C# MVP 架構（下）：把商品管理程式重構成 Model-View-Presenter

namespace MvpDemo.Core.Models;

public class Product
{
    public string Name { get; }
    public int Price { get; }

    public Product(string name, int price)
    {
        Name = name;
        Price = price;
    }
}

namespace MvpDemo.Core.Models;

public class AddResult
{
    public bool Success { get; }
    public string ErrorMessage { get; }

    private AddResult(bool success, string errorMessage)
    {
        Success = success;
        ErrorMessage = errorMessage;
    }

    public static AddResult Ok() => new AddResult(true, string.Empty);

    public static AddResult Fail(string message) => new AddResult(false, message);
}

namespace MvpDemo.Core.Models;

public class ProductService
{
    private readonly List<Product> _products = new List<Product>();

    public IReadOnlyList<Product> Products => _products;

    public int Count => _products.Count;

    public int TotalPrice
    {
        get
        {
            int total = 0;
            foreach (Product p in _products)
            {
                total += p.Price;
            }
            return total;
        }
    }

    public AddResult Add(string name, string priceText)
    {
        if (string.IsNullOrWhiteSpace(name))
        {
            return AddResult.Fail("請輸入商品名稱");
        }

        if (!int.TryParse(priceText, out int price))
        {
            return AddResult.Fail("價格必須是數字");
        }

        if (price <= 0)
        {
            return AddResult.Fail("價格必須大於 0");
        }

        foreach (Product p in _products)
        {
            if (p.Name == name)
            {
                return AddResult.Fail("這個商品已經存在");
            }
        }

        _products.Add(new Product(name, price));
        return AddResult.Ok();
    }

    public bool Remove(string name)
    {
        for (int i = 0; i < _products.Count; i++)
        {
            if (_products[i].Name == name)
            {
                _products.RemoveAt(i);
                return true;
            }
        }
        return false;
    }
}

namespace MvpDemo.Core.Views;

public interface IProductView
{
    string ProductNameText { get; }
    string PriceText { get; }

    event EventHandler AddClicked;

    void AddProductToList(string display);
    void ShowSummary(string text);
    void ShowError(string message);
    void ClearInputs();
}

using MvpDemo.Core.Models;
using MvpDemo.Core.Views;

namespace MvpDemo.Core.Presenters;

public class ProductPresenter
{
    private readonly IProductView _view;
    private readonly ProductService _service;

    public ProductPresenter(IProductView view, ProductService service)
    {
        _view = view;
        _service = service;

        _view.AddClicked += OnAddClicked;

        _view.ShowSummary(BuildSummary());
    }

    private void OnAddClicked(object? sender, EventArgs e)
    {
        AddProduct();
    }

    public void AddProduct()
    {
        AddResult result = _service.Add(_view.ProductNameText, _view.PriceText);

        if (!result.Success)
        {
            _view.ShowError(result.ErrorMessage);
            return;
        }

        Product added = _service.Products[_service.Count - 1];
        _view.AddProductToList($"{added.Name}  {added.Price} 元");
        _view.ShowSummary(BuildSummary());
        _view.ClearInputs();
    }

    private string BuildSummary()
    {
        return $"共 {_service.Count} 項，總價 {_service.TotalPrice} 元";
    }
}

using MvpDemo.Core.Views;

namespace MvpDemo.App;

public class MainForm : Form, IProductView
{
    private readonly TextBox txtName = new TextBox();
    private readonly TextBox txtPrice = new TextBox();
    private readonly Button btnAdd = new Button();
    private readonly ListBox lstItems = new ListBox();
    private readonly Label lblSummary = new Label();

    public MainForm()
    {
        Text = "商品管理（MVP 版）";
        ClientSize = new Size(380, 330);

        var lblName = new Label { Text = "商品名稱", Location = new Point(20, 20), AutoSize = true };
        txtName.Location = new Point(100, 17);
        txtName.Width = 250;

        var lblPrice = new Label { Text = "價格", Location = new Point(20, 55), AutoSize = true };
        txtPrice.Location = new Point(100, 52);
        txtPrice.Width = 250;

        btnAdd.Text = "新增";
        btnAdd.Location = new Point(100, 88);
        btnAdd.Width = 250;
        btnAdd.Click += (s, e) => AddClicked?.Invoke(this, EventArgs.Empty);

        lstItems.Location = new Point(20, 130);
        lstItems.Size = new Size(330, 130);

        lblSummary.Location = new Point(20, 275);
        lblSummary.AutoSize = true;

        Controls.AddRange(new Control[] { lblName, txtName, lblPrice, txtPrice, btnAdd, lstItems, lblSummary });
    }

    // ---- IProductView 實作 ----

    public string ProductNameText => txtName.Text;

    public string PriceText => txtPrice.Text;

    public event EventHandler? AddClicked;

    public void AddProductToList(string display) => lstItems.Items.Add(display);

    public void ShowSummary(string text) => lblSummary.Text = text;

    public void ShowError(string message) => MessageBox.Show(message);

    public void ClearInputs()
    {
        txtName.Clear();
        txtPrice.Clear();
        txtName.Focus();
    }
}

using MvpDemo.Core.Models;
using MvpDemo.Core.Presenters;

namespace MvpDemo.App;

static class Program
{
    [STAThread]
    static void Main()
    {
        ApplicationConfiguration.Initialize();

        var view = new MainForm();
        var service = new ProductService();
        _ = new ProductPresenter(view, service);

        Application.Run(view);
    }
}

using MvpDemo.Core.Views;

namespace MvpDemo.Tests;

public class FakeProductView : IProductView
{
    public string ProductNameText { get; set; } = string.Empty;
    public string PriceText { get; set; } = string.Empty;

    public event EventHandler? AddClicked;

    public List<string> ListItems { get; } = new List<string>();
    public List<string> Errors { get; } = new List<string>();
    public string Summary { get; private set; } = string.Empty;
    public int ClearInputsCallCount { get; private set; }

    public void AddProductToList(string display) => ListItems.Add(display);

    public void ShowSummary(string text) => Summary = text;

    public void ShowError(string message) => Errors.Add(message);

    public void ClearInputs() => ClearInputsCallCount++;

    public void ClickAdd() => AddClicked?.Invoke(this, EventArgs.Empty);
}

[Fact]
public void 刪除商品_統計應該正確減少()
{
    // 新增兩筆、刪除一筆，驗證 Summary 變成「共 1 項，總價 ... 元」
}

cd MvpDemo
dotnet build       # 建置三個專案
dotnet test        # 跑六個測試
cd MvpDemo.App
dotnet run         # 開啟視窗