DependencyObjectSelector.cs

using System;
using System.Linq;
using Windows.Foundation.Collections;
using Windows.UI.Xaml.Markup;
using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Data;
using Microsoft.UI.Xaml.Media;

namespace Coder.Desktop.App.Converters;

// This file uses manual DependencyProperty properties rather than
// DependencyPropertyGenerator since it doesn't seem to work properly with
// generics.

/// <summary>
///     An item in a DependencyObjectSelector. Each item has a key and a value.
///     The default item in a DependencyObjectSelector will be the only item
///     with a null key.
/// </summary>
/// <typeparam name="TK">Key type</typeparam>
/// <typeparam name="TV">Value type</typeparam>
public class DependencyObjectSelectorItem<TK, TV> : DependencyObject
    where TK : IEquatable<TK>
{
    public static readonly DependencyProperty KeyProperty =
        DependencyProperty.Register(nameof(Key),
            typeof(TK?),
            typeof(DependencyObjectSelectorItem<TK, TV>),
            new PropertyMetadata(null));

    public static readonly DependencyProperty ValueProperty =
        DependencyProperty.Register(nameof(Value),
            typeof(TV?),
            typeof(DependencyObjectSelectorItem<TK, TV>),
            new PropertyMetadata(null));

    public TK? Key
    {
        get => (TK?)GetValue(KeyProperty);
        set => SetValue(KeyProperty, value);
    }

    public TV? Value
    {
        get => (TV?)GetValue(ValueProperty);
        set => SetValue(ValueProperty, value);
    }
}

/// <summary>
///     Allows selecting between multiple value references based on a selected
///     key. This allows for dynamic mapping of model values to other objects.
///     The main use case is for selecting between other bound values, which
///     you cannot do with a simple ValueConverter.
/// </summary>
/// <typeparam name="TK">Key type</typeparam>
/// <typeparam name="TV">Value type</typeparam>
[ContentProperty(Name = nameof(References))]
public class DependencyObjectSelector<TK, TV> : DependencyObject
    where TK : IEquatable<TK>
{
    public static readonly DependencyProperty ReferencesProperty =
        DependencyProperty.Register(nameof(References),
            typeof(DependencyObjectCollection),
            typeof(DependencyObjectSelector<TK, TV>),
            new PropertyMetadata(null, ReferencesPropertyChanged));

    public static readonly DependencyProperty SelectedKeyProperty =
        DependencyProperty.Register(nameof(SelectedKey),
            typeof(TK?),
            typeof(DependencyObjectSelector<TK, TV>),
            new PropertyMetadata(null, SelectedKeyPropertyChanged));

    public static readonly DependencyProperty SelectedObjectProperty =
        DependencyProperty.Register(nameof(SelectedObject),
            typeof(TV?),
            typeof(DependencyObjectSelector<TK, TV>),
            new PropertyMetadata(null));

    public DependencyObjectCollection? References
    {
        get => (DependencyObjectCollection?)GetValue(ReferencesProperty);
        set
        {
            // Ensure unique keys and that the values are DependencyObjectSelectorItem<K, V>.
            if (value != null)
            {
                var items = value.OfType<DependencyObjectSelectorItem<TK, TV>>().ToArray();
                var keys = items.Select(i => i.Key).Distinct().ToArray();
                if (keys.Length != value.Count)
                    throw new ArgumentException("ObservableCollection Keys must be unique.");
            }

            SetValue(ReferencesProperty, value);
        }
    }

    /// <summary>
    ///     The key of the selected item. This should be bound to a property on
    ///     the model.
    /// </summary>
    public TK? SelectedKey
    {
        get => (TK?)GetValue(SelectedKeyProperty);
        set => SetValue(SelectedKeyProperty, value);
    }

    /// <summary>
    ///     The selected object. This can be read from to get the matching
    ///     object for the selected key. If the selected key doesn't match any
    ///     object, this will be the value of the null key. If there is no null
    ///     key, this will be null.
    /// </summary>
    public TV? SelectedObject
    {
        get => (TV?)GetValue(SelectedObjectProperty);
        set => SetValue(SelectedObjectProperty, value);
    }

    public DependencyObjectSelector()
    {
        References = [];
    }

    private void UpdateSelectedObject()
    {
        if (References != null)
        {
            // Look for a matching item a matching key, or fallback to the null
            // key.
            var references = References.OfType<DependencyObjectSelectorItem<TK, TV>>().ToArray();
            var item = references
                           .FirstOrDefault(i =>
                               (i.Key == null && SelectedKey == null) ||
                               (i.Key != null && SelectedKey != null && i.Key!.Equals(SelectedKey!)))
                       ?? references.FirstOrDefault(i => i.Key == null);
            if (item is not null)
            {
                // Bind the SelectedObject property to the reference's Value.
                // If the underlying Value changes, it will propagate to the
                // SelectedObject.
                BindingOperations.SetBinding
                (
                    this,
                    SelectedObjectProperty,
                    new Binding
                    {
                        Source = item,
                        Path = new PropertyPath(nameof(DependencyObjectSelectorItem<TK, TV>.Value)),
                    }
                );
                return;
            }
        }

        ClearValue(SelectedObjectProperty);
    }

    // Called when the References property is replaced.
    private static void ReferencesPropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
    {
        var self = obj as DependencyObjectSelector<TK, TV>;
        if (self == null) return;
        var oldValue = args.OldValue as DependencyObjectCollection;
        if (oldValue != null)
            oldValue.VectorChanged -= self.OnVectorChangedReferences;
        var newValue = args.NewValue as DependencyObjectCollection;
        if (newValue != null)
            newValue.VectorChanged += self.OnVectorChangedReferences;
    }

    // Called when the References collection changes without being replaced.
    private void OnVectorChangedReferences(IObservableVector<DependencyObject> sender, IVectorChangedEventArgs args)
    {
        UpdateSelectedObject();
    }

    // Called when SelectedKey changes.
    private static void SelectedKeyPropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
    {
        var self = obj as DependencyObjectSelector<TK, TV>;
        self?.UpdateSelectedObject();
    }
}

public sealed class StringToBrushSelectorItem : DependencyObjectSelectorItem<string, Brush>;

public sealed class StringToBrushSelector : DependencyObjectSelector<string, Brush>;