public static readonly DependencyProperty PositionXProperty =
            DependencyProperty.Register("PositionX", typeof(double), typeof(OnScreenJoystick), null);

.
.
.

public double PositionX
        {
            get { return Convert.ToDouble(GetValue(PositionXProperty )); }
            private set { SetValue(PositionXProperty , value); }
        }

.
.
.

private void Knob_MouseMove(object sender, MouseEventArgs e)
        {
            ...
            PositionX = deltaPos.X;
            ...
        }