ListBoxAssist.IsToggle - Ensure last clicked item is always selected (#3349)

* Ensures last clicked item is always selected

* Add attached property to allow toggling of selected item

Default behavior is now changed so the selected item can not be deselected. Setting this new AP will allow it, basically falling back to the old default behavior.

Author: nicolaihenriksen

MainDemo.Wpf/Toggles.xaml

@@ -338,11 +338,11 @@
 
       <smtx:XamlDisplay HorizontalAlignment="Left" UniqueKey="buttons_63">
         <!-- the following based on https://material.io/guidelines/components/buttons.html#buttons-toggle-buttons -->
-        <ListBox SelectedIndex="0" Style="{StaticResource MaterialDesignToolVerticalToggleListBox}">
+        <ListBox SelectedIndex="0" Style="{StaticResource MaterialDesignToolVerticalToggleListBox}" materialDesign:ListBoxAssist.CanUserToggleSelectedItem="True">
           <ListBox.ToolTip>
             <StackPanel>
               <TextBlock Text="MaterialDesignToolToggleListBox" />
-              <TextBlock Text="Exclusive selection" />
+              <TextBlock Text="Exclusive selection (allows un-toggling selected item)" />
               <TextBlock Text="ListBoxAssist.IsToggle allows more natural toggle behaviour" />
             </StackPanel>
           </ListBox.ToolTip>

MaterialDesign3.Demo.Wpf/Toggles.xaml

@@ -336,11 +336,11 @@
 
       <smtx:XamlDisplay HorizontalAlignment="Left" UniqueKey="buttons_63">
         <!-- the following based on https://material.io/guidelines/components/buttons.html#buttons-toggle-buttons -->
-        <ListBox SelectedIndex="0" Style="{StaticResource MaterialDesignToolVerticalToggleListBox}">
+        <ListBox SelectedIndex="0" Style="{StaticResource MaterialDesignToolVerticalToggleListBox}" materialDesign:ListBoxAssist.CanUserToggleSelectedItem="True">
           <ListBox.ToolTip>
             <StackPanel>
               <TextBlock Text="MaterialDesignToolToggleListBox" />
-              <TextBlock Text="Exclusive selection" />
+              <TextBlock Text="Exclusive selection (allows un-toggling selected item)" />
               <TextBlock Text="ListBoxAssist.IsToggle allows more natural toggle behaviour" />
             </StackPanel>
           </ListBox.ToolTip>

MaterialDesignThemes.Wpf/ListBoxAssist.cs

@@ -32,7 +32,7 @@ private static void ListBoxMouseButtonEvent(object sender, MouseButtonEventArgs
 
         if (listBoxItem is null || !listBoxItem.IsEnabled) return;
 
-        listBoxItem.SetCurrentValue(ListBoxItem.IsSelectedProperty, !listBoxItem.IsSelected);
+        listBoxItem.SetCurrentValue(ListBoxItem.IsSelectedProperty, !GetCanUserToggleSelectedItem(senderElement) || !listBoxItem.IsSelected);
         mouseButtonEventArgs.Handled = true;
 
         listBoxItem.Focus();
@@ -55,4 +55,13 @@ public static void SetIsToggle(DependencyObject element, bool value)
 
     public static bool GetIsToggle(DependencyObject element)
         => (bool)element.GetValue(IsToggleProperty);
+
+    public static readonly DependencyProperty CanUserToggleSelectedItemProperty = DependencyProperty.RegisterAttached(
+        "CanUserToggleSelectedItem", typeof(bool), typeof(ListBoxAssist), new FrameworkPropertyMetadata(default(bool)));
+
+    public static void SetCanUserToggleSelectedItem(DependencyObject element, bool value)
+        => element.SetValue(CanUserToggleSelectedItemProperty, value);
+
+    public static bool GetCanUserToggleSelectedItem(DependencyObject element)
+        => (bool)element.GetValue(CanUserToggleSelectedItemProperty);
 }