Set a property in Control Triggers using Binding WPF

Question

I have designed a TextBox in WPF which could be validated automatically and show the error message whenever validation is failed.

here is my ControlTemplate:

 <Setter Property="Template">
     <Setter.Value>
     <ControlTemplate TargetType="Controls:ShamsText">
         <Grid>
             <Border Name="container" HorizontalAlignment="Left" Padding="{TemplateBinding Padding}" Height="{TemplateBinding Height}" BorderThickness="1" BorderBrush="#ccc" Background="{TemplateBinding Background}">
                 <ScrollViewer Padding="0" x:Name="PART_ContentHost" Width="{TemplateBinding Width}" Height="{TemplateBinding Height}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" TextBlock.TextAlignment="Center" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"/>
             </Border>

             <Popup Name="Popup" 
                     Placement="Top"
                     Focusable="False"
                     AllowsTransparency="True"
                     IsOpen="{TemplateBinding ComboBox.IsDropDownOpen}"
                     PopupAnimation="Slide">
                 <Grid>
                     <Border Padding="0 0 0 10">
                         <Border Name="DropDownBorder" 
                             Background="#35403F"
                             Padding="12 10 12 10"
                             Margin="0, 1, 0, 0"
                             SnapsToDevicePixels="True"
                             CornerRadius="4"                                         
                             BorderThickness="1,1,1,1" 
                             BorderBrush="#35403F">
                             <ScrollViewer SnapsToDevicePixels="True">
                                 <TextBlock Name="PopupText" FontWeight="Thin" TextWrapping="Wrap" MaxWidth="{TemplateBinding Width}" Text=" " Foreground="#E8E8E8"/>
                             </ScrollViewer>
                         </Border>
                     </Border>
                     <TextBlock Foreground="#35403F" VerticalAlignment="Bottom" Margin="15 0 0 0" Text="▼"></TextBlock>
                 </Grid>
             </Popup>
         </Grid>
             <ControlTemplate.Triggers>

here is my trigger on it's ControlTemplate :

 <MultiTrigger>
     <MultiTrigger.Conditions>
         <Condition Property="IsShowRequired" Value="True"></Condition>
         <Condition Property="IsFocused" Value="True"></Condition>
         <Condition Property="IsReadOnly" Value="False"></Condition>
         <Condition Property="Text" Value=""></Condition>
     </MultiTrigger.Conditions>
     <Setter TargetName="container" Property="BorderBrush" Value="#66AFE9"/>
     <Setter TargetName="PopupText" Property="Text" Value="Required Error Messagge"/>
     <Setter TargetName="Popup" Property="IsOpen" Value="True"></Setter>
 </MultiTrigger>

I want to make " Required Error Message " Customizable so i added a property to my TextBox Class:

 public static readonly DependencyProperty RequiredMessageProperty = DependencyProperty.Register("RequiredMessage", typeof(string), typeof(ShamsText), new PropertyMetadata("این مقدار اجباری است."));
 public string RequiredMessage
 {
     get { return (string)GetValue(RequiredMessageProperty); }
     set { SetValue(RequiredMessageProperty, value); }
 }

so i changed my multi-trigger to:

 <MultiTrigger>
     <MultiTrigger.Conditions>
         <Condition Property="IsShowRequired" Value="True"></Condition>
         <Condition Property="IsFocused" Value="True"></Condition>
         <Condition Property="IsReadOnly" Value="False"></Condition>
         <Condition Property="Text" Value=""></Condition>
     </MultiTrigger.Conditions>
     <Setter TargetName="container" Property="BorderBrush" Value="#66AFE9"/>
     <Setter TargetName="PopupText" Property="Text" Value="{Binding RequiredMessage}"/>
     <Setter TargetName="Popup" Property="IsOpen" Value="True"></Setter>
 </MultiTrigger>

but it sets PopupText's Text to null whenever this trigger fires.

how can i set this value dynamically ?

Answer 1

Problem is with your Binding , using your code gave me binding errors. Change your binding to:

<MultiTrigger>
    <MultiTrigger.Conditions>
        <Condition Property="IsShowRequired" Value="True"></Condition>
        <Condition Property="IsFocused" Value="True"></Condition>
        <Condition Property="IsReadOnly" Value="False"></Condition>
        <Condition Property="Text" Value=""></Condition>
    </MultiTrigger.Conditions>
    <Setter TargetName="container" Property="BorderBrush" Value="#66AFE9"/>
    <Setter TargetName="PopupText" Property="Text" Value="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=RequiredMessage}"/>
    <Setter TargetName="Popup" Property="IsOpen" Value="True"></Setter>
</MultiTrigger>