struct TasksState: SubstateType {   
    var tasks: [Task]
    var addingTask: Bool
}

let tasksState = TasksState()
let action = StoreAction.add(states: [tasksState])
store.dispatch(action: action)

public enum StoreAction: ActionType {
    /// Adds substates to the application state.
    case add(states: [SubstateType])

    /// Removes all substates in the application state.
    case reset
}

let mainReducer: MainReducer = { (state: [SubstateType], action: ActionType) -> [SubstateType] in
    // Copy the `App State`
    var state: [SubstateType] = state
    
    // Cast to a specific `Action`.
    switch action {
    case let action as TasksAction:

        // Extract the `Substate`.
        guard var (tasksStateIndex, tasksState) = state
            .enumerated()
            .first(where: { (_, substate: SubstateType) -> Bool in
                return substate is Store.TasksState}
            ) as? (Int, Store.TasksState)
            else {
                fatalError("You need to register `TasksState` first")
        }

        // Reduce the `Substate` to get a new `Substate`.
        tasksState = Store.reduce(state: tasksState, action: action)
    
        // Replace the `Substate` in the `App State` with the new `Substate`.
        state[tasksStateIndex] = tasksState as SubstateType
    
    default:
        fatalError("Unknown action type")
    }
    
    // Return the new `App State`
    return state
}

protocol LoggingMiddlewareType: Middleware, HasDisposeBag {}

final class LoggingMiddleware: LoggingMiddlewareType {
    var disposeBag = DisposeBag()

    func observe(currentStateLastAction: Driver<CurrentStateLastAction>) {
        currentStateLastAction
            .drive(
                onNext: { (currentState: [SubstateType], lastAction: ActionType?) in
                    print(currentState)
                    print(lastAction)
                }, onCompleted: nil, onDisposed: nil)
            .disposed(by: disposeBag)
        }
    }
}