From bde380ebfa630f1d8df216025baf49904073eef8 Mon Sep 17 00:00:00 2001
From: Emmanuel Hansen <emmausssss@gmail.com>
Date: Tue, 17 Jan 2023 20:08:22 +0000
Subject: [PATCH] Add snapping sample to scroll viewer page

---
 .../Pages/ScrollViewerPage.xaml               | 139 +++++++++++++++++-
 .../Pages/ScrollViewerPage.xaml.cs            |  37 +++++
 .../Presenters/ScrollContentPresenter.cs      |   4 +-
 3 files changed, 176 insertions(+), 4 deletions(-)

diff --git a/samples/ControlCatalog/Pages/ScrollViewerPage.xaml b/samples/ControlCatalog/Pages/ScrollViewerPage.xaml
index 1903e50ed7..0eceb9e07d 100644
--- a/samples/ControlCatalog/Pages/ScrollViewerPage.xaml
+++ b/samples/ControlCatalog/Pages/ScrollViewerPage.xaml
@@ -3,8 +3,8 @@
              xmlns:pages="using:ControlCatalog.Pages"
              x:Class="ControlCatalog.Pages.ScrollViewerPage"
              x:DataType="pages:ScrollViewerPageViewModel">
-  <StackPanel Orientation="Vertical" Spacing="4">
-    <TextBlock Classes="h2">Allows for horizontal and vertical content scrolling.</TextBlock>
+  <StackPanel Orientation="Vertical" Spacing="20">
+    <TextBlock TextWrapping="Wrap" Classes="h2">Allows for horizontal and vertical content scrolling. Supports snapping on touch and pointer wheel scrolling.</TextBlock>
 
     <Grid ColumnDefinitions="Auto, *">
       <StackPanel Orientation="Vertical" Spacing="4">
@@ -31,6 +31,141 @@
                Source="/Assets/delicate-arch-896885_640.jpg" />
       </ScrollViewer>
     </Grid>
+    
+    <Grid ColumnDefinitions="Auto, *">
+      <StackPanel Orientation="Vertical" Spacing="4">
+        <StackPanel Orientation="Vertical" Spacing="4">
+          <TextBlock Text="Snap Point Type" />
+          <ComboBox Items="{Binding AvailableSnapPointsType}" SelectedItem="{Binding VerticalSnapPointsType}" />
+        </StackPanel>
+
+        <StackPanel Orientation="Vertical" Spacing="4">
+          <TextBlock Text="Snap Point Alignment" />
+          <ComboBox Items="{Binding AvailableSnapPointsAlignment}" SelectedItem="{Binding VerticalSnapPointsAlignment}" />
+        </StackPanel>
+
+        <ToggleSwitch IsChecked="{Binding AreSnapPointsRegular}"
+                      OffContent="No"
+                      OnContent="Yes"
+                      Content="Are Snap Points regular?" />
+      </StackPanel>
+
+      <ScrollViewer x:Name="VerticalSnapsScrollViewer"
+                    Grid.Column="1"
+                    Margin="5"
+                    VerticalSnapPointsType="{Binding VerticalSnapPointsType}"
+                    VerticalSnapPointsAlignment="{Binding VerticalSnapPointsAlignment}"
+                    Width="400" Height="200"
+                    HorizontalScrollBarVisibility="Disabled">
+        <StackPanel AreVerticalSnapPointsRegular="{Binding AreSnapPointsRegular}" Orientation="Vertical" HorizontalAlignment="Stretch">
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 1"/>
+          </Border>
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 2"/>
+          </Border>
+          <Border Padding="5, 10"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 3"/>
+          </Border>
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 4"/>
+          </Border>
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 5"/>
+          </Border>
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 6"/>
+          </Border>
+          <Border Padding="5,8"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 7"/>
+          </Border>
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 8"/>
+          </Border>
+          <Border Padding="5,4"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 9"/>
+          </Border>
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 10"/>
+          </Border>
+          <Border Padding="5,0"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 11"/>
+          </Border>
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 12"/>
+          </Border>
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 13"/>
+          </Border>
+          <Border Padding="5,20"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 14"/>
+          </Border>
+          <Border Padding="5"
+                  BorderBrush="Red"
+                  HorizontalAlignment="Stretch"
+                  BorderThickness="1">
+            <TextBlock FontWeight="Bold"
+                       Text="Child 15"/>
+          </Border>
+        </StackPanel>
+      </ScrollViewer>
+    </Grid>
 
   </StackPanel>
 </UserControl>
diff --git a/samples/ControlCatalog/Pages/ScrollViewerPage.xaml.cs b/samples/ControlCatalog/Pages/ScrollViewerPage.xaml.cs
index dcd7a88a56..10244af5f3 100644
--- a/samples/ControlCatalog/Pages/ScrollViewerPage.xaml.cs
+++ b/samples/ControlCatalog/Pages/ScrollViewerPage.xaml.cs
@@ -11,6 +11,9 @@ namespace ControlCatalog.Pages
         private bool _allowAutoHide;
         private ScrollBarVisibility _horizontalScrollVisibility;
         private ScrollBarVisibility _verticalScrollVisibility;
+        private SnapPointsType _verticalSnapPointsType;
+        private SnapPointsAlignment _verticalSnapPointsAlignment;
+        private bool _areSnapPointsRegular;
 
         public ScrollViewerPageViewModel()
         {
@@ -22,6 +25,20 @@ namespace ControlCatalog.Pages
                 ScrollBarVisibility.Disabled,
             };
 
+            AvailableSnapPointsType = new List<SnapPointsType>()
+            { 
+                SnapPointsType.None,
+                SnapPointsType.Mandatory,
+                SnapPointsType.MandatorySingle
+            };
+
+            AvailableSnapPointsAlignment = new List<SnapPointsAlignment>()
+            {
+                SnapPointsAlignment.Near,
+                SnapPointsAlignment.Center,
+                SnapPointsAlignment.Far,
+            };
+
             HorizontalScrollVisibility = ScrollBarVisibility.Auto;
             VerticalScrollVisibility = ScrollBarVisibility.Auto;
             AllowAutoHide = true;
@@ -33,6 +50,12 @@ namespace ControlCatalog.Pages
             set => this.RaiseAndSetIfChanged(ref _allowAutoHide, value);
         }
 
+        public bool AreSnapPointsRegular
+        {
+            get => _areSnapPointsRegular;
+            set => this.RaiseAndSetIfChanged(ref _areSnapPointsRegular, value);
+        }
+
         public ScrollBarVisibility HorizontalScrollVisibility
         {
             get => _horizontalScrollVisibility;
@@ -45,7 +68,21 @@ namespace ControlCatalog.Pages
             set => this.RaiseAndSetIfChanged(ref _verticalScrollVisibility, value);
         }
 
+        public SnapPointsType VerticalSnapPointsType
+        {
+            get => _verticalSnapPointsType;
+            set => this.RaiseAndSetIfChanged(ref _verticalSnapPointsType, value);
+        }
+
+        public SnapPointsAlignment VerticalSnapPointsAlignment
+        {
+            get => _verticalSnapPointsAlignment;
+            set => this.RaiseAndSetIfChanged(ref _verticalSnapPointsAlignment, value);
+        }
+
         public List<ScrollBarVisibility> AvailableVisibility { get; }
+        public List<SnapPointsType> AvailableSnapPointsType { get; }
+        public List<SnapPointsAlignment> AvailableSnapPointsAlignment { get; }
     }
 
     public class ScrollViewerPage : UserControl
diff --git a/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs b/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs
index 2326699c36..3b291841fe 100644
--- a/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs
@@ -613,7 +613,7 @@ namespace Avalonia.Controls.Presenters
                         {
                             height = _verticalSnapPoint;
                         }
-                        else
+                        else if(_verticalSnapPoints != null)
                         {
                             double yOffset = Offset.Y;
                             switch (VerticalSnapPointsAlignment)
@@ -645,7 +645,7 @@ namespace Avalonia.Controls.Presenters
                         {
                             width = _horizontalSnapPoint;
                         }
-                        else
+                        else if(_horizontalSnapPoints != null)
                         {
                             double xOffset = Offset.X;
                             switch (VerticalSnapPointsAlignment)