Binding to a different dataContexts

Windows Phone application (Silverlight 3)

I have a textblock

<TextBlock Text="{Binding Key}"  FontSize="40" Foreground="{Binding propertyOnAMainViewModel}" />

DataContext of a TextBlock is set to a class Grou开发者_如何学Gop instance, that exposes Key property.

I need to bind the foreground property of the TextBlock to a dynamic (settable from code) property, but on a different ViewModel, not the Group.

Is it possible to bind different properties on one element to a different data contexts?

You can do this,, but it is not terribly elegant! each binding has a Source, which, if not specified, is the control's DataContext. You can set the source explicitly if you construct the binding in code-behind. Within XAML, your only options are default (i.e. DataContext), or ElementName bindings.

What I would do is create a ViewModel that exposes both the properties you wish to bind to, and use that as your DataContext.

It's easiest if you can host one VM inside of another:

public class TextBoxViewModel : ViewModelBase
{
  private ChildViewModel _childVM;

  public ChildViewModel ChildVM
  {
    get { return _childVM; }
    set
    {
      if (_childVM == value)
        return;

      if (_childVM != null)
        _childVM.PropertyChanged -= OnChildChanged;

      _childVM = value;

      if (_childVM != null)
        _childVM.PropertyChanged += OnChildChanged;

      OnPropertyChanged("ChildVM");
    }
  }

  public Brush TextBoxBackground
  {
    get
    {
      if(ChildVM == null)
        return null;

      return ChildVM.MyBackground;
    }
    set
    {
      if (ChildVM != null)
        ChildVM.MyBackground = value;
    }
  }

  private void OnChildChanged(object sender, PropertyChangedEventArgs e)
  {
    if (string.IsNullOrEmpty(e.PropertyName) || e.PropertyName == "MyBackground")
      OnPropertyChanged("TextBoxBackground");
  }
}