WPF：在不中断 IsEnabled 的情况下实现 ICommandSource

Question

我创建了一个实现 ICommandSource 的 TextBox，在大多数情况下，我使用 Slider 遵循 Microsoft 的示例。此文本框将在按下“Enter”键时执行绑定命令。这部分行为正常工作，我遇到的问题是 IsEnabled 不能再在 XAML 中设置？我不确定这是为什么，您仍然可以在 Button 等原生 Microsoft 类上设置 IsEnabled。

public class CommandTextBox : TextBox, ICommandSource
{
    // The DependencyProperty for the Command.
    public static readonly DependencyProperty CommandProperty = DependencyProperty.Register("Command", typeof(ICommand), typeof(CommandTextBox), new PropertyMetadata(OnCommandChanged));

    // The DependencyProperty for the CommandParameter.
    public static readonly DependencyProperty CommandParameterProperty = DependencyProperty.Register("CommandParameter", typeof(object), typeof(CommandTextBox));

    // The DependencyProperty for the CommandTarget.
    public static readonly DependencyProperty CommandTargetProperty = DependencyProperty.Register("CommandTarget", typeof(IInputElement), typeof(CommandTextBox));

    public ICommand Command
    {
        get { return (ICommand)GetValue(CommandProperty); }
        set { SetValue(CommandProperty, value); }
    }

    public object CommandParameter
    {
        get { return GetValue(CommandParameterProperty); }
        set { SetValue(CommandParameterProperty, value); }
    }

    public bool CommandResetsText { get; set; }

    public IInputElement CommandTarget
    {
        get { return (IInputElement)GetValue(CommandTargetProperty); }
        set { SetValue(CommandTargetProperty, value); }
    }

    // Command dependency property change callback. 
    private static void OnCommandChanged(DependencyObject dp, DependencyPropertyChangedEventArgs e)
    {
        CommandTextBox ctb = (CommandTextBox)dp;
        ctb.HookUpCommand((ICommand)e.OldValue, (ICommand)e.NewValue);
    }

    // Add a new command to the Command Property. 
    private void HookUpCommand(ICommand oldCommand, ICommand newCommand)
    {
        // If oldCommand is not null, then we need to remove the handlers. 
        if (oldCommand != null)
            RemoveCommand(oldCommand);

        AddCommand(newCommand);
    }

    // Remove an old command from the Command Property. 
    private void RemoveCommand(ICommand command)
    {
        EventHandler handler = CanExecuteChanged;
        command.CanExecuteChanged -= handler;
    }

    // Add the command. 
    private void AddCommand(ICommand command)
    {
        var handler = new EventHandler(CanExecuteChanged);
        canExecuteChangedHandler = handler;
        if (command != null)
            command.CanExecuteChanged += canExecuteChangedHandler;
    }

    private void CanExecuteChanged(object sender, EventArgs e)
    {
        if (Command != null)
        {
            RoutedCommand command = Command as RoutedCommand;

            // If a RoutedCommand. 
            if (command != null)
            {
                if (command.CanExecute(CommandParameter, CommandTarget))
                    this.IsEnabled = true;
                else
                    this.IsEnabled = false;
            }
            // If a not RoutedCommand. 
            else
            {
                if (Command.CanExecute(CommandParameter))
                    this.IsEnabled = true;
                else
                    this.IsEnabled = false;
            }
        }
    }

    // If Command is defined, pressing the enter key will invoke the command; 
    // Otherwise, the textbox will behave normally. 
    protected override void OnKeyDown(KeyEventArgs e)
    {
        base.OnKeyDown(e);

        if (e.Key == Key.Enter)
        {
            if (Command != null)
            {
                RoutedCommand command = Command as RoutedCommand;

                if (command != null)
                    command.Execute(CommandParameter, CommandTarget);
                else
                    Command.Execute(CommandParameter);

                if (CommandResetsText)
                    this.Text = String.Empty;
            }
        }
    }

    private EventHandler canExecuteChangedHandler;
}

XAML

<Button Command="{Binding GenericCommand}" IsEnabled="False" /> // This is disabled(desired).
<CommandTextBox Command="{Binding GenericCommand}" IsEnabled="False" /> // This is enabled?

我们将不胜感激。

Answer 1

首先，您需要创建一个私有属性或一个字段，如果该命令可以执行，则将设置该属性：

private bool _canExecute;

其次，需要覆盖IsEnabledCore属性：

protected override bool IsEnabledCore
{
    get
    {
        return ( base.IsEnabledCore && _canExecute );
    }
}

最后，您只需将IsEnabled 替换为_canExecute 并强制IsEnabledCore 即可修复CanExecuteChanged 方法：

private void CanExecuteChanged(object sender, EventArgs e)
{
    if (Command != null)
    {
        RoutedCommand command = Command as RoutedCommand;

        // If a RoutedCommand. 
        if (command != null)
        {
            _canExecute = command.CanExecute(CommandParameter, CommandTarget);
        }
        // If a not RoutedCommand. 
        else
        {
            _canExecute = Command.CanExecute(CommandParameter);
        }
    }

    CoerceValue(UIElement.IsEnabledProperty);
}