FrameworkElement.PredictFocus(FocusNavigationDirection) Method

Reference

Definition

Namespace:: System.Windows

Assembly:: PresentationFramework.dll

Important

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Determines the next element that would receive focus relative to this element for a provided focus movement direction, but does not actually move the focus.

public:
 override System::Windows::DependencyObject ^ PredictFocus(System::Windows::Input::FocusNavigationDirection direction);

public override sealed System.Windows.DependencyObject PredictFocus (System.Windows.Input.FocusNavigationDirection direction);

override this.PredictFocus : System.Windows.Input.FocusNavigationDirection -> System.Windows.DependencyObject

Public Overrides NotOverridable Function PredictFocus (direction As FocusNavigationDirection) As DependencyObject

Parameters

direction: FocusNavigationDirection

The direction for which a prospective focus change should be determined.

Returns

DependencyObject

The next element that focus would move to if focus were actually traversed. May return null if focus cannot be moved relative to this element for the provided direction.

Exceptions

InvalidEnumArgumentException

Specified one of the following directions in the TraversalRequest: Next, Previous, First, Last. These directions are not legal for PredictFocus(FocusNavigationDirection) (but they are legal for MoveFocus(TraversalRequest)).

Examples

The following example implements a handler that handles several possible button inputs, each button representing a possible FocusNavigationDirection. The handler tracks the element with current keyboard focus, and calls PredictFocus on that element, and specifies the appropriate FocusNavigationDirection as initialization for the TraversalRequest type parameter provided. Instead of moving to that element as MoveFocus would do, the handler changes the physical dimensions of the predicted focus destination for visualization purposes.

    private void OnPredictFocus(object sender, RoutedEventArgs e)
    {
        DependencyObject predictionElement = null;

        UIElement elementWithFocus = Keyboard.FocusedElement as UIElement;

        if (elementWithFocus != null)
        {
            // Only these four directions are currently supported
            // by PredictFocus, so we need to filter on these only.
            if ((_focusMoveValue == FocusNavigationDirection.Up) ||
                (_focusMoveValue == FocusNavigationDirection.Down) ||
                (_focusMoveValue == FocusNavigationDirection.Left) ||
                (_focusMoveValue == FocusNavigationDirection.Right))
            {

                // Get the element which would receive focus if focus were changed.
                predictionElement = elementWithFocus.PredictFocus(_focusMoveValue);
               
                Control controlElement = predictionElement as Control;

                // If a ContentElement.
                if (controlElement != null)
                {
                    controlElement.Foreground = Brushes.DarkBlue;
                    controlElement.FontSize += 10;
                    controlElement.FontWeight = FontWeights.ExtraBold;

                    // Fields used to reset the UI when the mouse 
                    // button is released.
                    _focusPredicted = true;
                    _predictedControl = controlElement;
                }
            }
        }
    }

Private Sub OnPredictFocus(ByVal sender As Object, ByVal e As RoutedEventArgs)
    Dim predictionElement As DependencyObject = Nothing

    Dim elementWithFocus As UIElement = TryCast(Keyboard.FocusedElement, UIElement)

    If elementWithFocus IsNot Nothing Then
        ' Only these four directions are currently supported
        ' by PredictFocus, so we need to filter on these only.
        If (_focusMoveValue = FocusNavigationDirection.Up) OrElse (_focusMoveValue = FocusNavigationDirection.Down) OrElse (_focusMoveValue = FocusNavigationDirection.Left) OrElse (_focusMoveValue = FocusNavigationDirection.Right) Then

            ' Get the element which would receive focus if focus were changed.
            predictionElement = elementWithFocus.PredictFocus(_focusMoveValue)

            Dim controlElement As Control = TryCast(predictionElement, Control)

            ' If a ContentElement.
            If controlElement IsNot Nothing Then
                controlElement.Foreground = Brushes.DarkBlue
                controlElement.FontSize += 10
                controlElement.FontWeight = FontWeights.ExtraBold

                ' Fields used to reset the UI when the mouse 
                ' button is released.
                _focusPredicted = True
                _predictedControl = controlElement
            End If
        End If
    End If
End Sub

Remarks

MoveFocus is the related method that actually does move focus.

Applies to

Jaa