Added a Trigger to MouseOver Property of ValidationMessage

src/MahApps.Metro.Samples/MahApps.Metro.Demo/App.xaml

@@ -19,6 +19,7 @@
 
             <Style BasedOn="{StaticResource MetroValidationPopup}" TargetType="{x:Type controls:CustomValidationPopup}">
                 <Setter Property="CloseOnMouseLeftButtonDown" Value="False" />
+                <Setter Property="ShowValidationErrorOnMouseOver" Value="False" />
             </Style>
 
             <!--  quick change of the collapse storyboard duration  -->

src/MahApps.Metro.Samples/MahApps.Metro.Demo/ExampleViews/ButtonsExample.xaml

@@ -239,8 +239,9 @@
                                    OnLabel="Enabled"
                                    Style="{StaticResource MahApps.Metro.Styles.ToggleSwitch.Win10}" />
             <Controls:ToggleSwitch x:Name="visibleSwitch"
+                                   Controls:ValidationHelper.ShowValidationErrorOnMouseOver="True"
                                    Margin="{StaticResource ControlMargin}"
-                                   IsChecked="False"
+                                   IsChecked="{Binding IsToggleSwitchVisible, ValidatesOnDataErrors=True}"
                                    OffLabel="Collapsed"
                                    OnLabel="Visible"
                                    Style="{StaticResource MahApps.Metro.Styles.ToggleSwitch.Win10}" />

src/MahApps.Metro.Samples/MahApps.Metro.Demo/MainWindowViewModel.cs

@@ -300,6 +300,11 @@ public bool IsHamburgerMenuPaneOpen
                     return "No time given!";
                 }
 
+                if (columnName == nameof(IsToggleSwitchVisible) && !IsToggleSwitchVisible)
+                {
+                    return "There is something hidden... \nActivate me to show it up.";
+                }
+
                 return null;
             }
         }
@@ -455,5 +460,7 @@ private void ToggleIconScaling(object obj)
         public bool IsScaleDownLargerFrame => ((MetroWindow)Application.Current.MainWindow).IconScalingMode == MultiFrameImageMode.ScaleDownLargerFrame;
 
         public bool IsNoScaleSmallerFrame => ((MetroWindow)Application.Current.MainWindow).IconScalingMode == MultiFrameImageMode.NoScaleSmallerFrame;
+
+        public bool IsToggleSwitchVisible { get; set; }
     }
 }

src/MahApps.Metro/Controls/CustomValidationPopup.cs

@@ -18,15 +18,13 @@ namespace MahApps.Metro.Controls
     /// </summary>
     public class CustomValidationPopup : Popup
     {
-        public static readonly DependencyProperty CloseOnMouseLeftButtonDownProperty = DependencyProperty.Register("CloseOnMouseLeftButtonDown", typeof(bool), typeof(CustomValidationPopup), new PropertyMetadata(true));
-
         private Window hostWindow;
 
-        public CustomValidationPopup()
-        {
-            this.Loaded += this.CustomValidationPopup_Loaded;
-            this.Opened += this.CustomValidationPopup_Opened;
-        }
+        public static readonly DependencyProperty CloseOnMouseLeftButtonDownProperty
+            = DependencyProperty.Register(nameof(CloseOnMouseLeftButtonDown),
+                                          typeof(bool),
+                                          typeof(CustomValidationPopup),
+                                          new PropertyMetadata(true));
 
         /// <summary>
         /// Gets/sets if the popup can be closed by left mouse button down.
@@ -37,6 +35,24 @@ public bool CloseOnMouseLeftButtonDown
             set { SetValue(CloseOnMouseLeftButtonDownProperty, value); }
         }
 
+        public static readonly DependencyProperty ShowValidationErrorOnMouseOverProperty
+            = DependencyProperty.RegisterAttached(nameof(ShowValidationErrorOnMouseOver),
+                                                  typeof(bool),
+                                                  typeof(CustomValidationPopup),
+                                                  new PropertyMetadata(false));
+
+        public bool ShowValidationErrorOnMouseOver
+        {
+            get { return (bool)GetValue(ShowValidationErrorOnMouseOverProperty); }
+            set { SetValue(ShowValidationErrorOnMouseOverProperty, value); }
+        }
+
+        public CustomValidationPopup()
+        {
+            this.Loaded += this.CustomValidationPopup_Loaded;
+            this.Opened += this.CustomValidationPopup_Opened;
+        }
+
         protected override void OnPreviewMouseLeftButtonDown(MouseButtonEventArgs e)
         {
             if (CloseOnMouseLeftButtonDown)
@@ -98,6 +114,7 @@ private void CustomValidationPopup_Unloaded(object sender, RoutedEventArgs e)
             {
                 target.SizeChanged -= this.hostWindow_SizeOrLocationChanged;
             }
+
             if (this.hostWindow != null)
             {
                 this.hostWindow.LocationChanged -= this.hostWindow_SizeOrLocationChanged;
@@ -106,6 +123,7 @@ private void CustomValidationPopup_Unloaded(object sender, RoutedEventArgs e)
                 this.hostWindow.Activated -= this.hostWindow_Activated;
                 this.hostWindow.Deactivated -= this.hostWindow_Deactivated;
             }
+
             this.Unloaded -= this.CustomValidationPopup_Unloaded;
             this.Opened -= this.CustomValidationPopup_Opened;
             this.hostWindow = null;
@@ -157,6 +175,7 @@ private void SetTopmostState(bool isTop)
             {
                 return;
             }
+
             var hwnd = hwndSource.Handle;
 
 #pragma warning disable 618

src/MahApps.Metro/Controls/Helper/ValidationHelper.cs

@@ -0,0 +1,28 @@
+using System.ComponentModel;
+using System.Windows;
+
+namespace MahApps.Metro.Controls
+{
+    public static class ValidationHelper
+    {
+        public static readonly DependencyProperty ShowValidationErrorOnMouseOverProperty
+            = DependencyProperty.RegisterAttached("ShowValidationErrorOnMouseOver",
+                                                  typeof(bool),
+                                                  typeof(ValidationHelper),
+                                                  new PropertyMetadata(false));
+
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(UIElement))]
+        public static bool GetShowValidationErrorOnMouseOver(UIElement element)
+        {
+            return (bool)element.GetValue(ShowValidationErrorOnMouseOverProperty);
+        }
+
+        [Category(AppName.MahApps)]
+        [AttachedPropertyBrowsableForType(typeof(UIElement))]
+        public static void SetShowValidationErrorOnMouseOver(UIElement element, bool value)
+        {
+            element.SetValue(ShowValidationErrorOnMouseOverProperty, value);
+        }
+    }
+}

src/MahApps.Metro/Styles/ValidationErrorTemplate.xaml

@@ -1,8 +1,8 @@
 <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-                    xmlns:Controls="clr-namespace:MahApps.Metro.Controls">
+                    xmlns:mah="clr-namespace:MahApps.Metro.Controls">
 
-    <Style x:Key="MetroValidationPopup" TargetType="{x:Type Controls:CustomValidationPopup}">
+    <Style x:Key="MetroValidationPopup" TargetType="{x:Type mah:CustomValidationPopup}">
         <Setter Property="HorizontalAlignment" Value="Right" />
         <Setter Property="HorizontalOffset" Value="0" />
         <Setter Property="Placement" Value="Right" />
@@ -22,7 +22,8 @@
                         BorderThickness="1"
                         DataContext="{Binding ElementName=placeholder}">
 
-                    <Grid Width="12"
+                    <Grid x:Name="RedTriangle"
+                          Width="12"
                           Height="12"
                           Margin="1 -4 -4 0"
                           HorizontalAlignment="Right"
@@ -37,10 +38,10 @@
                     </Grid>
                 </Border>
 
-                <Controls:CustomValidationPopup x:Name="ValidationPopup"
-                                                AllowsTransparency="True"
-                                                IsOpen="False"
-                                                PlacementTarget="{Binding ElementName=PopupTargetElement, Mode=OneWay}">
+                <mah:CustomValidationPopup x:Name="ValidationPopup"
+                                           AllowsTransparency="True"
+                                           IsOpen="False"
+                                           PlacementTarget="{Binding ElementName=PopupTargetElement, Mode=OneWay}">
                     <Grid x:Name="Root" Margin="4">
                         <Grid.Resources>
                             <Style TargetType="Border">
@@ -113,7 +114,7 @@
                             <ItemsControl ItemsSource="{Binding}" />
                         </Border>
                     </Grid>
-                </Controls:CustomValidationPopup>
+                </mah:CustomValidationPopup>
             </Grid>
         </AdornedElementPlaceholder>
 
@@ -125,6 +126,22 @@
                 </MultiDataTrigger.Conditions>
                 <Setter TargetName="ValidationPopup" Property="IsOpen" Value="True" />
             </MultiDataTrigger>
+            <MultiDataTrigger>
+                <MultiDataTrigger.Conditions>
+                    <Condition Binding="{Binding ElementName=RedTriangle, Path=IsMouseOver, Mode=OneWay}" Value="True" />
+                    <Condition Binding="{Binding ElementName=placeholder, Path=AdornedElement.(Validation.HasError), Mode=OneWay}" Value="True" />
+                    <Condition Binding="{Binding ElementName=placeholder, Path=AdornedElement.(mah:ValidationHelper.ShowValidationErrorOnMouseOver), Mode=OneWay}" Value="True" />
+                </MultiDataTrigger.Conditions>
+                <Setter TargetName="ValidationPopup" Property="IsOpen" Value="True" />
+            </MultiDataTrigger>
+            <MultiDataTrigger>
+                <MultiDataTrigger.Conditions>
+                    <Condition Binding="{Binding ElementName=RedTriangle, Path=IsMouseOver, Mode=OneWay}" Value="True" />
+                    <Condition Binding="{Binding ElementName=placeholder, Path=AdornedElement.(Validation.HasError), Mode=OneWay}" Value="True" />
+                    <Condition Binding="{Binding ElementName=ValidationPopup, Path=ShowValidationErrorOnMouseOver, Mode=OneWay}" Value="True" />
+                </MultiDataTrigger.Conditions>
+                <Setter TargetName="ValidationPopup" Property="IsOpen" Value="True" />
+            </MultiDataTrigger>
         </ControlTemplate.Triggers>
     </ControlTemplate>