Change a WPF context menu on button click event to MVVM

I have a WPF button that currently does two functions that I would like to put into a MVVM pattern. If a condition is met when clicked, it opens a context menu that we build dynamically, otherwise we simply execute another function.

private void Button_MouseDown(object sender, MouseButtonEventArgs e)
{
    if (e.ChangedButton == MouseButton.Left)
    {
        if (MyTest())
        {
          Button b = sender as Button;
          ContextMenu contextMenu = MyContextMenuBuilder();
          contextMenu.PlacementTarget = b;
          contextMenu.Placement = System.Windows.Controls.Primitives.PlacementMode.Bottom;
          contextMenu.IsOpen = true;
        }
        else
       {
          MyOtherFunction(开发者_运维问答);
       }
    }
}

How do I MV-VM this code since the VM does not have knowledge of the button to place the menu, and the menu itself to open it?

---

You could put MyTest() into an object that you place within the DataContext of the button, then have a DataTrigger defined in XAML that decides what's enabled. EG:

public class ButtonAnnotation
{
    public object Foo { get; set; }

    public bool SatisfiesTest {
        get {
            ... logic for MyTest goes here ...
        }
    }

}

...
this.TheButtonOrItsParent.DataContext = new ButtonAnnotation();

Then in XAML:

<DataTrigger Binding="{Binding Path=SatisfiesTest}" Value="True">
    <DataTrigger.Setters>
        <Setter Property="ContextMenu" Value="{StaticResource TheContextMenu}"/>
    </DataTrigger.Setters>
</DataTrigger>