add horizontal alignment support

Author: vgromfeld

Microsoft.Toolkit.Uwp.SampleApp/SamplePages/WrapPanel/WrapPanel.bind

@@ -47,13 +47,15 @@
                               Padding="@[Padding:Thickness:0,0,0,0]@"
                               VerticalSpacing="@[VerticalSpacing:Slider:5:0-200]@"
                               HorizontalSpacing="@[HorizontalSpacing:Slider:5:0-200]@"
-                              VerticalContentAlignment="@[VerticalContentAlignment:Enum:VerticalAlignment.Top]"/>
+                              VerticalContentAlignment="@[VerticalContentAlignment:Enum:VerticalAlignment.Top]@"
+                              HorizontalContentAlignment="@[HorizontalContentAlignment:Enum:HorizontalAlignment.Left]@" />
         </ItemsPanelTemplate>
       </ItemsControl.ItemsPanel>
       <ListView.ItemContainerStyle>
         <Style TargetType="ListViewItem">
           <Setter Property="VerticalContentAlignment" Value="Stretch" />
           <Setter Property="HorizontalContentAlignment" Value="Stretch" />
+          <Setter Property="Padding" Value="0" />
         </Style>
       </ListView.ItemContainerStyle>
     </ListView>

Microsoft.Toolkit.Uwp.UI.Controls/WrapPanel/WrapPanel.Data.cs

@@ -85,6 +85,38 @@ public UvRect WithVerticalAlignment(VerticalAlignment alignment, double maxHeigh
                 }
             }
 
+            public UvRect WithHorizontalAlignment(HorizontalAlignment alignment, double maxHeight)
+            {
+                switch (alignment)
+                {
+                    case HorizontalAlignment.Center:
+                        return new UvRect
+                        {
+                            Position = Position.Add(
+                                u: 0,
+                                v: Math.Max((maxHeight - Size.V) / 2.0, 0.0)),
+                            Size = Size,
+                        };
+                    case HorizontalAlignment.Right:
+                        return new UvRect
+                        {
+                            Position = Position.Add(
+                                u: 0,
+                                v: Math.Max(maxHeight - Size.V, 0.0)),
+                            Size = Size,
+                        };
+                    case HorizontalAlignment.Stretch:
+                        return new UvRect
+                        {
+                            Position = Position,
+                            Size = new UvMeasure { U = Size.U, V = maxHeight },
+                        };
+                    case HorizontalAlignment.Left:
+                    default:
+                        return this;
+                }
+            }
+
             public Rect ToRect(Orientation orientation)
             {
                 switch (orientation)

Microsoft.Toolkit.Uwp.UI.Controls/WrapPanel/WrapPanel.cs

@@ -73,6 +73,24 @@ public VerticalAlignment VerticalContentAlignment
             typeof(WrapPanel),
             new PropertyMetadata(VerticalAlignment.Top, LayoutPropertyChanged));
 
+        /// <summary>
+        /// Gets or sets the horizontal alignment to use when the control <see cref="Orientation"/> is set to <see cref="Orientation.Vertical"/>.
+        /// </summary>
+        public HorizontalAlignment HorizontalContentAlignment
+        {
+            get => (HorizontalAlignment)GetValue(HorizontalContentAlignmentProperty);
+            set => SetValue(HorizontalContentAlignmentProperty, value);
+        }
+
+        /// <summary>
+        /// The DP to store the <see cref="HorizontalContentAlignment"/> property value.
+        /// </summary>
+        public static readonly DependencyProperty HorizontalContentAlignmentProperty = DependencyProperty.Register(
+            nameof(HorizontalContentAlignment),
+            typeof(HorizontalAlignment),
+            typeof(WrapPanel),
+            new PropertyMetadata(HorizontalAlignment.Left, LayoutPropertyChanged));
+
         /// <summary>
         /// Gets or sets the orientation of the WrapPanel.
         /// Horizontal means that child controls will be added horizontally until the width of the panel is reached, then a new row is added to add new child controls.
@@ -287,10 +305,11 @@ void Arrange(UIElement child, bool isLast = false)
                         }
                         else
                         {
-                            arrangeRect = rect;
+                            arrangeRect = rect.WithHorizontalAlignment(HorizontalContentAlignment, row.Size.V);
                         }
 
-                        child.Arrange(arrangeRect.ToRect(Orientation));
+                        var finalRect = arrangeRect.ToRect(Orientation);
+                        child.Arrange(finalRect);
                     }
                 }
             }