modeling_testable_dependencies_1.swift

/// Defines the `SomeRepository` contract.
protocol SomeRepositoryProtocol {
    /// Returns the cached data.
    var cachedList: [String]? { get }
    
    /// Gets the list from service or cache.
    /// - Parameter completion: returns the list
    func getList(then completion: @escaping ([String]?) -> Void)
}

final class SomeRepository: SomeRepositoryProtocol {
    // MARK: - Dependencies
    
    private let session: URLSession
    private let jsonDecoder: JSONDecoder
    
    // MARK: - Properties
    
    private(set) var cachedList: [String]?
    
    // MARK: - Initialization
    
    init(
        session: URLSession = .shared,
        jsonDecoder: JSONDecoder = .init()
    ) {
        self.session = session
        self.jsonDecoder = jsonDecoder
    }
    
    // MARK: - Public API
    
    func getList(then completion: @escaping ([String]?) -> Void) {
        if let cached = cachedList {
            completion(cached)
            return
        }
        
        session.dataTask(with: .init(string: "www.myapi.com/list")!) { [weak self] data, response, error in
            guard
                let data = data,
                let decodedValue = try? self?.jsonDecoder.decode([String].self, from: data)
            else {
                completion(nil)
                return
            }
            self?.cachedList = decodedValue
            completion(decodedValue)
        }
    }
}