Refactor PasswordBox reveal style TextBox focus- and text selection code

The old focus code did not work that well because keyboard focus was not really possible to set via the Style. With the introduction of behaviors, we can now improve that a lot.

MaterialDesignThemes.Wpf/Behaviors/PasswordBoxRevealTextBoxBehavior.cs

@@ -0,0 +1,92 @@
+using System.Reflection;
+using System.Windows.Documents;
+using Microsoft.Xaml.Behaviors;
+
+namespace MaterialDesignThemes.Wpf.Behaviors;
+
+internal class PasswordBoxRevealTextBoxBehavior : Behavior<TextBox>
+{
+    private static readonly DependencyProperty SelectionProperty = DependencyProperty.RegisterAttached(
+        "Selection", typeof(TextSelection), typeof(PasswordBoxRevealTextBoxBehavior), new UIPropertyMetadata(default(TextSelection)));
+    private static void SetSelection(DependencyObject obj, TextSelection? value) => obj.SetValue(SelectionProperty, value);
+    private static TextSelection? GetSelection(DependencyObject obj) => (TextSelection?)obj.GetValue(SelectionProperty);
+
+    internal static readonly DependencyProperty PasswordBoxProperty = DependencyProperty.Register(
+        nameof(PasswordBox), typeof(PasswordBox), typeof(PasswordBoxRevealTextBoxBehavior), new PropertyMetadata(default(PasswordBox)));
+
+    internal PasswordBox? PasswordBox
+    {
+        get => (PasswordBox) GetValue(PasswordBoxProperty);
+        set => SetValue(PasswordBoxProperty, value);
+    }
+
+    protected override void OnAttached()
+    {
+        base.OnAttached();
+        AssociatedObject.IsVisibleChanged += AssociatedObjectOnIsVisibleChanged;
+        if (PasswordBox != null)
+        {
+            var info = typeof(PasswordBox).GetProperty("Selection", BindingFlags.NonPublic | BindingFlags.Instance);
+            var selection = info?.GetValue(PasswordBox, null) as TextSelection;
+            SetSelection(AssociatedObject, selection);
+        }
+    }
+
+    protected override void OnDetaching()
+    {
+        base.OnDetaching();
+        if (AssociatedObject != null)
+        {
+            AssociatedObject.ClearValue(SelectionProperty);
+            AssociatedObject.IsVisibleChanged -= AssociatedObjectOnIsVisibleChanged;
+        }
+    }
+
+    private void AssociatedObjectOnIsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
+    {
+        if (AssociatedObject.IsVisible)
+        {
+            AssociatedObject.SelectionLength = 0;
+            var selection = GetPasswordBoxSelection();
+            AssociatedObject.SelectionStart = selection.SelectionStart;
+            AssociatedObject.SelectionLength = selection.SelectionEnd;
+            Keyboard.Focus(AssociatedObject);
+        }
+        else if (PasswordBox != null)
+        {
+            SetPasswordBoxSelection(AssociatedObject.SelectionStart, AssociatedObject.SelectionLength);
+            Keyboard.Focus(PasswordBox);
+        }
+    }
+
+    private PasswordBoxSelection GetPasswordBoxSelection()
+    {
+        var selection = GetSelection(AssociatedObject);
+        var typeTextRange = selection?.GetType().GetInterfaces().FirstOrDefault(i => i.Name == "ITextRange");
+        object? start = typeTextRange?.GetProperty("Start")?.GetGetMethod()?.Invoke(selection, null);
+        object? end = typeTextRange?.GetProperty("End")?.GetGetMethod()?.Invoke(selection, null);
+        int? startValue = start?.GetType().GetProperty("Offset", BindingFlags.NonPublic | BindingFlags.Instance)?.GetValue(start, null) as int?;
+        int? endValue = end?.GetType().GetProperty("Offset", BindingFlags.NonPublic | BindingFlags.Instance)?.GetValue(end, null) as int?;
+        int selectionStart = startValue.GetValueOrDefault(0);
+        int selectionLength = 0;
+        if (endValue.HasValue)
+        {
+            selectionLength = endValue.Value - startValue.GetValueOrDefault(0);
+        }
+        return new PasswordBoxSelection(selectionStart, selectionLength);
+    }
+
+    private void SetPasswordBoxSelection(int selectionStart, int selectionLength) => typeof(PasswordBox).GetMethod("Select", BindingFlags.Instance | BindingFlags.NonPublic)?.Invoke(PasswordBox, new object[] { selectionStart, selectionLength });
+
+    private struct PasswordBoxSelection
+    {
+        public readonly int SelectionStart;
+        public readonly int SelectionEnd;
+
+        public PasswordBoxSelection(int selectionStart, int selectionEnd)
+        {
+            SelectionStart = selectionStart;
+            SelectionEnd = selectionEnd;
+        }
+    }
+}

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.PasswordBox.xaml

@@ -1,4 +1,4 @@
-<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                     xmlns:converters="clr-namespace:MaterialDesignThemes.Wpf.Converters"
                     xmlns:internal="clr-namespace:MaterialDesignThemes.Wpf.Internal"
@@ -640,7 +640,11 @@
                                Style="{StaticResource MaterialDesignRawTextBox}"
                                Text="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:PasswordBoxAssist.Password), UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
                                UseLayoutRounding="{TemplateBinding UseLayoutRounding}"
-                               Visibility="{Binding ElementName=ContentGrid, Path=(wpf:PasswordBoxAssist.IsPasswordRevealed), Converter={StaticResource BooleanToVisibilityConverter}}" />
+                               Visibility="{Binding ElementName=ContentGrid, Path=(wpf:PasswordBoxAssist.IsPasswordRevealed), Converter={StaticResource BooleanToVisibilityConverter}}">
+                        <behaviors:Interaction.Behaviors>
+                          <internalbehaviors:PasswordBoxRevealTextBoxBehavior PasswordBox="{Binding RelativeSource={RelativeSource TemplatedParent}}" />
+                        </behaviors:Interaction.Behaviors>
+                      </TextBox>
                     </Grid>
 
                     <wpf:SmartHint x:Name="Hint"
@@ -723,15 +727,6 @@
             </Canvas>
           </Grid>
           <ControlTemplate.Triggers>
-            <!-- Transfer focus to the RevealPasswordTextBox when the password is revealed and the PasswordBox gains focus -->
-            <MultiTrigger>
-              <MultiTrigger.Conditions>
-                <Condition Property="IsFocused" Value="True" />
-                <Condition Property="wpf:PasswordBoxAssist.IsPasswordRevealed" Value="True" />
-              </MultiTrigger.Conditions>
-              <Setter TargetName="ContentGrid" Property="FocusManager.FocusedElement" Value="{Binding ElementName=RevealPasswordTextBox}" />
-            </MultiTrigger>
-
             <MultiTrigger>
               <MultiTrigger.Conditions>
                 <Condition Property="wpf:HintAssist.IsFloating" Value="True" />