Code sample for Medium article about Caching and Protocols

Cache.swift

//This is basically a description of the cache along with the type
public struct Cache<T> {
    let fileName: String
    let transform: (T) throws -> Data
    let reverse: (Data) throws -> T
}

extension Cache {
    //You can define a "convenience" initializer that takes a fileName,
    //and provides PList serialization as transform and reverse transform operations
    public init(plistName: String) {
        self.fileName = plistName
        self.transform = { try PropertyListSerialization.data(fromPropertyList: $0, format: .xml, options: 0) }
        self.reverse = { try PropertyListSerialization.propertyList(from: $0, options: [], format: nil) as! T }
    }
}

Cacher.swift

public final 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)")
        }
    }

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

        // Create an operation to process the request.
        let operation = BlockOperation {
            do {
                try cache.transform(item).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)
    }

    public func load<T>(from cache: Cache<T>, completion: @escaping (_ item: T) -> Void) {
        let url = destination.appendingPathComponent(cache.fileName, isDirectory: false)

        // Create an operation to process the request.
        let operation = BlockOperation {
            do {
                let data = try Data(contentsOf: url, options: [])
                let item = try cache.reverse(data)
                completion(item)
            } catch {
                fatalError("Failed to read item from cache: \(error)")
            }
        }

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

main.swift

let cacher = Cacher(destination: .temporary)

//You can have a cache that stores any valid JSON, not only dictionaries
let jsonCache = Cache(
    fileName: "cached_server_response.json",
    transform: { try! JSONSerialization.data(withJSONObject: $0, options: []) },
    reverse: { try! JSONSerialization.jsonObject(with: $0, options: []) }
)

//You can restrict the type of the config to be a dictionary
//So that you always get a valid dictionary with String keys
//Or you can further restrict it to have only Int values, if that is your use-case
let configPlist = Cache<[String: Any]>(plistName: "config.plist")

//You can have caches that store the same underlying type (String)
//But have diferent strategies for turning those types into data, and back
let noteCache = Cache<String>(
    fileName: "note.txt",
    transform: { $0.data(using: .utf8)! },
    reverse: { String(data: $0, encoding: .utf8)! }
)
let plistStringCache = Cache<String>(plistName: "note.plist")


// Contents of file will be:
//"<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<!DOCTYPE plist PUBLIC \"-//Apple//DTD PLIST 1.0//EN\" \"http://www.apple.com/DTDs/PropertyList-1.0.dtd\">\n<plist version=\"1.0\">\n<string>Hello, World!</string>\n</plist>\n"
cacher.persist("Hello, World!", at: plistStringCache) {
    //dump($0)
    let data = try! Data(contentsOf: $0)
    let string = String(data: data, encoding: .utf8)!
    dump(string)
}
// Contents of file will be:
// "Hello, World!"
cacher.persist("Hello, World!", at: noteCache) {
    //dump($0)
    let data = try! Data(contentsOf: $0)
    let string = String(data: data, encoding: .utf8)!
    dump(string)
}

cacher.load(from: plistStringCache) { dump($0) }
cacher.load(from: noteCache) { dump($0) }

thanks man but I am having a little of something from this line: cacher.load(from: plistStringCache) { dump($0) }, it throws file "note.plist" Not found