Develop Reference

Binding an element's property in MainWindow to a property in an embedded UserControl

In my UserControl I have a DataGrid and a Button. I can easily bind the IsEnabled property of the Button to the SelectedItems.Count property of the DataGrid.
For example, in my UserControl I have...
<Button IsEnabled="{Binding ElementName=MyDataGrid, Path=SelectedItems.Count}" />
However, I'm wanting to do the same with a Button in my MainWindow where I also have the same UserControl embedded.
I would like to do this, but it doesn't work:
<Button IsEnabled="{Binding ElementName=MyUserControl, Path=MyDataGrid.SelectedItems.Count}" />
Is there a way to do this purely in XAML or do I need to create a property in my ViewModel that reflects the SelectedItems.Count property of the DataGrid and bind that to my Button in the MainWindow?
Thanks!

Define public property in UserControl (to keep selected items), and bind it to SelectedItem of datagrid,
Now you will be able to get public property in main window by ElementName and XPath or Path.

Call A Method From A Control That Is Not In The DataContext

I have a button in a UserControl that I want to call a method in another class (which happens to be my main window's view model).
<ToggleButton cal:Message.Attach="[Event Click] = [Action ToggleWatch]">
The user control's DataContext is tied to a DataTemplate. This makes everything beautiful, unless I run into cases like this where implementing the ToggleWatch method in the DataTemplate class doesn't make much sense, since the DataTemplate should only contain data.
Is the best way to get around this is by setting the DataContext of this control to the MainWindowViewModel? That workaround fails when I want to bind a value from the DataTemplate to the same button, since the DataContext modification will then make it look for the value in the MainWindowViewModel.
In case I've over complicated the question, I will summarise. I have a UserControl whose DataContext is a seperate DataTemplate. I want to call a method from a button when it is clicked, but I want the method seperated from the DataTemplate. I want the method in a completely different class. What is the best way to solve this?
Here is how I'm setting the DataTemplate:
<WrapPanel>
<ItemsControl ItemsSource="{Binding Devices}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<local:DeviceCleanerBox/>
...

UserControl must exist as child of your MainWindow and if i am right you must have set DataContext of MainWindow to be MainWindowViewModel.
So, what you can do is declare ICommand in MainWindowViewModel and bind to button's Command using RelativeSource to search for window's DataContext:
<ToggleButton Command="{Binding DataContext.CommandName,
RelativeSource={RelativeSource Mode=FindAncestor,
AncestorType=Window}}"/>

How do I avoid binding to 2 datacontext

My application looks like the following
The black is my MainWindow, the red is a tab control and the yellow is a UserControl.
The UserControl has many Dependency Properties defined and they bind to the DataContext (Which is set in the MainWindow's code behind, using this.DataContext = this).
To bind my UserControl to the same DataContext as my MainWindow, in my UserControl xaml I have the following
DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorLevel=1,AncestorType=Window}}"
This works great, and when I interact with my UserControl, due to the two way binding, it updates the Properties of my MainWindow, which in turn updates my TabControl!
The issue is, my UserControl now has some extra functionality and as such, needs to bind to the UserControl's code behind (such as values for the GUI).
This is where I'm stuck. I can't bind from my UserControl to my code behind because I've already created a DataContext.
I know I could use the WinForms approach, and name each control with x:Name="MyControl" like
MyControl.Text = "This value";
or
MyControl.DataContext = this;
Yeuk I think!!
My question is, is this the only way to go, or can I still use binding.

First of all you don't need to manually set DataContext on UserControl. DataContext is an inheritable property so it will inherit DataContext from its parent unless you have explicitly set it.
Get rid of DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorLevel=1,AncestorType=Window}}" from your UserControl.
And now, in case you want to bind to code behind for some controls in your UserControl, you can bind using RelativeSource or can set DataContext on control:
DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor,
AncestorType=UserControl}}"
If controls can be clubbed together under one panel, set DataContext on parent panel say Grid and child controls will inherit from it:
<Grid DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor,
AncestorType=UserControl}}">
..Child Controls here will inherit DataContext
</Grid>
And to answer your question:
MyControl.DataContext = this; <-- Is this possible
Yes, it is possible like i mentioned above.

You can just use another RelativeSource Binding as you have for the MainWindow properties... to access the properties defined in the UserControl, try the following XAML in your UserControl:
<TextBlock Text="{Binding UserControlProperty, RelativeSource={RelativeSource
AncestorType={x:Type YourXmlNamespacePrefix:YourUserControl}}}" />
Obviously, you'll need to update YourXmlNamespacePrefix:YourUserControl to a valid XML Namespace and control type to get this to work.
I'm not saying either that you should set the DataContext anywhere, or change any properties. This is a RelativeSource Binding... you do not need to set any DataContext to make it work. I thought you would have known that seeing as you're already using one. Just try the example out.

Bind ComboBox in DataGrid to collection in view

I have a view that is bound to my usercontrol. On my usercontrol I have a datagrid which is bound to a collection from my view. One column in that datagrid is a DataGridComboBoxColumn and I need to bind it to a different collection in my view. The binding of the datagrid looks like this:
DataContext="{Binding .}" ItemsSource="{Binding VictimVillages}"
The datacontext is the view and VictimVillages is the collection I want to display in the datagrid.
I have tried to access the collection I want to display in the combobox with
ItemsSource="{Binding Path=Patterns,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type Views:ViewPlanner}}}"
The name of the collection is Patterns, the type of the view is "ViewPlanner", which is in the Views namespace.
I have create a diagram of the dependencies. I need help to figure out the yellow part.

when using RelativeSource Binding you(mostly) have to use DataContext.YourProperty in your Binding Path.
if your datacontext from the DataGrid also have the Patterns Collection you can do this
ItemsSource="{Binding Path=DataContext.Patterns,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type DataGrid}}}"
i often use "MarkerInterface" for Relative bindings. that mean empty interfaces that i can add to my views.
public interface IDataContextMarker4PatternCollection {}
now add this interface to your view/usercontrol where the Pattern Collection Property is in the Datacontext.
and now you can simply use RelativeSourceBinding
ItemsSource="{Binding Path=DataContext.Patterns,RelativeSource={RelativeSource AncestorType={x:Type local:IDataContextMarker4PatternCollection }}}"

Binding Label content to Self string field but no result

I'm trying what should be an easy bind in my user control:
<UserControl x:Class="MyApp.FlowNode" ...>
<StackPanel>
<Label Content="{Binding Path=Header, RelativeSource={RelativeSource Self}}" />
</StackPanel>
</UserControl>
With the underlying code being:
public partial class FlowNode : UserControl
{
public FlowNode()
{
InitializeComponent();
}
public string Header { get { return "Testing"; } };
}
However, the label stays blank. What am I doing wrong?

{RelativeSource Self} refers to the Label instance, not your user control.
Instead of using a RelativeSource, you should set the UserControl's DataContext to itself, as karmicpuppet suggested.

Try to use
{Binding Path=Header, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type FlowNode}}}.

In your constructor, set the DataContext of the UserControl to itself. "this.DataContext = this". And remove the RelativeSource in your binding.

The most convenient way to get access to 'Me'/'this' in xaml in my opinion is to put a Name on your control :
< UserControl
x:Class="MyApp.FlowNode"
...
Name="MyFlowNode" >
And then, if you bind to your control's property or dp, you use ElementName syntax.
So you don't break the DataContext of your control.
There might be a way with RelativeSource FindAncestor binding, but i don't see advantages over that one.
But do not set DataContext of your Control to itself, otherwise it will loose awareness of its DataContext.