How design patterns can help you in developing unit testing-enabled applications

using Northwind.Services;
using Northwind.TransferObjects.Model;
using Northwind.TransferObjects.Presentation;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;

namespace Northwind.Presentation
{
    /// <summary>
    /// Implements a presenter in a Model-View-Presenter Pattern
    /// </summary>
    public class ViewCustomerPresenter
    {
        private readonly IViewCustomerView view;
        private ICustomerServices services;

        public ViewCustomerPresenter()
        {
        }

        public ViewCustomerPresenter(IViewCustomerView view) : this(view, new CustomerServices()) {}

        /// <summary>
        /// Constructs a Presenter with injection of dependencies for layers view and services
        /// </summary>
        /// <param name="view"></param>
        /// <param name="services"></param>
        public ViewCustomerPresenter(IViewCustomerView view, ICustomerServices services)
        {
            this.view = view;
            this.services = services;
        }

        private void DisplayOrders()
        {
            string customerId = SelectedCustomerId;

            CustomerTO customerTO = services.GetDetailsForCustomer(customerId);
            UpdateViewFrom(customerTO);
        }

        public void Initialize()
        {
            ChangeViewMode(ViewMode.ChooseRepository);
        }

        /// <summary>
        /// Initialize presenter by presenting customer list and setting view to master list mode
        /// </summary>
        public void ShowDatabase()
        {
            services = new CustomerServices(RepositoryMode.Database);
            services.GetCustomerList().BindTo(view.CustomerList);
            ChangeViewMode(ViewMode.MasterList);
        }

        public void ShowInMemory()
        {
            services = new CustomerServices(RepositoryMode.InMemory);
            services.GetCustomerList().BindTo(view.CustomerList);
            ChangeViewMode(ViewMode.MasterList);
        }

        private void DisplayCustomerDetails(string customerId)
        {
            if (customerId.Length > 0)
            {
                CustomerTO customerTO = services.GetDetailsForCustomer(customerId);
                UpdateViewFrom(customerTO);
            }
        }

        public void NewCustomer()
        {
            string customerId = view.RequestInputBox(CustomerMessages.INFORM_NEW_CUSTOMER_ID, CustomerMessages.CUSTOMER);
            if (customerId.Length > 0)
            {
                CustomerTO customerTO = services.NewCustomer(customerId);
                UpdateViewFrom(customerTO);
                ChangeViewMode(ViewMode.New);
            }
        }

        public void EditCustomer()
        {
            string customerID = view.Id;
            DisplayCustomerDetails(customerID);
            ChangeViewMode(ViewMode.Edit);
        }

        public void DeleteCustomer()
        {
            services.DeleteCustomer(view.Id);
            services.GetCustomerList().BindTo(view.CustomerList);
            ChangeViewMode(ViewMode.MasterList);
        }

        public void SaveCustomer()
        {
            CustomerTO customerTO = GetTOFromView();
            try
            {
                services.UpdateCustomer(customerTO);
                services.GetCustomerList().BindTo(view.CustomerList);
                ChangeViewMode(ViewMode.MasterList);
            }
            catch (Exception exc)
            {
                if (exc.InnerException == null)
                {
                    view.DisplayError(exc.Message);
                }
                else
                {
                    view.DisplayError(exc.InnerException.Message);
                }
            }
        }

        public void CancelCustomer()
        {
            ChangeViewMode(ViewMode.MasterList);
        }

        public void SearchCustomer()
        {
            services.GetCustomerList(view.NameToSearch).BindTo(view.CustomerList);
            ChangeViewMode(ViewMode.MasterList);
        }

        public void SelectedCustomerChanged()
        {
            ILookupTO selectedItem = view.CustomerList.SelectedItem;
            if (selectedItem != null)
            {
                view.Id = selectedItem.Value;
                view.EditActionButton.Enabled = true;
                view.DeleteActionButton.Enabled = true;
            }
        }

        private string SelectedCustomerId
        {
            get
            {
                string id = "";
                ILookupTO selectedItem = view.CustomerList.SelectedItem;
                
                if (selectedItem != null)
                {
                    string selectedId = selectedItem.Value;

                    if (String.IsNullOrEmpty(selectedId)) return null;

                    id = selectedId;
                }

                return id;
            }
        }

        private void UpdateViewFrom(CustomerTO customerTO)
        {
            view.Id = customerTO.ID;
            view.Company = customerTO.CompanyName;
            view.ContactName = customerTO.ContactName;
            view.ContactTitle = customerTO.ContactTitle;
            view.Address = customerTO.Address;
            view.City = customerTO.City;
            view.Region = customerTO.Region;
            view.Country = customerTO.Country;
            view.Phone = customerTO.Phone;
            view.Fax = customerTO.Fax;
            view.PostalCode = customerTO.PostalCode;

            ILookupCollection orderList = new LookupCollection(new OrderToLookupConverter().ConvertAllFrom(customerTO.Orders));
            orderList.BindTo(view.OrderList);
        }

        private CustomerTO GetTOFromView()
        {
            CustomerTO customerTO = new CustomerTO(
                                                    view.Id,
                                                    view.Company,
                                                    view.ContactName,
                                                    view.ContactTitle,
                                                    view.Address,
                                                    view.City,
                                                    view.Region,
                                                    view.PostalCode,
                                                    view.Country,
                                                    view.Phone,
                                                    view.Fax,
                                                    null
                                                   );
            return customerTO;
        }

        private void ChangeViewMode(ViewMode viewMode)
        {
            switch (viewMode)
            {
                case ViewMode.ChooseRepository:
                    view.RepositoryPanel.Visible = true;
                    view.MasterPanel.Visible = false;
                    view.DetailsPanel.Visible = false;

                    view.NewActionButton.Enabled = false;
                    view.EditActionButton.Enabled = false;
                    view.DeleteActionButton.Enabled = false;
                    view.SaveActionButton.Enabled = false;
                    view.CancelActionButton.Enabled = false;

                    break;
                case ViewMode.MasterList:
                    view.MasterPanel.Visible = true;
                    view.RepositoryPanel.Visible = false;
                    view.NewActionButton.Enabled = true;
                    if (view.CustomerList.SelectedItem == null)
                    {
                        view.EditActionButton.Enabled = false;
                        view.DeleteActionButton.Enabled = false;
                    }
                    else
                    {
                        view.EditActionButton.Enabled = true;
                        view.DeleteActionButton.Enabled = true;
                    }

                    view.DetailsPanel.Visible = false;
                    view.SaveActionButton.Enabled = false;
                    view.CancelActionButton.Enabled = false;
                    break;
                case ViewMode.New:
                case ViewMode.Edit: 
                    view.MasterPanel.Visible = false;
                    view.NewActionButton.Enabled = false;
                    view.EditActionButton.Enabled = false;
                    view.DeleteActionButton.Enabled = false;

                    view.DetailsPanel.Visible = true;
                    view.SaveActionButton.Enabled = true;
                    view.CancelActionButton.Enabled = true;
                    break;
            }
            view.Mode = viewMode;
         }

        public ICustomerServices Services
        {
            get { return this.services; }
        }

    }

    public enum ViewMode
    {
        ChooseRepository,
        MasterList,
        New,
        Edit
    }

    public class CustomerMessages
    {
        public static string INFORM_NEW_CUSTOMER_ID = "Inform new customer id";
        public static string CUSTOMER = "Inform new customer id";
    }
}