【C#/WPF実践入門編(8)】コマンドの基礎 ～ICommandによる操作の分離～【MVVMパターン理解の基礎】

...  
<Button Content="+" Click="OnIncrement"/>
...

private void OnIncrement(object sender, RoutedEventArgs e)
{
    Count++;
}

public interface ICommand
{
    bool CanExecute(object? parameter);
    void Execute(object? parameter); 
    event EventHandler? CanExecuteChanged; 
}

public class Counter : INotifyPropertyChanged
{
    public int Count{
      ...
    }

    public ICommand DecrementCommand { get; }
    
    public Counter()
    {
        // Count > 0の時のみ実行可能なコマンドを作成
        // Executeとして「_ => Count--」を設定
        // CanExecuteとして「_ => Count > 0」を設定
        // (SimpleCommandはICommandを実装している。具体的な実装方法は演習で…)
        DecrementCommand = new SimpleCommand(_ => Count--, _ => Count > 0);
    }
}

<!-- DecrementCommandのCanExecuteがfalseを返すと、ボタンが自動的に無効化される -->
<Button Content="－" Command="{Binding DecrementCommand}"/>

using System.Windows.Input;

namespace WpfApp1
{
    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);
    }
}

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

namespace WpfApp1
{
    public class Counter : INotifyPropertyChanged
    {
        private int _count;

        public int Count
        {
            get => _count;
            set
            {
                if (_count != value)
                {
                    _count = value;
                    OnPropertyChanged();
                }
            }
        }

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

        private readonly SimpleCommand _incrementCommand;
        private readonly SimpleCommand _decrementCommand;

        public Counter()
        {
            _incrementCommand = new SimpleCommand(_ => Count++);
            _decrementCommand = new SimpleCommand(_ => Count--, _ => Count > 0);
        }

        public event PropertyChangedEventHandler? PropertyChanged;

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

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

 public class SimpleCommand : ICommand
    {
       ...
        public event EventHandler? CanExecuteChanged;

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

<Window x:Class="WpfApp1.MainWindow"
        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:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" 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 WpfApp1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new Counter(); //ここだけ追加
        }
    }
}

public Counter()
{
    ...
    IncrementCommand = new RelayCommand(() => Count++);
    DecrementCommand = new RelayCommand(() => Count--, () => Count > 0);
    ...
}

public partial class Counter : ObservableObject
{
    [ObservableProperty]
    private int count;
...