From 733238a8afe68ea128874eec0f86b1510c9cdc5d Mon Sep 17 00:00:00 2001
From: Max Katz <maxkatz6@outlook.com>
Date: Sun, 26 Feb 2023 11:48:42 -0500
Subject: [PATCH] Implement PlacementMode.Center

---
 .../ControlCatalog/Pages/FlyoutsPage.axaml    |  9 ++++
 src/Avalonia.Controls/PlacementMode.cs        |  5 +++
 .../PopupPositioning/IPopupPositioner.cs      | 45 ++++++++++++++-----
 3 files changed, 47 insertions(+), 12 deletions(-)

diff --git a/samples/ControlCatalog/Pages/FlyoutsPage.axaml b/samples/ControlCatalog/Pages/FlyoutsPage.axaml
index 54aa9d1b67..8dbc6d283f 100644
--- a/samples/ControlCatalog/Pages/FlyoutsPage.axaml
+++ b/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">
diff --git a/src/Avalonia.Controls/PlacementMode.cs b/src/Avalonia.Controls/PlacementMode.cs
index fa4f029cf3..20e4e5470d 100644
--- a/src/Avalonia.Controls/PlacementMode.cs
+++ b/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.
diff --git a/src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs b/src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs
index 0bb596d778..3f1b8008b1 100644
--- a/src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs
+++ b/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);
+        }
     }
 
 }