Gradient editor - Initial implementation

src/Avalonia/Artemis.UI.Shared/Controls/GradientPicker/GradientPicker.cs

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+using System;
+using System.Collections.Specialized;
+using System.Linq;
+using Artemis.Core;
+using Avalonia;
+using Avalonia.Controls.Primitives;
+using Avalonia.Data;
+using Avalonia.Input;
+using Avalonia.Media;
+
+namespace Artemis.UI.Shared.Controls.GradientPicker;
+
+public class GradientPicker : TemplatedControl
+{
+    private LinearGradientBrush _linearGradientBrush = new();
+
+    /// <summary>
+    ///     Gets or sets the color gradient.
+    /// </summary>
+    public static readonly StyledProperty<ColorGradient> ColorGradientProperty =
+        AvaloniaProperty.Register<GradientPicker, ColorGradient>(nameof(Core.ColorGradient), notifying: ColorGradientChanged, defaultValue: ColorGradient.GetUnicornBarf());
+
+    /// <summary>
+    ///     Gets or sets the currently selected color stop.
+    /// </summary>
+    public static readonly StyledProperty<ColorGradientStop?> SelectedColorStopProperty =
+        AvaloniaProperty.Register<GradientPicker, ColorGradientStop?>(nameof(SelectedColorStop), defaultBindingMode: BindingMode.TwoWay);
+
+    /// <summary>
+    /// Gets the linear gradient brush representing the color gradient.
+    /// </summary>
+    public static readonly DirectProperty<GradientPicker, LinearGradientBrush> LinearGradientBrushProperty =
+        AvaloniaProperty.RegisterDirect<GradientPicker, LinearGradientBrush>(nameof(LinearGradientBrush), g => g.LinearGradientBrush);
+
+    /// <summary>
+    ///     Gets or sets the color gradient.
+    /// </summary>
+    public ColorGradient ColorGradient
+    {
+        get => GetValue(ColorGradientProperty);
+        set => SetValue(ColorGradientProperty, value);
+    }
+
+    /// <summary>
+    ///     Gets or sets the currently selected color stop.
+    /// </summary>
+    public ColorGradientStop? SelectedColorStop
+    {
+        get => GetValue(SelectedColorStopProperty);
+        set => SetValue(SelectedColorStopProperty, value);
+    }
+
+    /// <summary>
+    /// Gets the linear gradient brush representing the color gradient.
+    /// </summary>
+    public LinearGradientBrush LinearGradientBrush
+    {
+        get => _linearGradientBrush;
+        private set => SetAndRaise(LinearGradientBrushProperty, ref _linearGradientBrush, value);
+    }
+
+    private static void ColorGradientChanged(IAvaloniaObject sender, bool before)
+    {
+        if (before)
+            (sender as GradientPicker)?.Unsubscribe();
+        else
+            (sender as GradientPicker)?.Subscribe();
+    }
+
+    private void Subscribe()
+    {
+        ColorGradient.CollectionChanged += ColorGradientOnCollectionChanged;
+        ColorGradient.StopChanged += ColorGradientOnStopChanged;
+
+        UpdateGradient();
+        SelectedColorStop = ColorGradient.FirstOrDefault();
+    }
+
+    private void Unsubscribe()
+    {
+        ColorGradient.CollectionChanged -= ColorGradientOnCollectionChanged;
+        ColorGradient.StopChanged -= ColorGradientOnStopChanged;
+    }
+
+    private void ColorGradientOnCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
+    {
+        UpdateGradient();
+    }
+
+    private void ColorGradientOnStopChanged(object? sender, EventArgs e)
+    {
+        UpdateGradient();
+    }
+
+    private void UpdateGradient()
+    {
+        // Remove old stops
+
+        // ReSharper disable once ConditionIsAlwaysTrueOrFalse
+        if (ColorGradient == null)
+            return;
+
+        // Add new stops
+
+        // Update the display gradient
+        GradientStops collection = new();
+        foreach (ColorGradientStop c in ColorGradient.OrderBy(s => s.Position))
+            collection.Add(new GradientStop(Color.FromArgb(c.Color.Alpha, c.Color.Red, c.Color.Green, c.Color.Blue), c.Position));
+        LinearGradientBrush = new LinearGradientBrush {GradientStops = collection};
+    }
+
+    private void SelectColorStop(object? sender, PointerReleasedEventArgs e)
+    {
+        if (sender is IDataContextProvider dataContextProvider && dataContextProvider.DataContext is ColorGradientStop colorStop)
+            SelectedColorStop = colorStop;
+    }
+
+    #region Overrides of Visual
+
+    /// <inheritdoc />
+    protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        Subscribe();
+        base.OnAttachedToVisualTree(e);
+    }
+
+    /// <inheritdoc />
+    protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
+    {
+        Unsubscribe();
+        base.OnDetachedFromVisualTree(e);
+    }
+
+    #endregion
+}