when using mvvm and further when you define a viewmodel with:
a viewmodel should not know/reference the view
then you cant set focus through the viewmodel.
but what i do in mvvm is the following in the viewmodel:
set the focus to the element which is bind to the viewmodel property
for this i create a behavior which simply walk through all control in the visual tree and look for the binding expressions path. and if i find a path expression then simply focus the uielement.
EDIT:
xaml usage
<UserControl>
<i:Interaction.Behaviors>
<Behaviors:OnLoadedSetFocusToBindingBehavior BindingName="MyFirstPropertyIWantToFocus" SetFocusToBindingPath="{Binding Path=FocusToBindingPath, Mode=TwoWay}"/>
</i:Interaction.Behaviors>
</UserControl>
viemodel in any method
this.FocusToBindingPath = "MyPropertyIWantToFocus";
behavior
public class SetFocusToBindingBehavior : Behavior<FrameworkElement>
{
public static readonly DependencyProperty SetFocusToBindingPathProperty =
DependencyProperty.Register("SetFocusToBindingPath", typeof(string), typeof(SetFocusToBindingBehavior ), new FrameworkPropertyMetadata(SetFocusToBindingPathPropertyChanged));
public string SetFocusToBindingPath
{
get { return (string)GetValue(SetFocusToBindingPathProperty); }
set { SetValue(SetFocusToBindingPathProperty, value); }
}
private static void SetFocusToBindingPathPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var behavior = d as SetFocusToBindingBehavior;
var bindingpath = (e.NewValue as string) ?? string.Empty;
if (behavior == null || string.IsNullOrWhiteSpace(bindingpath))
return;
behavior.SetFocusTo(behavior.AssociatedObject, bindingpath);
//wenn alles vorbei ist dann binding path zurücksetzen auf string.empty,
//ansonsten springt PropertyChangedCallback nicht mehr an wenn wieder zum gleichen Propertyname der Focus gesetzt werden soll
behavior.SetFocusToBindingPath = string.Empty;
}
private void SetFocusTo(DependencyObject obj, string bindingpath)
{
if (string.IsNullOrWhiteSpace(bindingpath))
return;
var ctrl = CheckForBinding(obj, bindingpath);
if (ctrl == null || !(ctrl is IInputElement))
return;
var iie = (IInputElement) ctrl;
ctrl.Dispatcher.BeginInvoke((Action)(() =>
{
if (!iie.Focus())
{
//zb. bei IsEditable=true Comboboxen funzt .Focus() nicht, daher Keyboard.Focus probieren
Keyboard.Focus(iie);
if (!iie.IsKeyboardFocusWithin)
{
Debug.WriteLine("Focus konnte nicht auf Bindingpath: " + bindingpath + " gesetzt werden.");
var tNext = new TraversalRequest(FocusNavigationDirection.Next);
var uie = iie as UIElement;
if (uie != null)
{
uie.MoveFocus(tNext);
}
}
}
}), DispatcherPriority.Background);
}
public string BindingName { get; set; }
protected override void OnAttached()
{
base.OnAttached();
AssociatedObject.Loaded += AssociatedObjectLoaded;
}
protected override void OnDetaching()
{
base.OnDetaching();
AssociatedObject.Loaded -= AssociatedObjectLoaded;
}
private void AssociatedObjectLoaded(object sender, RoutedEventArgs e)
{
SetFocusTo(AssociatedObject, this.BindingName);
}
private DependencyObject CheckForBinding(DependencyObject obj, string bindingpath)
{
var properties = TypeDescriptor.GetProperties(obj, new Attribute[] { new PropertyFilterAttribute(PropertyFilterOptions.All) });
if (obj is IInputElement && ((IInputElement) obj).Focusable)
{
foreach (PropertyDescriptor property in properties)
{
var prop = DependencyPropertyDescriptor.FromProperty(property);
if (prop == null) continue;
var ex = BindingOperations.GetBindingExpression(obj, prop.DependencyProperty);
if (ex == null) continue;
if (ex.ParentBinding.Path.Path == bindingpath)
return obj;
}
}
for (int i = 0; i < VisualTreeHelper.GetChildrenCount(obj); i++)
{
var result = CheckForBinding(VisualTreeHelper.GetChild(obj, i),bindingpath);
if (result != null)
return result;
}
return null;
}
}