viernes, 31 de agosto de 2012

Modifying a ControlTemplate in a Style at runtime (programmatically)

My problem this time was modifying a ControlTemplate at runtime. Let me explain: 

I have a WPF Button which I create programmatically and I set a pre-defined Style to it.

Button btn = new Button();
btn.Content = "Category";
btn.Style = this.Resources["catButtonStyle"] as Style;
btn.Click += Button_Click;

This style defines a ControlTemplate with a VisualTree as follows:


The ContextMenu contains two MenuItems but I wanted to add a third MenuItem to be displayed as a Contextual menu at runtime. 

Here is my Style:
<Style x:Key="catButtonStyle" TargetType="Button">
    <Setter Property="OverridesDefaultStyle" Value="True" />
    <Setter Property="Margin" Value="2" />
    <Setter Property="FontFamily" Value="Verdana" />
    <Setter Property="FontSize" Value="11px" />
    <Setter Property="FontWeight" Value="Bold" />
    <Setter Property="Template">
            <ControlTemplate TargetType="Button">
                <DockPanel Name="CategoryDockPanel">
                    <Border Name="border" BorderThickness="1" Padding="4,2" BorderBrush="DarkGray" CornerRadius="3" Background="{TemplateBinding Background}">
                        <Grid Name="CategoryGrid">
                            <TextBlock Text="My button TextBlock" Name="block">
                            <!-- Context Menus -->
                                    <MenuItem Header="Properties" />
                                    <MenuItem Header="Remove"  />
                    <Trigger Property="IsMouseOver" Value="True">
                        <Setter TargetName="border" Property="BorderBrush" Value="#FF4788c8" />
                        <Setter Property="Foreground" Value="#FF4788c8" />
So the main logic I had to follow to that was the following:
  1. Find the target Control inside of my ControlTemplate
    I struggled a little bit with this step because I was not able to Find my target control inside of my ControlTemplate.  Note that it is very important to call the ApplyTemplate() method BEFORE trying to find a Control in a ControlTemplate otherwise you will always get null back. This step I was missing and gave me a headache.

    Once you ApplyTemplate, there are basically two ways of finding controls:

    a) By Control Name:  If you choose this route you need to give a Name to the immediate child of your ControlTemplate, in my case was <DockPanel Name="CategoryControlPanel". This link gives a good explanation on how you can Find Controls in a ControlTemplate: How to: Find ControlTemplate-Generated Elements

    b) By Control Type.  If you don't want to give a Name to your ControlTemplate Controls then you can use a helper method to find VisualChildren, like this: Find all controls in WPF Window by type
  2. Manipulate the content of my target control. This step is trivial
Here is how my code looks like:
MenuItem mi = new MenuItem();
mi.Header = "Add Mapping";
//Important: ApplyTemplate First so our ControlTemplate controls are available
//Find by Name
DockPanel dp = btn.Template.FindName("CategoryDockPanel",btn) as DockPanel;
//Or Find target control directly by Type
var tmp = btn.FindVisualChildren<TextBlock>().ToList();
if(dp != null)
    //Get a list of TextBlock childrens in the Visual Tree
    var tb_list = dp.FindVisualChildren<TextBlock>();
    if(tb_list != null && tb_list.Count() > 0)
        //Get the first TextBlock
        TextBlock tb = tb_list.First();
        if (tb != null && tb.ContextMenu != null)
            //Manipulate Context Menus

And now I'm able to see the added MenuItem after right-clicking on that button:

Notice that using the "FindVisualChildren" helper method is a quicker way of finding a control by type. But that will depend on your visual tree. There may be situations where you have multiple controls of the same type and you want to access a specific one. Here is where finding by name could be a good option.

That's it for now. Hope you find this useful.


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