PersistentDataStore+Loading.swift

func loadData<T>(forIdentifier identifier: Identifier, dataTransformer: @escaping (Data) -> (T?), completionHandler block: @escaping (T?) -> ()) {
    queue.async {
        let url = self.url(forIdentifier: identifier)
        guard FileManager.default.fileExists(atPath: url.path) else {
            DispatchQueue.main.async {
                block(nil)
            }
            return
        }
        do {
            let data = try Data(contentsOf: url, options: .mappedIfSafe)
            let object = dataTransformer(data)
            DispatchQueue.main.async {
                block(object)
            }
        }
        catch {
            print("Failed reading data at URL \(url).")
            DispatchQueue.main.async {
                block(nil)
            }
        }
    }
}

// Example
persistentStore.loadData(forIdentifier: "my_identifier", dataTransformer: { UIImage(data: $0) }) { (image) in
    guard let image = image else {
        print("Failed loading image.")
        return
    }
    print(image)
}