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/// Enum that contains states for general data loading.
///
/// - initial: Initial state. Before something will happen.
/// - loading: Loading state. When data loading is started.
/// - result: Result state from a specific data source with result.
/// - error: Error state with a specific error.
/// - empty: Empty state. When data was requested and empty result was received.
public enum GeneralDataLoadingState<DS: DataSource> {

    case initial
    case loading
    case result(newResult: DS.ResultType, from: DS)
    case error(error: Error)
    case empty
}

extension GeneralDataLoadingState: DataLoadingState {

    public typealias DataSourceType = DS

    public static var initialState: GeneralDataLoadingState<DS> {
        .initial
    }

    public static var emptyState: GeneralDataLoadingState<DS> {
        .empty
    }

    public static func initialLoadingState(after: GeneralDataLoadingState<DS>) -> GeneralDataLoadingState<DS> {
        .loading
    }

    public static func resultState(result: DS.ResultType,
                                   from: DS,
                                   after: GeneralDataLoadingState<DS>) -> GeneralDataLoadingState<DS> {

        .result(newResult: result, from: from)
    }

    public static func errorState(error: Error,
                                  after: GeneralDataLoadingState<DS>) -> GeneralDataLoadingState<DS> {

        .error(error: error)
    }

    public var isInitialState: Bool {
        switch self {
        case .initial:
            return true

        default:
            return false
        }
    }

    public var result: DS.ResultType? {
        switch self {
        case .result(let newResult, _):
            return newResult

        default:
            return nil
        }
    }

    public var error: Error? {
        switch self {
        case .error(let error):
            return error

        default:
            return nil
        }
    }
}