【C#/WPF実践入門編(9)】MVVMパターン/アーキテクチャの基礎①～Model・View・ViewModelの3層で役割分担～

public class Counter : INotifyPropertyChanged
{
...
  public ICommand IncrementCommand => _incrementCommand;
  ...
  public event PropertyChangedEventHandler? PropertyChanged;
...
}

WpfCounterApp/
├── CounterWindow.xaml          (View)
├── CounterWindow.xaml.cs       (View - コードビハインド)
├── CounterViewModel.cs         (ViewModel)
├── CounterModel.cs            (Model)
└── SimpleCommand.cs           (汎用コマンドクラス)
…

using System.Windows.Input;

namespace WpfCounterApp
{
    public class SimpleCommand : ICommand
    {
        private readonly Action<object?> _execute;
        private readonly Predicate<object?>? _canExecute;

        public SimpleCommand(Action<object?> execute, Predicate<object?>? canExecute = null)
        {
            _execute = execute ?? throw new ArgumentNullException(nameof(execute));
            _canExecute = canExecute;
        }

        public bool CanExecute(object? parameter) => _canExecute?.Invoke(parameter) ?? true;
        public void Execute(object? parameter) => _execute(parameter);

        public event EventHandler? CanExecuteChanged;

        public void RaiseCanExecuteChanged() => CanExecuteChanged?.Invoke(this, EventArgs.Empty);
    }
}

namespace WpfCounterApp
{
    public class CounterModel
    {
        public int Value { get; private set; }

        public void Increment()
        {
            Value++;
        }

        public bool CanDecrement()
        {
            return Value > 0;
        }

        public void Decrement()
        {
            if (CanDecrement())
            {
                Value--;
            }
        }
    }
}

using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows.Input;

namespace WpfCounterApp
{
    public class CounterViewModel : INotifyPropertyChanged
    {
        private readonly CounterModel _model;
        private readonly SimpleCommand _incrementCommand;
        private readonly SimpleCommand _decrementCommand;

        public CounterViewModel()
        {
            _model = new CounterModel();
            _incrementCommand = new SimpleCommand(_ => ExecuteIncrement());
            _decrementCommand = new SimpleCommand(_ => ExecuteDecrement(), _ => _model.CanDecrement());
        }

        public int Count => _model.Value;

        public ICommand IncrementCommand => _incrementCommand;
        public ICommand DecrementCommand => _decrementCommand;

        private void ExecuteIncrement()
        {
            _model.Increment();
            OnPropertyChanged(nameof(Count));
        }

        private void ExecuteDecrement()
        {
            _model.Decrement();
            OnPropertyChanged(nameof(Count));
        }

        public event PropertyChangedEventHandler? PropertyChanged;

        protected void OnPropertyChanged([CallerMemberName] string? propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));

            if (propertyName == nameof(Count))
            {
                _decrementCommand.RaiseCanExecuteChanged();
            }
        }
    }
}

<Window x:Class="WpfCounterApp.CounterWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfCounterApp"
        mc:Ignorable="d"
        Title="CounterWindow" Height="450" Width="800">
    <StackPanel Margin="20" VerticalAlignment="Center">
        <!-- カウンター表示 -->
        <TextBlock Text="{Binding Count, StringFormat='Count: {0}'}" 
               FontSize="24" 
               HorizontalAlignment="Center" 
               Margin="0,0,0,20"/>

        <!-- ボタン群 -->
        <StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
            <Button Content="－" 
                Command="{Binding DecrementCommand}" 
                Width="40" Height="30"/>
            <Button Content="＋" 
                Command="{Binding IncrementCommand}" 
                Width="40" Height="30"/>
        </StackPanel>

        <!-- 説明テキスト -->
        <TextBlock Text="「－」ボタンは0以下で自動無効化されます" 
               HorizontalAlignment="Center" 
               Margin="0,15,0,0" />
    </StackPanel>
</Window>

using System.Windows;

namespace WpfCounterApp
{
    /// <summary>
    /// CounterWindow.xaml の相互作用ロジック
    /// </summary>
    public partial class CounterWindow : Window
    {
        public CounterWindow()
        {
            InitializeComponent();
            DataContext = new CounterViewModel();
        }
    }
}

<Application x:Class="WpfCounterApp.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfCounterApp"
             StartupUri="CounterWindow.xaml">
    <Application.Resources>
         
    </Application.Resources>
</Application>