Closure based image loader

ImageLoader.swift

import UIKit
import Combine


final class ImageLoader {
	typealias LoadPublisher = AnyPublisher<UIImage, Error>
	typealias Cache = NSCache<CacheKey, UIImage>

	private let loadImage: (URL, Cache) -> LoadPublisher
	private let cachedImage: (URL, Cache) -> UIImage?

	init(
		loadImage: @escaping (URL, Cache) -> LoadPublisher,
		cachedImage: @escaping (URL, Cache) -> UIImage?
	) {
		self.loadImage = loadImage
		self.cachedImage = cachedImage
	}
}

extension ImageLoader {
	final class CacheKey: NSObject {
		let key: URL

		init(_ key: URL) {
			self.key = key
		}

		override func isEqual(_ object: Any?) -> Bool {
			guard let object = object as? Self else { return false }
			return object.key == key
		}

		override var hash: Int {
			key.hashValue
		}
	}
}

// The live version can be accessed easily from the environment via .live
extension ImageLoader {
	static var live: ImageLoader = {
		let liveImageCache = Cache()
		Self(
			loadImage: { url, cache in
				enum Error: Swift.Error {
					case noData
					case invalidImageData
				}
				// Fetch images for real here
				// One can use a dispatch queue internally or just a lock
				let image = UIImage()
				cache.setObject(image, forKey: .init(url))
				return Just(image)
					.setFailureType(to: Swift.Error.self)
					.eraseToAnyPublisher()
			},
			cachedImage: { url, cache in
				cache.object(forKey: .init(url))
			}
		)
	}()
}

// It's so easy to create a preview version...
#if DEBUG
extension ImageLoader {
	static var preview: ImageLoader = {
		let liveImageCache = Cache()
		Self(
			loadImage: { url, cache in
				let image = UIImage()
				cache.setObject(image, forKey: .init(url))
				return Just(image)
					.setFailureType(to: Error.self)
					.eraseToAnyPublisher()
			},
			cachedImage: { url, cache in
				cache.object(forKey: .init(url))
			}
		)
	}()
}
#endif