Implement PlacementMode.Center

samples/ControlCatalog/Pages/FlyoutsPage.axaml

@@ -136,6 +136,15 @@
                                 </Flyout>
                             </Button.Flyout>
                         </Button>
+                        <Button Content="Placement=Center">
+                            <Button.Flyout>
+                                <Flyout Placement="Center">
+                                    <Panel Width="100" Height="100">
+                                        <TextBlock Text="Flyout Content!" />
+                                    </Panel>
+                                </Flyout>
+                            </Button.Flyout>
+                        </Button>
                         <Button Content="Placement=TopEdgeAlignedLeft">
                             <Button.Flyout>
                                 <Flyout Placement="TopEdgeAlignedLeft">

src/Avalonia.Controls/PlacementMode.cs

@@ -31,6 +31,11 @@ namespace Avalonia.Controls
         /// Preferred location is above the target element.
         /// </summary>
         Top,
+        
+        /// <summary>
+        /// Preferred location is centered over the target element.
+        /// </summary>
+        Center,
 
         /// <summary>
         /// The popup is placed according to <see cref="Popup.PlacementAnchor"/> and <see cref="Popup.PlacementGravity"/> rules.

src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs

@@ -464,21 +464,25 @@ namespace Avalonia.Controls.Primitives.PopupPositioning
                 positionerParameters.Anchor = PopupAnchor.TopLeft;
                 positionerParameters.Gravity = PopupGravity.BottomRight;
             }
-            else
+            else if (placement == PlacementMode.Center)
             {
-                if (target == null)
-                    throw new InvalidOperationException("Placement mode is not Pointer and PlacementTarget is null");
-                var matrix = target.TransformToVisual(topLevel);
-                if (matrix == null)
+                // Start with top left position, and add offset to move it to the center
+                var anchorRectangle = GetAnchorRectangle(topLevel, target, rect);
+                positionerParameters.AnchorRectangle = anchorRectangle; 
+                positionerParameters.Anchor = PopupAnchor.TopLeft;
+                positionerParameters.Gravity = PopupGravity.BottomRight;
+
+                var targetSize = positionerParameters.Size;
+                if (targetSize != default)
                 {
-                    if (target.GetVisualRoot() == null)
-                        throw new InvalidOperationException("Target control is not attached to the visual tree");
-                    throw new InvalidOperationException("Target control is not in the same tree as the popup parent");
+                    positionerParameters.Offset += new Point(
+                        (anchorRectangle.Width - targetSize.Width) / 2,
+                        (anchorRectangle.Height - targetSize.Height) / 2);
                 }
-
-                var bounds = new Rect(default, target.Bounds.Size);
-                var anchorRect = rect ?? bounds;
-                positionerParameters.AnchorRectangle = anchorRect.Intersect(bounds).TransformToAABB(matrix.Value);
+            }
+            else
+            {
+                positionerParameters.AnchorRectangle = GetAnchorRectangle(topLevel, target, rect);
 
                 var parameters = placement switch
                 {
@@ -528,6 +532,23 @@ namespace Avalonia.Controls.Primitives.PopupPositioning
                 }
             }
         }
+
+        private static Rect GetAnchorRectangle(TopLevel topLevel, Visual target, Rect? rect)
+        {
+            if (target == null)
+                throw new InvalidOperationException("Placement mode is not Pointer and PlacementTarget is null");
+            var matrix = target.TransformToVisual(topLevel);
+            if (matrix == null)
+            {
+                if (target.GetVisualRoot() == null)
+                    throw new InvalidOperationException("Target control is not attached to the visual tree");
+                throw new InvalidOperationException("Target control is not in the same tree as the popup parent");
+            }
+
+            var bounds = new Rect(default, target.Bounds.Size);
+            var anchorRect = rect ?? bounds;
+            return anchorRect.Intersect(bounds).TransformToAABB(matrix.Value);
+        }
     }
 
 }