Code sample for Medium article about Caching and Protocols

Cacher.swift

public protocol Cachable {
	var fileName: String { get }
	func transform() -> Data
}

final public class Cacher {
	let destination: URL
	private let queue = OperationQueue()

	public enum CacheDestination {
		case temporary
		case atFolder(String)
	}

	// MARK: Initialization

	public init(destination: CacheDestination) {
		// Create the URL for the location of the cache resources
		switch destination {
		case .temporary:
			self.destination = URL(fileURLWithPath: NSTemporaryDirectory())
		case .atFolder(let folder):
			let documentFolder = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
			self.destination = URL(fileURLWithPath: documentFolder).appendingPathComponent(folder, isDirectory: true)
		}

		let fileManager = FileManager.default

		do {
			try fileManager.createDirectory(at: self.destination, withIntermediateDirectories: true, attributes: nil)
		}
		catch {
			fatalError("Unable to create cache URL: \(error)")
		}
	}

	// MARK

	public func persist(item: Cachable, completion: @escaping (_ url: URL) -> Void) {
		let url = destination.appendingPathComponent(item.fileName, isDirectory: false)

		// Create an operation to process the request.
		let operation = BlockOperation {
			do {
				try item.transform().write(to: url, options: [.atomicWrite])
			} catch {
				fatalError("Failed to write item to cache: \(error)")
			}
		}

		// Set the operation's completion block to call the request's completion handler.
		operation.completionBlock = {
			completion(url)
		}

		// Add the operation to the queue to start the work.
		queue.addOperation(operation)
	}
}

Checkout the Swift 4 and multi platform (iOS, tvOS, macOS, watchOS) at https://github.com/raulriera/Cacher