Merge pull request #13 from danwalmsley/item-nudge-behavior

Behavior to nudge items out the way during drag drop operations.

Author: wieslawsoltes

src/Avalonia.Xaml.Interactions.Custom/ItemsControl/ItemNudgeDropBehavior.cs

@@ -0,0 +1,97 @@
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Interactivity;
+using Avalonia.Layout;
+using Avalonia.Media.Transformation;
+using Avalonia.Xaml.Interactivity;
+
+namespace Avalonia.Xaml.Interactions.Custom;
+
+/// <summary>
+/// 
+/// </summary>
+public class ItemNudgeDropBehavior : StyledElementBehavior<ItemsControl>
+{
+    /// <summary>
+    /// 
+    /// </summary>
+    public static readonly StyledProperty<Orientation> OrientationProperty =
+        AvaloniaProperty.Register<ItemNudgeDropBehavior, Orientation>(nameof(Orientation),
+            defaultValue: Orientation.Vertical);
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public Orientation Orientation
+    {
+        get => GetValue(OrientationProperty);
+        set => SetValue(OrientationProperty, value);
+    }
+
+    /// <inheritdoc />
+    protected override void OnAttachedToVisualTree()
+    {
+        AssociatedObject?.AddHandler(DragDrop.DragLeaveEvent, OnDragLeave);
+        AssociatedObject?.AddHandler(DragDrop.DragOverEvent, OnDragOver);
+        AssociatedObject?.AddHandler(DragDrop.DropEvent, OnDrop);
+    }
+
+    /// <inheritdoc />
+    protected override void OnDetachedFromVisualTree()
+    {
+        AssociatedObject?.RemoveHandler(DragDrop.DragLeaveEvent, OnDragLeave);
+        AssociatedObject?.RemoveHandler(DragDrop.DragOverEvent, OnDragOver);
+        AssociatedObject?.RemoveHandler(DragDrop.DropEvent, OnDrop);
+    }
+    
+    private void ApplyTranslation(Control control, double x, double y)
+    {
+        var transformBuilder = new TransformOperations.Builder(1);
+        transformBuilder.AppendTranslate(x, y);
+        control.RenderTransform = transformBuilder.Build();
+    }
+    
+    private void RemoveTranslations(object? sender)
+    {
+        if (sender is ItemsControl itemsControl)
+        {
+            foreach (var container in itemsControl.GetRealizedContainers())
+            {
+                ApplyTranslation(container, 0, 0);
+            }
+        }
+    }
+    
+    private void OnDrop(object? sender, DragEventArgs e)
+    {
+        RemoveTranslations(sender);
+    }
+
+    private void OnDragLeave(object? sender, RoutedEventArgs e)
+    {
+        RemoveTranslations(sender);
+    }
+
+    private void OnDragOver(object? sender, DragEventArgs e)
+    {
+        if (sender is not ItemsControl itemsControl) return;
+
+        var isHorizontal = Orientation == Orientation.Horizontal;
+
+        var dragPosition = e.GetPosition(itemsControl);
+
+        for (int index = 0; index < itemsControl.ItemCount; index++)
+        {
+            var container = itemsControl.ContainerFromIndex(index);
+
+            if (container == null) continue;
+            
+            var containerMidPoint = isHorizontal ? container.Bounds.Center.X : container.Bounds.Center.Y;
+            
+            var translationX = isHorizontal && dragPosition.X <= containerMidPoint ? container.Bounds.Width : 0;
+            var translationY = !isHorizontal && dragPosition.Y <= containerMidPoint ? container.Bounds.Height : 0;
+
+            ApplyTranslation(container, translationX, translationY);
+        }
+    }
+}