DiffView.axaml.cs

using System;

using Avalonia;
using Avalonia.Controls;
using Avalonia.Media;
using Avalonia.Media.Imaging;
using Avalonia.Styling;

namespace SourceGit.Views
{
    public class ImageDiffView : Control
    {
        public static readonly StyledProperty<double> AlphaProperty =
            AvaloniaProperty.Register<ImageDiffView, double>(nameof(Alpha), 0.5);

        public double Alpha
        {
            get => GetValue(AlphaProperty);
            set => SetValue(AlphaProperty, value);
        }

        public static readonly StyledProperty<Bitmap> OldImageProperty =
            AvaloniaProperty.Register<ImageDiffView, Bitmap>(nameof(OldImage), null);

        public Bitmap OldImage
        {
            get => GetValue(OldImageProperty);
            set => SetValue(OldImageProperty, value);
        }

        public static readonly StyledProperty<Bitmap> NewImageProperty =
            AvaloniaProperty.Register<ImageDiffView, Bitmap>(nameof(NewImage), null);

        public Bitmap NewImage
        {
            get => GetValue(NewImageProperty);
            set => SetValue(NewImageProperty, value);
        }

        static ImageDiffView()
        {
            AffectsMeasure<ImageDiffView>(OldImageProperty, NewImageProperty);
            AffectsRender<ImageDiffView>(AlphaProperty);
        }

        public override void Render(DrawingContext context)
        {
            if (_bgBrush == null)
            {
                var maskBrush = new SolidColorBrush(ActualThemeVariant == ThemeVariant.Dark ? 0xFF404040 : 0xFFBBBBBB);
                var bg = new DrawingGroup()
                {
                    Children =
                    {
                        new GeometryDrawing() { Brush = maskBrush, Geometry = new RectangleGeometry(new Rect(0, 0, 12, 12)) },
                        new GeometryDrawing() { Brush = maskBrush, Geometry = new RectangleGeometry(new Rect(12, 12, 12, 12)) },
                    }
                };

                _bgBrush = new DrawingBrush(bg)
                {
                    AlignmentX = AlignmentX.Left,
                    AlignmentY = AlignmentY.Top,
                    DestinationRect = new RelativeRect(new Size(24, 24), RelativeUnit.Absolute),
                    Stretch = Stretch.None,
                    TileMode = TileMode.Tile,
                };
            }

            context.FillRectangle(_bgBrush, new Rect(Bounds.Size));

            var alpha = Alpha;
            var w = Bounds.Width - 16;
            var h = Bounds.Height - 16;
            var x = w * alpha;
            var left = OldImage;
            if (left != null && alpha > 0)
            {
                var src = new Rect(0, 0, left.Size.Width * alpha, left.Size.Height);
                var dst = new Rect(8, 8, x, h);
                context.DrawImage(left, src, dst);
            }

            var right = NewImage;
            if (right != null && alpha < 1)
            {
                var src = new Rect(right.Size.Width * alpha, 0, right.Size.Width * (1 - alpha), right.Size.Height);
                var dst = new Rect(x + 8, 8, w - x, h);
                context.DrawImage(right, src, dst);
            }

            context.DrawLine(new Pen(Brushes.DarkGreen, 2), new Point(x + 8, 0), new Point(x + 8, Bounds.Height));
        }

        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
        {
            base.OnPropertyChanged(change);

            if (change.Property.Name == "ActualThemeVariant")
            {
                _bgBrush = null;
                InvalidateVisual();
            }
        }

        protected override Size MeasureOverride(Size availableSize)
        {
            var left = OldImage;
            var right = NewImage;

            if (left != null)
            {
                var lSize = GetDesiredSize(left.Size, availableSize);
                if (right != null)
                {
                    var rSize = GetDesiredSize(right.Size, availableSize);
                    if (rSize.Width > lSize.Width)
                        return rSize;
                    return lSize;
                }
                else
                {
                    return lSize;
                }
            }
            else if (right != null)
            {
                return GetDesiredSize(right.Size, availableSize);
            }
            else
            {
                return availableSize;
            }
        }

        private Size GetDesiredSize(Size img, Size available)
        {
            var w = available.Width - 16;
            var h = available.Height - 16;

            if (img.Width <= w)
            {
                if (img.Height <= h)
                {
                    return new Size(img.Width + 16, img.Height + 16);
                }
                else
                {
                    return new Size(h * img.Width / img.Height + 16, available.Height);
                }
            }
            else
            {
                var s = Math.Max(img.Width / w, img.Height / h);
                return new Size(img.Width / s + 16, img.Height / s + 16);
            }
        }

        private DrawingBrush _bgBrush = null;
    }

    public partial class DiffView : UserControl
    {
        public DiffView()
        {
            InitializeComponent();
        }
    }
}