Binding two dependency properties from two different usercontrols - wpf

Question

I have created two separate usercontrols, they are meant to work together.
The first one is a simple usercontrol with a thumb attached to it, the thumb makes the control move around by dragging, this is simple and working.
XAML:
<Canvas>
<Thumb x:Name="Thumb" Width="15" Height="15" DragDelta="Thumb_DragDelta"/>
</Canvas>
Code-Behind: A dependency property called Position, when Setter is called it updates the usercontrol's margin.
public partial class ThumbPoint : UserControl
{
public Point Position
{
get { return (Point)GetValue(PositionProperty); }
set { SetValue(PositionProperty, value); this.Margin = new Thickness(value.X, value.Y, 0, 0); }
}
// Using a DependencyProperty as the backing store for Position. This enables animation, styling, binding, etc...
public static readonly DependencyProperty PositionProperty =
DependencyProperty.Register("Position", typeof(Point), typeof(ThumbPoint), new PropertyMetadata(new Point()));
public ThumbPoint()
{
InitializeComponent();
Position = new Point(0, 0);
}
private void Thumb_DragDelta(object sender, System.Windows.Controls.Primitives.DragDeltaEventArgs e)
{
Position = new Point(Position.X + e.HorizontalChange, Position.Y + e.VerticalChange);
}
}
The second UserControl is called StraightLine, its composed of a Line control
XAML:
<Canvas>
<Line x:Name="Line" Stroke="Gray" StrokeThickness="1"/>
</Canvas>
Code-Behind: A dependency property called StartPosition, when Setter is called it updates the Line X1 and Y1 (starting position of the line).
public partial class StraightLine : UserControl
{
public Point StartPosition
{
get { return (Point)GetValue(StartPositionProperty); }
set { SetValue(StartPositionProperty, value); Line.X1 = value.X; Line.Y1 = value.Y; }
}
// Using a DependencyProperty as the backing store for StartPosition. This enables animation, styling, binding, etc...
public static readonly DependencyProperty StartPositionProperty =
DependencyProperty.Register("StartPosition", typeof(Point), typeof(StraightLine), new PropertyMetadata(new Point()));
public StraightLine()
{
InitializeComponent();
Line.X1 = 0;
Line.Y1 = 0;
Line.X2 = 300;
Line.Y2 = 200;
}
}
Here I am trying to bind them together on the mainwindow.xaml:
<Canvas>
<local:ThumbPoint x:Name="ThumbPoint"/>
<local:StraightLine StartPosition="{Binding Position, ElementName=ThumbPoint}"/>
</Canvas>
Desired effect: DependencyProperty StartPosition of the StraightLine should be updated.
Whats happening: It's not being updated so only the ThumbPoint is moving.

Answer 1

binding doesn't use common property wrappers for DP (public Point StartPosition), it uses SetValue() directly, so code in setter isn't invoked.
What is needed is propertyChangedCallback:
public Point StartPosition
{
get { return (Point)GetValue(StartPositionProperty); }
set { SetValue(StartPositionProperty, value); }
}
public static readonly DependencyProperty StartPositionProperty =
DependencyProperty.Register("StartPosition", typeof(Point), typeof(StraightLine), new PropertyMetadata(new Point(), OnStartPositionChanged));
private static void OnStartPositionChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
StraightLine c = (StraightLine) d;
c.Line.X1 = c.StartPosition.X;
c.Line.Y1 = c.StartPosition.Y;
}
in ThumbPoint public Point Position property has the same issue but it works there because you use setter directly: Position = new Point()
Alternatively bind X1 and Y1 value in xaml:
<Canvas>
<Line x:Name="Line" Stroke="Gray" StrokeThickness="1"
X1="{Binding StartPosition.X, RelativeSource={RelativeSource AncestorType=StraightLine}}"
Y1="{Binding StartPosition.Y, RelativeSource={RelativeSource AncestorType=StraightLine}}"/>
</Canvas>
(or use ElementName instead of RelativeSource)