache
public protocol ImageCacheType: class {
    // Returns the image associated with a given url
    func image(for url: URL) -> UIImage?
    // Inserts the image of the specified url in the cache
    func insertImage(_ image: UIImage?, for url: URL)
    // Removes the image of the specified url in the cache
    func removeImage(for url: URL)
    // Removes all images from the cache
    func removeAllImages()
    // Accesses the value associated with the given key for reading and writing
    subscript(_ url: URL) -> UIImage? { get set }
}

public final class ImageCache: ImageCacheType {

    // 1st level cache, that contains encoded images
    private lazy var imageCache: NSCache<AnyObject, AnyObject> = {
        let cache = NSCache<AnyObject, AnyObject>()
        cache.countLimit = config.countLimit
        return cache
    }()
    // 2nd level cache, that contains decoded images
    private lazy var decodedImageCache: NSCache<AnyObject, AnyObject> = {
        let cache = NSCache<AnyObject, AnyObject>()
        cache.totalCostLimit = config.memoryLimit
        return cache
    }()
    private let lock = NSLock()
    private let config: Config

    public struct Config {
        public let countLimit: Int
        public let memoryLimit: Int

        public static let defaultConfig = Config(countLimit: 100, memoryLimit: 1024 * 1024 * 100) // 100 MB
    }

    public init(config: Config = Config.defaultConfig) {
        self.config = config
    }

    public func image(for url: URL) -> UIImage? {
        lock.lock(); defer { lock.unlock() }
        // the best case scenario -> there is a decoded image in memory
        if let decodedImage = decodedImageCache.object(forKey: url as AnyObject) as? UIImage {
            return decodedImage
        }
        // search for image data
        if let image = imageCache.object(forKey: url as AnyObject) as? UIImage {
            let decodedImage = image.decodedImage()
            decodedImageCache.setObject(image as AnyObject, forKey: url as AnyObject, cost: decodedImage.diskSize)
            return decodedImage
        }
        return nil
    }

    public func insertImage(_ image: UIImage?, for url: URL) {
        guard let image = image else { return removeImage(for: url) }
        let decompressedImage = image.decodedImage()

        lock.lock(); defer { lock.unlock() }
        imageCache.setObject(decompressedImage, forKey: url as AnyObject, cost: 1)
        decodedImageCache.setObject(image as AnyObject, forKey: url as AnyObject, cost: decompressedImage.diskSize)
    }

    public func removeImage(for url: URL) {
        lock.lock(); defer { lock.unlock() }
        imageCache.removeObject(forKey: url as AnyObject)
        decodedImageCache.removeObject(forKey: url as AnyObject)
    }

    public func removeAllImages() {
        lock.lock(); defer { lock.unlock() }
        imageCache.removeAllObjects()
        decodedImageCache.removeAllObjects()
    }

    public subscript(_ key: URL) -> UIImage? {
        get {
            return image(for: key)
        }
        set {
            return insertImage(newValue, for: key)
        }
    }
}

fileprivate extension UIImage {

    func decodedImage() -> UIImage {
        guard let cgImage = cgImage else { return self }
        let size = CGSize(width: cgImage.width, height: cgImage.height)
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        let context = CGContext(data: nil, width: Int(size.width), height: Int(size.height), bitsPerComponent: 8, bytesPerRow: cgImage.bytesPerRow, space: colorSpace, bitmapInfo: CGImageAlphaInfo.noneSkipFirst.rawValue)
        context?.draw(cgImage, in: CGRect(origin: .zero, size: size))
        guard let decodedImage = context?.makeImage() else { return self }
        return UIImage(cgImage: decodedImage)
    }

    // Rough estimation of how much memory image uses in bytes
    var diskSize: Int {
        guard let cgImage = cgImage else { return 0 }
        return cgImage.bytesPerRow * cgImage.height
    }
}

public final class ImageLoader {
    public static let shared = ImageLoader()

    private let cache: ImageCacheType
    private lazy var backgroundQueue: OperationQueue = {
        let queue = OperationQueue()
        queue.maxConcurrentOperationCount = 5
        return queue
    }()

    public init(cache: ImageCacheType = ImageCache()) {
        self.cache = cache
    }

    public func loadImage(from url: URL) -> AnyPublisher<UIImage?, Never> {
        if let image = cache[url] {
            return Just(image).eraseToAnyPublisher()
        }
        return URLSession.shared.dataTaskPublisher(for: url)
            .map { (data, response) -> UIImage? in return UIImage(data: data) }
            .catch { error in return Just(nil) }
            .handleEvents(receiveOutput: {[unowned self] image in
                guard let image = image else { return }
                self.cache[url] = image
            })
            .print("Image loading \(url):")
            .subscribe(on: backgroundQueue)
            .receive(on: RunLoop.main)
            .eraseToAnyPublisher()
    }
}