Dynamic width binding to the dropdown(Popup) used in custom combobox - wpf

Question

I am using custom ComboBox with style, I want to set width of a Popup dynamically via coading so that to auto adjust the width of popup
so I wanted to change Popup like second image dynamically(whatever may be thesize of ComboBox)
I am using style as follows
<Style x:Key="ComboBoxStyle" TargetType="{x:Type ComboBox}">
<Setter Property="Foreground" Value="#666666"/>
<Setter Property="FontFamily" Value="Arial"/>
<Setter Property="FontSize" Value="13"/>
<Setter Property="Height" Value="28"/>
<Setter Property="BorderThickness" Value="1.5"/>
<Setter Property="Padding" Value="4,3"/>
<Setter Property="Margin" Value="5"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ComboBox}">
<Grid>
<Popup Margin="1" x:Name="PART_Popup" AllowsTransparency="true" IsOpen="{Binding Path=IsDropDownOpen, RelativeSource={RelativeSource TemplatedParent}}" Placement="Bottom" PopupAnimation="{DynamicResource {x:Static SystemParameters.ComboBoxPopupAnimationKey}}" Grid.ColumnSpan="2" Width="{Binding ActualWidth,RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=Grid}}">
<Border Name="DropDownBorder" Width="Auto" Height="Auto" BorderThickness="1,0,1,1" CornerRadius="0,0,4,4" BorderBrush="#FFbbbbbb">
<Border.Background>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="White" Offset="0" />
<GradientStop Color="#FFE9E9E9" Offset="1" />
</LinearGradientBrush>
</Border.Background>
<ScrollViewer CanContentScroll="true">
<ItemsPresenter />
</ScrollViewer>
</Border>
</Popup>
<ToggleButton Style="{StaticResource cmbToggle}" Grid.ColumnSpan="2" IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"/>
<ContentPresenter HorizontalAlignment="Left" Margin="5,0,0,0" VerticalAlignment="Center" IsHitTestVisible="false" Content="{TemplateBinding SelectionBoxItem}" ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="cmbToggle" TargetType="{x:Type ToggleButton}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<Border Name="cmbBorder" CornerRadius="3" BorderBrush="#FFaaaaaa" BorderThickness="1.5">
<Border.Background>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="White" Offset="0" />
<GradientStop Color="#FFE9E9E9" Offset="1" />
</LinearGradientBrush>
</Border.Background>
<Border BorderBrush="#FFaaaaaa" BorderThickness="1,0,0,0" Width="20" HorizontalAlignment="Right">
<Polygon Name="pol" Fill="#FF787878" Points="4,9 8,14 12,9" Stroke="#FF787878" StrokeThickness="0" Margin="1 1 0 0">
</Polygon>
</Border>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter Property="CornerRadius" TargetName="cmbBorder" Value="4,4,0,0"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
So My idea is to change width of Popup dynamically(Combobox width - togglebutton width = popup width).I have written style in App.xaml
How to do this, Please help me. Thanks in advance.

Answer 1

Ok so in your ToggleButton Style we can see the Border holding Polygon arrow is 20 in Width. That's the Width to be removed from the Popup
Thus we can do something like:
Firstly add 2 Column's to the Grid in ComboBox ControlTemplate
<ControlTemplate TargetType="{x:Type ComboBox}">
<Grid>
<!-- New Bit -->
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="20" />
</Grid.ColumnDefinitions>
<!-- End of New Bit -->
<Popup x:Name="PART_Popup"
Grid.ColumnSpan="2"
...
and then update the Width of Popup to
<Popup x:Name="PART_Popup"
Grid.ColumnSpan="2"
Width="{Binding Path=ColumnDefinitions[0].ActualWidth,
RelativeSource={RelativeSource Mode=FindAncestor,
AncestorType=Grid}}"
...
Your Style already has ColumnSpan mentioned on appropriate controls, so nothing else is needed. This should give you the output your looking for.

Answer 2

you can directly use
<ComboBox.Resources>
<Style TargetType="{x:Type Popup}">
<Setter Property="Width" Value="110"/>
</Style>
</ComboBox.Resources>

Answer 3

Using Dhaval Patel's solution bound to a view model property did the trick.
I calculate the max width using FormattedText when the collection changes and it bind it below.
<ComboBox.Resources>
<Style TargetType="{x:Type Popup}">
<Setter Property="Width" Value="{Binding MaxWidthOfMyCollection"/>
</Style>
</ComboBox.Resources>