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/// Protocol that describes possible states of network operation process.
public protocol NetworkOperationState {

    associatedtype DataSourceType: DataSource

    /// Initial state. Before something will happen.
    static var initialState: Self { get }

    /// Method that returns loading state after a given state.
    ///
    /// - Parameter after: Previous state of data loading process.
    /// - Returns: Instance of loading state with given argument.
    static func initialLoadingState(after: Self) -> Self

    /// Method that returns result state from a specific data source after a given state.
    ///
    /// - Parameters:
    ///   - result: DataSource result.
    ///   - from: DataSource from that a result was received.
    ///   - after: Previous state of data loading process.
    /// - Returns: Instance of result state with given arguments.
    static func resultState(result: DataSourceType.ResultType, from: DataSourceType, after: Self) -> Self

    /// Method that returns error state with a specific error after a given state.
    ///
    /// - Parameters:
    ///   - error: An error that happens due data loading process.
    ///   - after: Previous state of data loading process.
    /// - Returns: Instance of error state with given arguments.
    static func errorState(error: Error, after: Self) -> Self

    /// Returns true if current state is initial state.
    var isInitialState: Bool { get }

    /// Returns result if current state is result state.
    var result: DataSourceType.ResultType? { get }

    /// Returns error if current state is error state.
    var error: Error? { get }
}