Update one Dependency Property based on another

Question

I have a custom control I have set up in c#/wpf. I am using Dependency properties for items, some of these are optional, but my current code seems very convoluted:

public class PopupMenu : ContentControl
{
    public static readonly DependencyProperty AddWordCommandProperty =
        DependencyProperty.Register(nameof(AddWordCommand), typeof(ICommand), typeof(PopupMenu),
            new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, OnAddWordCommandChanged));
            
    public static readonly DependencyProperty ShowAddWordButtonProperty =
        DependencyProperty.Register(nameof(ShowAddWordButton), typeof(bool), typeof(PopupMenu),
            new FrameworkPropertyMetadata(false, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, OnShowAddWordButtonChanged));
            
    public ICommand AddWordCommand
    {
        get => (ICommand)GetValue(AddWordCommandProperty);
        set => SetValue(AddWordCommandProperty, value);
    }
    
    public bool ShowAddWordButton
    {
        get => (bool)GetValue(ShowAddWordButtonProperty);
        set => SetValue(ShowAddWordButtonProperty, value);
    }
    
    private static void OnAddWordCommandChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not PopupMenu ctrl)
            return;

        ctrl.AddWordCommand = (ICommand)e.NewValue;
    }

    private static void OnShowAddWordButtonChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not PopupMenu ctrl)
            return;

        ctrl.ShowAddWordButton = (bool)e.NewValue;
    }
}

When I use this control I must pass a command and also a bool for this button.

I have tried (and failed) ways of not having to pass the bool and instead letting 'ShowAddWordButton' be self-calculating, but is there a correct way to deal with this?

Attempt 1 - failed:

    public ICommand AddWordCommand
    {
        get => (ICommand)GetValue(AddWordCommandProperty);
        set
        {
            SetValue(AddWordCommandProperty, value);

            SetValue(ShowAddWordButtonProperty, value != null);
        }
    }

Attempt 2 - Failed:

    private static void OnAddWordCommandChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is not PopupMenu ctrl)
            return;
            
        var cmd = (ICommand)e.NewValue;

        ctrl.AddWordCommand = cmd;
        
        ctrl.ShowAddWordButton = cmd != null;
    }

I have this code in place as this menu is going to grow with more buttons and they will not always be shown.

TIA

Answer 1

You haven't clearly stated what you want to get. I hope my guess is correct and this code will help you.

    public class PopupMenu : ContentControl
    {
        public static readonly DependencyProperty AddWordCommandProperty =
            DependencyProperty.Register(nameof(AddWordCommand), typeof(ICommand), typeof(PopupMenu),
                new FrameworkPropertyMetadata(null,
                    (d,e)=> d.SetValue(ShowAddWordButtonPropertyKey, e.NewValue is not null)));

        private static readonly DependencyPropertyKey ShowAddWordButtonPropertyKey =
            DependencyProperty.RegisterReadOnly(nameof(ShowAddWordButton), typeof(bool), typeof(PopupMenu),
                new FrameworkPropertyMetadata(false));

        public static readonly DependencyProperty ShowAddWordButtonProperty = ShowAddWordButtonPropertyKey.DependencyProperty;

        public ICommand AddWordCommand
        {
            get => (ICommand)GetValue(AddWordCommandProperty);
            set => SetValue(AddWordCommandProperty, value);
        }

        public bool ShowAddWordButton => (bool)GetValue(ShowAddWordButtonProperty);
    }

Implementation option using style triggers:

<Style TargetType="MenuItem">
    <Style.Triggers>
        <Trigger Property="Command" Value="{x:Null}">
           <Setter Property="Visibility" Value="Collapsed"/>
        </Trigger>
    </Style.Triggers>
</Style>