Fix issue by for HorizontalScroll

Author: nicolaihenriksen

src/MaterialDesignThemes.Wpf/ScrollViewerAssist.cs

@@ -64,9 +64,6 @@ public static PaddingMode GetPaddingMode(DependencyObject element)
     private static readonly DependencyProperty HorizontalScrollHookProperty = DependencyProperty.RegisterAttached(
         "HorizontalScrollHook", typeof(HwndSourceHook), typeof(ScrollViewerAssist), new PropertyMetadata(null));
 
-    private static readonly DependencyProperty HorizontalScrollSourceProperty = DependencyProperty.RegisterAttached(
-        "HorizontalScrollSource", typeof(HwndSource), typeof(ScrollViewerAssist), new PropertyMetadata(null));
-
     private static readonly DependencyProperty BubbleVerticalScrollHookProperty = DependencyProperty.RegisterAttached(
         "BubbleVerticalScrollHook", typeof(HwndSourceHook), typeof(ScrollViewerAssist), new PropertyMetadata(null));
 
@@ -76,16 +73,16 @@ public static PaddingMode GetPaddingMode(DependencyObject element)
     private static void OnSupportHorizontalScrollChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
     {
         //Based on: https://blog.walterlv.com/post/handle-horizontal-scrolling-of-touchpad-en.html
-        if (d is ScrollViewer scrollViewer)
+        if (d is ScrollViewer sv)
         {
-            if (scrollViewer.IsLoaded)
+            if (sv.IsLoaded)
             {
-                DoOnLoaded(scrollViewer);
+                DoOnLoaded(sv);
             }
             else
             {
-                WeakEventManager<ScrollViewer, RoutedEventArgs>.AddHandler(scrollViewer, nameof(ScrollViewer.Loaded), OnLoaded);
-                WeakEventManager<ScrollViewer, RoutedEventArgs>.AddHandler(scrollViewer, nameof(ScrollViewer.Unloaded), OnUnloaded);
+                RegisterForLoadedEvent(sv);
+                RegisterForUnloadedEvent(sv);
             }
         }
 
@@ -97,10 +94,35 @@ static void OnLoaded(object? sender, RoutedEventArgs e)
             }
         }
 
+        static void UnregisterForLoadedEvent(ScrollViewer sv)
+        {
+            WeakEventManager<ScrollViewer, RoutedEventArgs>.RemoveHandler(sv, nameof(ScrollViewer.Loaded), OnLoaded);
+        }
+
+        static void RegisterForLoadedEvent(ScrollViewer sv)
+        {
+            // Avoid double registrations
+            UnregisterForLoadedEvent(sv);
+            WeakEventManager<ScrollViewer, RoutedEventArgs>.AddHandler(sv, nameof(ScrollViewer.Loaded), OnLoaded);
+        }
+
+        static void UnregisterForUnloadedEvent(ScrollViewer sv)
+        {
+            WeakEventManager<ScrollViewer, RoutedEventArgs>.RemoveHandler(sv, nameof(ScrollViewer.Unloaded), OnUnloaded);
+        }
+
+        static void RegisterForUnloadedEvent(ScrollViewer sv)
+        {
+            // Avoid double registrations
+            UnregisterForUnloadedEvent(sv);
+            WeakEventManager<ScrollViewer, RoutedEventArgs>.AddHandler(sv, nameof(ScrollViewer.Unloaded), OnUnloaded);
+        }
+
         static void DoOnLoaded(ScrollViewer sv)
         {
             if (GetSupportHorizontalScroll(sv))
             {
+                RegisterForUnloadedEvent(sv);
                 RegisterHook(sv);
             }
             else
@@ -117,30 +139,23 @@ static void OnUnloaded(object? sender, RoutedEventArgs e)
             }
         }
 
-        static void RemoveHandlers(ScrollViewer scrollViewer)
-        {
-            WeakEventManager<ScrollViewer, RoutedEventArgs>.RemoveHandler(scrollViewer, nameof(ScrollViewer.Loaded), OnLoaded);
-            WeakEventManager<ScrollViewer, RoutedEventArgs>.RemoveHandler(scrollViewer, nameof(ScrollViewer.Unloaded), OnUnloaded);
-        }
-
-        static void RemoveHook(ScrollViewer scrollViewer)
+        static void RemoveHook(ScrollViewer sv)
         {
-            if (scrollViewer.GetValue(HorizontalScrollHookProperty) is HwndSourceHook hook &&
-                scrollViewer.GetValue(HorizontalScrollSourceProperty) is HwndSource source)
+            var source = PresentationSource.FromVisual(sv) as HwndSource;
+            if (source is not null && sv.GetValue(HorizontalScrollHookProperty) is HwndSourceHook hook)
             {
                 source.RemoveHook(hook);
-                scrollViewer.SetValue(HorizontalScrollHookProperty, null);
+                sv.SetValue(HorizontalScrollHookProperty, null);
             }
         }
 
-        static void RegisterHook(ScrollViewer scrollViewer)
+        static void RegisterHook(ScrollViewer sv)
         {
-            RemoveHook(scrollViewer);
-            if (PresentationSource.FromVisual(scrollViewer) is HwndSource source)
+            RemoveHook(sv);
+            if (PresentationSource.FromVisual(sv) is HwndSource source)
             {
                 HwndSourceHook hook = Hook;
-                scrollViewer.SetValue(HorizontalScrollSourceProperty, source);
-                scrollViewer.SetValue(HorizontalScrollHookProperty, hook);
+                sv.SetValue(HorizontalScrollHookProperty, hook);
                 source.AddHook(hook);
             }
 
@@ -151,15 +166,15 @@ IntPtr Hook(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled
                 const int WM_NCDESTROY = 0x0082;
                 switch (msg)
                 {
-                    case WM_MOUSEHWHEEL when scrollViewer.IsMouseOver:
+                    case WM_MOUSEHWHEEL when sv.IsMouseOver:
                         int tilt = (short)((wParam.ToInt64() >> 16) & 0xFFFF);
-                        scrollViewer.ScrollToHorizontalOffset(scrollViewer.HorizontalOffset + tilt);
+                        sv.ScrollToHorizontalOffset(sv.HorizontalOffset + tilt);
                         return (IntPtr)1;
                     case WM_DESTROY:
                     case WM_NCDESTROY:
-                        RemoveHandlers(scrollViewer);
-                        var source = PresentationSource.FromVisual(scrollViewer) as HwndSource;
-                        source?.RemoveHook(Hook);
+                        UnregisterForLoadedEvent(sv);
+                        UnregisterForUnloadedEvent(sv);
+                        RemoveHook(sv);
                         break;
                 }
                 return IntPtr.Zero;