Show selected item when combobox uses classic mode

-Rename Default to Classic
-Make ComboBoxPopup as internal, because it isn't a part of public api
-Show selected item when a combobox uses classic mode
-Add PopupPlacement property in comboboxpopup

Author: l1pton17

MaterialDesignThemes.Wpf/ComboBoxPopup.cs

@@ -10,8 +10,18 @@
 
 namespace MaterialDesignThemes.Wpf
 {
-    public class ComboBoxPopup : Popup
+    internal enum ComboBoxPopupPlacement
     {
+        Undefined,
+        Down,
+        Up, 
+        Classic
+    }
+
+    internal class ComboBoxPopup : Popup
+    {
+        #region UpContentTemplate property
+
         public static readonly DependencyProperty UpContentTemplateProperty
             = DependencyProperty.Register(nameof(UpContentTemplate),
                 typeof(ControlTemplate),
@@ -24,6 +34,10 @@ public ControlTemplate UpContentTemplate
             set { SetValue(UpContentTemplateProperty, value); }
         }
 
+        #endregion
+
+        #region DownContentTemplate region
+
         public static readonly DependencyProperty DownContentTemplateProperty
             = DependencyProperty.Register(nameof(DownContentTemplate),
                 typeof(ControlTemplate),
@@ -36,18 +50,26 @@ public ControlTemplate DownContentTemplate
             set { SetValue(DownContentTemplateProperty, value); }
         }
 
-        public static readonly DependencyProperty DefaultContentTemplateProperty
-            = DependencyProperty.Register(nameof(DefaultContentTemplate),
+        #endregion
+
+        #region ClassicContentTemplate property
+
+        public static readonly DependencyProperty ClassicContentTemplateProperty
+            = DependencyProperty.Register(nameof(ClassicContentTemplate),
                 typeof(ControlTemplate),
                 typeof(ComboBoxPopup),
                 new UIPropertyMetadata(null));
 
-        public ControlTemplate DefaultContentTemplate
+        public ControlTemplate ClassicContentTemplate
         {
-            get { return (ControlTemplate)GetValue(DefaultContentTemplateProperty); }
-            set { SetValue(DefaultContentTemplateProperty, value); }
+            get { return (ControlTemplate)GetValue(ClassicContentTemplateProperty); }
+            set { SetValue(ClassicContentTemplateProperty, value); }
         }
 
+        #endregion
+
+        #region UpVerticalOffset property
+
         public static readonly DependencyProperty UpVerticalOffsetProperty
             = DependencyProperty.Register(nameof(UpVerticalOffset),
                 typeof(double),
@@ -60,6 +82,10 @@ public double UpVerticalOffset
             set { SetValue(UpVerticalOffsetProperty, value); }
         }
 
+        #endregion
+
+        #region DownVerticalOffset property
+
         public static readonly DependencyProperty DownVerticalOffsetProperty
             = DependencyProperty.Register(nameof(DownVerticalOffset),
                 typeof(double),
@@ -72,6 +98,24 @@ public double DownVerticalOffset
             set { SetValue(DownVerticalOffsetProperty, value); }
         }
 
+        #endregion
+
+        #region PopupPlacement property
+
+        public static readonly DependencyProperty PopupPlacementProperty
+            = DependencyProperty.Register(nameof(PopupPlacement),
+                typeof(ComboBoxPopupPlacement),
+                typeof(ComboBoxPopup),
+                new PropertyMetadata(ComboBoxPopupPlacement.Undefined, PopupPlacementPropertyChangedCallback));
+
+        public ComboBoxPopupPlacement PopupPlacement
+        {
+            get { return (ComboBoxPopupPlacement)GetValue(PopupPlacementProperty); }
+            set { SetValue(PopupPlacementProperty, value); }
+        }
+
+        #endregion
+        
         #region Background property
 
         private static readonly DependencyPropertyKey BackgroundPropertyKey =
@@ -127,17 +171,6 @@ public ComboBoxPopup()
             this.CustomPopupPlacementCallback = ComboBoxCustomPopupPlacementCallback;
         }
 
-        private void SetChildTemplateIfNeed(ControlTemplate template)
-        {
-            var contentControl = Child as ContentControl;
-            if (contentControl == null) throw new InvalidOperationException("Child must be ContentControl");
-
-            if (!ReferenceEquals(contentControl.Template, template))
-            {
-                contentControl.Template = template;
-            }
-        }
-
         private void SetupBackground(IEnumerable<DependencyObject> visualAncestry)
         {
             var background = visualAncestry
@@ -191,7 +224,7 @@ private CustomPopupPlacement[] ComboBoxCustomPopupPlacementCallback(
             if (locationX + popupSize.Width - realOffsetX > screenWidth
                 || locationX - realOffsetX < 0)
             {
-                SetChildTemplateIfNeed(DefaultContentTemplate);
+                PopupPlacement = ComboBoxPopupPlacement.Classic;
 
                 var defaultVerticalOffsetIndepent = DpiHelper.TransformToDeviceY(mainVisual, DefaultVerticalOffset);
                 var newY = locationY + popupSize.Height > screenHeight
@@ -203,7 +236,7 @@ private CustomPopupPlacement[] ComboBoxCustomPopupPlacementCallback(
 
             if (locationY + popupSize.Height > screenHeight)
             {
-                SetChildTemplateIfNeed(UpContentTemplate);
+                PopupPlacement = ComboBoxPopupPlacement.Up;
 
                 var upVerticalOffsetIndepent = DpiHelper.TransformToDeviceY(mainVisual, UpVerticalOffset);
                 var newY = upVerticalOffsetIndepent - popupSize.Height + targetSize.Height;
@@ -212,13 +245,53 @@ private CustomPopupPlacement[] ComboBoxCustomPopupPlacementCallback(
             }
             else
             {
-                SetChildTemplateIfNeed(DownContentTemplate);
+                PopupPlacement = ComboBoxPopupPlacement.Down;
 
                 var downVerticalOffsetIndepent = DpiHelper.TransformToDeviceY(mainVisual, DownVerticalOffset);
                 var newY = downVerticalOffsetIndepent;
 
                 return new[] { new CustomPopupPlacement(new Point(offsetX, newY), PopupPrimaryAxis.None) };
             }
         }
+
+        private void SetChildTemplateIfNeed(ControlTemplate template)
+        {
+            var contentControl = Child as ContentControl;
+            if (contentControl == null) throw new InvalidOperationException("Child must be ContentControl");
+
+            if (!ReferenceEquals(contentControl.Template, template))
+            {
+                contentControl.Template = template;
+            }
+        }
+
+        private void SetChildTemplate(ComboBoxPopupPlacement placement)
+        {
+            switch (placement)
+            {
+                case ComboBoxPopupPlacement.Classic:
+                    SetChildTemplateIfNeed(ClassicContentTemplate);
+                    return;
+                case ComboBoxPopupPlacement.Down:
+                    SetChildTemplateIfNeed(DownContentTemplate);
+                    return;
+                case ComboBoxPopupPlacement.Up:
+                    SetChildTemplateIfNeed(UpContentTemplate);
+                    return;
+                default:
+                    throw new NotImplementedException($"Unexpected value {placement} of the {nameof(PopupPlacement)} property inside the {nameof(ComboBoxPopup)} control.");
+            }
+        }
+
+        private static void PopupPlacementPropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
+        {
+            var popup = d as ComboBoxPopup;
+            if (popup == null) return;
+
+            if (!(e.NewValue is ComboBoxPopupPlacement)) return;
+            var placement = (ComboBoxPopupPlacement)e.NewValue;
+
+            popup.SetChildTemplate(placement);
+        }
     }
 }

MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.ComboBox.xaml

@@ -2,7 +2,8 @@
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                     xmlns:converters="clr-namespace:MaterialDesignThemes.Wpf.Converters"
                     xmlns:system="clr-namespace:System;assembly=mscorlib"
-                    xmlns:wpf="clr-namespace:MaterialDesignThemes.Wpf">
+                    xmlns:wpf="clr-namespace:MaterialDesignThemes.Wpf"
+                    xmlns:materialdesign="clr-namespace:MaterialDesignThemes.Wpf">
 
     <ResourceDictionary.MergedDictionaries>
         <ResourceDictionary Source="pack://application:,,,/MaterialDesignThemes.Wpf;component/Themes/MaterialDesignTheme.Shadows.xaml" />
@@ -149,7 +150,7 @@
         </Grid>
     </ControlTemplate>
 
-    <ControlTemplate x:Key="PopupContentDefaultTemplate" TargetType="ContentControl">
+    <ControlTemplate x:Key="PopupContentClassicTemplate" TargetType="ContentControl">
         <Grid MinWidth="{Binding ElementName=templateRoot, Path=ActualWidth, Converter={StaticResource MathAddConverter}, ConverterParameter=32}"
               Margin="6">
             <Grid.RowDefinitions>
@@ -459,7 +460,7 @@
                                    UpVerticalOffset="15"
                                    UpContentTemplate="{StaticResource PopupContentUpTemplate}"
                                    DownContentTemplate="{StaticResource PopupContentDownTemplate}"
-                                   DefaultContentTemplate="{StaticResource PopupContentDefaultTemplate}">
+                                   ClassicContentTemplate="{StaticResource PopupContentClassicTemplate}">
                     <ContentControl>
                         <ScrollViewer MaxHeight="{TemplateBinding MaxDropDownHeight}"
                                       Background="{Binding Background, ElementName=PART_Popup}">
@@ -470,6 +471,9 @@
             </Grid>
         </Grid>
         <ControlTemplate.Triggers>
+            <Trigger SourceName="PART_Popup" Property="PopupPlacement" Value="{x:Static materialdesign:ComboBoxPopupPlacement.Classic}">
+                <Setter Property="ItemContainerStyle" Value="{StaticResource MaterialDesignComboBoxItemStyle}" />
+            </Trigger>
             <Trigger Property="IsEnabled" Value="False">
                 <Setter TargetName="templateRoot" Property="Opacity" Value="0.56"/>
             </Trigger>