protocol SearchMoviesUseCase {
    func execute(requestValue: SearchMoviesUseCaseRequestValue,
                 completion: @escaping (Result<MoviesPage, Error>) -> Void) -> Cancellable?
}

final class DefaultSearchMoviesUseCase: SearchMoviesUseCase {

    private let moviesRepository: MoviesRepository
    private let moviesQueriesRepository: MoviesQueriesRepository
    
    init(moviesRepository: MoviesRepository, moviesQueriesRepository: MoviesQueriesRepository) {
        self.moviesRepository = moviesRepository
        self.moviesQueriesRepository = moviesQueriesRepository
    }
    
    func execute(requestValue: SearchMoviesUseCaseRequestValue,
                 completion: @escaping (Result<MoviesPage, Error>) -> Void) -> Cancellable? {
        return moviesRepository.fetchMoviesList(query: requestValue.query, page: requestValue.page) { result in
            
            if case .success = result {
                self.moviesQueriesRepository.saveRecentQuery(query: requestValue.query) { _ in }
            }

            completion(result)
        }
    }
}

// Repository Interfaces
protocol MoviesRepository {
    func fetchMoviesList(query: MovieQuery, page: Int, completion: @escaping (Result<MoviesPage, Error>) -> Void) -> Cancellable?
}

protocol MoviesQueriesRepository {
    func fetchRecentsQueries(maxCount: Int, completion: @escaping (Result<[MovieQuery], Error>) -> Void)
    func saveRecentQuery(query: MovieQuery, completion: @escaping (Result<MovieQuery, Error>) -> Void)
}

// Note: We cannot have any UI frameworks(like UIKit or SwiftUI) imports here. 
protocol MoviesListViewModelInput {
    func didSearch(query: String)
    func didSelect(at indexPath: IndexPath)
}

protocol MoviesListViewModelOutput {
    var items: Observable<[MoviesListItemViewModel]> { get }
    var error: Observable<String> { get }
}

protocol MoviesListViewModel: MoviesListViewModelInput, MoviesListViewModelOutput { }

struct MoviesListViewModelActions {
    // Note: if you would need to edit movie inside Details screen and update this 
    // MoviesList screen with Updated movie then you would need this closure:
    //  showMovieDetails: (Movie, @escaping (_ updated: Movie) -> Void) -> Void
    let showMovieDetails: (Movie) -> Void
}

final class DefaultMoviesListViewModel: MoviesListViewModel {
    
    private let searchMoviesUseCase: SearchMoviesUseCase
    private let actions: MoviesListViewModelActions?
    
    private var movies: [Movie] = []
    
    // MARK: - OUTPUT
    let items: Observable<[MoviesListItemViewModel]> = Observable([])
    let error: Observable<String> = Observable("")
    
    init(searchMoviesUseCase: SearchMoviesUseCase,
         actions: MoviesListViewModelActions) {
        self.searchMoviesUseCase = searchMoviesUseCase
        self.actions = actions
    }
    
    private func load(movieQuery: MovieQuery) {
        
        searchMoviesUseCase.execute(movieQuery: movieQuery) { result in
            switch result {
            case .success(let moviesPage):
                // Note: We must map here from Domain Entities into Item View Models. Separation of Domain and View
                self.items.value += moviesPage.movies.map(MoviesListItemViewModel.init)
                self.movies += moviesPage.movies
            case .failure:
                self.error.value = NSLocalizedString("Failed loading movies", comment: "")
            }
        }
    }
}

// MARK: - INPUT. View event methods
extension MoviesListViewModel {
    
    func didSearch(query: String) {
        load(movieQuery: MovieQuery(query: query))
    }
    
    func didSelect(at indexPath: IndexPath) {
        actions?.showMovieDetails(movies[indexPath.row])
    }
}

// Note: This item view model is to display data and does not contain any domain model to prevent views accessing it
struct MoviesListItemViewModel: Equatable {
    let title: String
}

extension MoviesListItemViewModel {
    init(movie: Movie) {
        self.title = movie.title ?? ""
    }
}

import UIKit

final class MoviesListViewController: UIViewController, StoryboardInstantiable, UISearchBarDelegate {
    
    private var viewModel: MoviesListViewModel!
    
    final class func create(with viewModel: MoviesListViewModel) -> MoviesListViewController {
        let vc = MoviesListViewController.instantiateViewController()
        vc.viewModel = viewModel
        return vc
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        bind(to: viewModel)
    }
    
    private func bind(to viewModel: MoviesListViewModel) {
        viewModel.items.observe(on: self) { [weak self] items in
            self?.moviesTableViewController?.items = items
        }
        viewModel.error.observe(on: self) { [weak self] error in
            self?.showError(error)
        }
    }
    
    func searchBarSearchButtonClicked(_ searchBar: UISearchBar) {
        guard let searchText = searchBar.text, !searchText.isEmpty else { return }
        viewModel.didSearch(query: searchText)
    }
}

protocol MoviesSearchFlowCoordinatorDependencies  {
    func makeMoviesListViewController() -> UIViewController
    func makeMoviesDetailsViewController(movie: Movie) -> UIViewController
}

final class MoviesSearchFlowCoordinator {
    
    private weak var navigationController: UINavigationController?
    private let dependencies: MoviesSearchFlowCoordinatorDependencies

    init(navigationController: UINavigationController,
         dependencies: MoviesSearchFlowCoordinatorDependencies) {
        self.navigationController = navigationController
        self.dependencies = dependencies
    }
    
    func start() {
        // Note: here we keep strong reference with actions closures, this way this flow do not need to be strong referenced
        let actions = MoviesListViewModelActions(showMovieDetails: showMovieDetails)
        let vc = dependencies.makeMoviesListViewController(actions: actions)
        
        navigationController?.pushViewController(vc, animated: false)
    }
    
    private func showMovieDetails(movie: Movie) {
        let vc = dependencies.makeMoviesDetailsViewController(movie: movie)
        navigationController?.pushViewController(vc, animated: true)
    }
}

// MARK: - Movies Queries Suggestions List
func makeMoviesQueriesSuggestionsListViewController(didSelect: @escaping MoviesQueryListViewModelDidSelectAction) -> UIViewController {
   if #available(iOS 13.0, *) { // SwiftUI
       let view = MoviesQueryListView(viewModelWrapper: makeMoviesQueryListViewModelWrapper(didSelect: didSelect))
       return UIHostingController(rootView: view)
   } else { // UIKit
       return MoviesQueriesTableViewController.create(with: makeMoviesQueryListViewModel(didSelect: didSelect))
   }
}

final class DefaultMoviesRepository {
    
    private let dataTransferService: DataTransfer
    
    init(dataTransferService: DataTransfer) {
        self.dataTransferService = dataTransferService
    }
}

extension DefaultMoviesRepository: MoviesRepository {
    
    public func fetchMoviesList(query: MovieQuery, page: Int, completion: @escaping (Result<MoviesPage, Error>) -> Void) -> Cancellable? {
        
        let endpoint = APIEndpoints.getMovies(with: MoviesRequestDTO(query: query.query,
                                                                     page: page))
        return dataTransferService.request(with: endpoint) { (response: Result<MoviesResponseDTO, Error>) in
            switch response {
            case .success(let moviesResponseDTO):
                completion(.success(moviesResponseDTO.toDomain()))
            case .failure(let error):
                completion(.failure(error))
            }
        }
    }
}

// MARK: - Data Transfer Object (DTO)
// It is used as intermediate object to encode/decode JSON response into domain, inside DataTransferService
struct MoviesRequestDTO: Encodable {
    let query: String
    let page: Int
}

struct MoviesResponseDTO: Decodable {
    private enum CodingKeys: String, CodingKey {
        case page
        case totalPages = "total_pages"
        case movies = "results"
    }
    let page: Int
    let totalPages: Int
    let movies: [MovieDTO]
}
...
// MARK: - Mappings to Domain
extension MoviesResponseDTO {
    func toDomain() -> MoviesPage {
        return .init(page: page,
                     totalPages: totalPages,
                     movies: movies.map { $0.toDomain() })
    }
}

struct APIEndpoints {
    
    static func getMovies(with moviesRequestDTO: MoviesRequestDTO) -> Endpoint<MoviesResponseDTO> {

        return Endpoint(path: "search/movie/",
                        method: .get,
                        queryParametersEncodable: moviesRequestDTO)
    }
}


let config = ApiDataNetworkConfig(baseURL: URL(string: appConfigurations.apiBaseURL)!,
                                  queryParameters: ["api_key": appConfigurations.apiKey])
let apiDataNetwork = DefaultNetworkService(session: URLSession.shared,
                                           config: config)

let endpoint = APIEndpoints.getMovies(with: MoviesRequestDTO(query: query.query,
                                                             page: page))
dataTransferService.request(with: endpoint) { (response: Result<MoviesResponseDTO, Error>) in
    let moviesPage = try? response.get()
}

public final class Observable<Value> {
    
    private var closure: ((Value) -> ())?

    public var value: Value {
        didSet { closure?(value) } // 값이 변경되면 전달받은 클로저를 호출할 겁니다
    }

    public init(_ value: Value) {
        self.value = value
    }

    public func observe(_ closure: @escaping (Value) -> Void) {
        self.closure = closure
        closure(value)
    }
}

final class ExampleViewController: UIViewController {
    
    private var viewModel: MoviesListViewModel!
    
    private func bind(to viewModel: ViewModel) {
        self.viewModel = viewModel
        viewModel.items.observe(on: self) { [weak self] items in
            self?.tableViewController?.items = items
            // Important: You cannot use viewModel inside this closure, it will cause retain cycle memory leak (viewModel.items.value not allowed)
            // self?.tableViewController?.items = viewModel.items.value // This would be retain cycle. You can access viewModel only with self?.viewModel
        }
        // Or in one line
        viewModel.items.observe(on: self) { [weak self] in self?.tableViewController?.items = $0 } // observable 객체의 값이 변경되면 이 클로저를 실행시킬겁니다
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        bind(to: viewModel) 
        viewModel.viewDidLoad()
    }
}


protocol ViewModelInput {
    func viewDidLoad()
}

protocol ViewModelOutput {
    var items: Observable<[ItemViewModel]> { get }
}

protocol ViewModel: ViewModelInput, ViewModelOutput {}

final class MoviesListItemCell: UITableViewCell {

    private var viewModel: MoviesListItemViewModel! { didSet { unbind(from: oldValue) } }
  
    func fill(with viewModel: MoviesListItemViewModel) { 
        self.viewModel = viewModel
        bind(to: viewModel)
    }
    
    private func bind(to viewModel: MoviesListItemViewModel) {
        viewModel.posterImage.observe(on: self) { [weak self] in self?.imageView.image = $0.flatMap(UIImage.init) }
    }
    
    private func unbind(from item: MoviesListItemViewModel?) {
        item?.posterImage.remove(observer: self)
    }
}

// Step 1: Delegate를 선언하고 첫번째 ViewModel을 weak로 할당한다.
protocol MoviesQueryListViewModelDelegate: class {
    func moviesQueriesListDidSelect(movieQuery: MovieQuery)
}
...
final class DefaultMoviesQueryListViewModel: MoviesListViewModel {
    private weak var delegate: MoviesQueryListViewModelDelegate?
    
    func didSelect(item: MoviesQueryListViewItemModel) { 
        // Note: We have to map here from View Item Model to Domain Enity
        delegate?.moviesQueriesListDidSelect(movieQuery: MovieQuery(query: item.query))
    }
}

// Step 2:  두번째 viewModel이 이 delegate 프토토콜을 구현하게 한다.
extension MoviesListViewModel: MoviesQueryListViewModelDelegate {
    func moviesQueriesListDidSelect(movieQuery: MovieQuery) {
        update(movieQuery: movieQuery)
    }
}

// MoviesQueryList.swift
// Step 1: 다른 ViewModel과 communicate할 action 클로저를 정의한다. 여기서는 query가 선택되었을 때 MovieList를 알린다.
typealias MoviesQueryListViewModelDidSelectAction = (MovieQuery) -> Void

// Step 2: 필요할 때 action closure를 호출한다.
class MoviesQueryListViewModel {
    init(didSelect: MoviesQueryListViewModelDidSelectAction? = nil) {
        self.didSelect = didSelect
    }
    func didSelect(item: MoviesQueryListItemViewModel) {
        didSelect?(MovieQuery(query: item.query))
    }
}

// MoviesQueryList.swift
// Step 3: MoviesQueryListView를 출력할때, 파라미터 (_ didSelect: MovieQuery) -> Void의 타입인 action closure를 전달한다.
struct MoviesListViewModelActions {
    let showMovieQueriesSuggestions: (@escaping (_ didSelect: MovieQuery) -> Void) -> Void
}

class MoviesListViewModel { 
    var actions: MoviesListViewModelActions?

    func showQueriesSuggestions() {
        actions?.showMovieQueriesSuggestions { self.update(movieQuery: $0) } 
        //or simpler actions?.showMovieQueriesSuggestions(update)
    }
}

// FlowCoordinator.swift
// Step 4: FlowCoordinator 안에서 self function으로 action closure를 주입함으로써 두 viewModel 간의 communication을 설정한다.
class MoviesSearchFlowCoordinator {
    func start() {
        let actions = MoviesListViewModelActions(showMovieQueriesSuggestions: self.showMovieQueriesSuggestions)
        let vc = dependencies.makeMoviesListViewController(actions: actions)  
        present(vc)
    }

    private func showMovieQueriesSuggestions(didSelect: @escaping (MovieQuery) -> Void) {
        let vc = dependencies.makeMoviesQueriesSuggestionsListViewController(didSelect: didSelect)
        present(vc)
    }
}

// Define Dependencies protocol for class or struct that needs it
protocol MoviesSearchFlowCoordinatorDependencies  {
    func makeMoviesListViewController() -> MoviesListViewController
}

class MoviesSearchFlowCoordinator {
    
    private let dependencies: MoviesSearchFlowCoordinatorDependencies

    init(dependencies: MoviesSearchFlowCoordinatorDependencies) {
        self.dependencies = dependencies
    }
...
}

// Make the DIContainer to conform to this protocol
extension MoviesSceneDIContainer: MoviesSearchFlowCoordinatorDependencies {}

// And inject MoviesSceneDIContainer `self` into class that needs it
final class MoviesSceneDIContainer {
    ...
    // MARK: - Flow Coordinators
    func makeMoviesSearchFlowCoordinator(navigationController: UINavigationController) -> MoviesSearchFlowCoordinator {
        return MoviesSearchFlowCoordinator(navigationController: navigationController,
                                           dependencies: self)
    }
}

// MoviesListViewController를 반환하는 makeMoviesListViewController 클로저를 정의한다.
class MoviesSearchFlowCoordinator {
   
    private var makeMoviesListViewController: () -> MoviesListViewController

    init(navigationController: UINavigationController,
         makeMoviesListViewController: @escaping () -> MoviesListViewController) {
        ...
        self.makeMoviesListViewController = makeMoviesListViewController
    }
    ...
}

// MoviesSceneDiContainer의 `self`.makeMoviesListViewController 함수를 필요한 곳에 주입한다.
final class MoviesSceneDIContainer {
    ...
    // MARK: - Flow Coordinators
    func makeMoviesSearchFlowCoordinator(navigationController: UINavigationController) -> MoviesSearchFlowCoordinator {
        return MoviesSearchFlowCoordinator(navigationController: navigationController,
                                           makeMoviesListViewController: self.makeMoviesListViewController)
    }
    
    // MARK: - Movies List
    func makeMoviesListViewController() -> MoviesListViewController {
        ...
    }
}