making the thumb class easier to use (reference frame for dragdelta)

[trebahl]

At the moment, the position changes passed to the DragDelta event are computed from positions relative to the Thumb itself. This is problematic because the event handlers will typically move the Thumb in response, making the delta values erratic.

How about being able to specify a FrameworkElement (typically one would pass the parent of the thumb) relative to which the positions are retrieved before being used to compute deltas? By default, that FE would be the Thumb itself, to achieve backwards compatibility.

[trebahl]

I have written this derived class, redefining the mouse event handlers with slightly modified code.

    public class FixedThumb : Thumb
    {
        static readonly DependencyPropertyKey IsDraggingPropertyKey;
        static FixedThumb()
        {
            IsDraggingPropertyKey = typeof(Thumb).GetField("IsDraggingPropertyKey", System.Reflection.BindingFlags.Static | System.Reflection.BindingFlags.NonPublic).GetValue(null) as DependencyPropertyKey;
        }
        public Point DragStart { get; private set; }

        public FrameworkElement MotionReferenceFrame { get; set; }

        public FixedThumb(FrameworkElement refframe) { MotionReferenceFrame = refframe; }

        Point GetPos(MouseEventArgs e) => e.GetPosition(MotionReferenceFrame);
        Vector GetRelPos(MouseEventArgs e) => GetPos(e) - DragStart;
        protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
        {
            if (!IsDragging)
            {
                e.Handled = true;
                Focus();
                CaptureMouse();
                IsDragging = true;
                DragStart = GetPos(e);
                bool flag = true;
                try
                {
                    RaiseEvent(new DragStartedEventArgs(0, 0));
                    flag = false;
                }
                finally
                {
                    if (flag)
                    {
                        CancelDrag();
                    }
                }
            }
        }

        protected override void OnMouseMove(MouseEventArgs e)
        {
            OnMouseMove(e, e.MouseDevice.LeftButton == MouseButtonState.Pressed);
        }

        // for testing purposed as the button press states can't be faked on the mousedevice
        public void OnMouseMove(MouseEventArgs e, bool pressed)
        {
            if (!IsDragging)
            {
                return;
            }

            if (pressed)
            {
                e.Handled = true;
                var delta = GetRelPos(e);
                RaiseEvent(new DragDeltaEventArgs(delta.X, delta.Y));
            }
            else
            {
                if (e.MouseDevice.Captured == this)
                {
                    ReleaseMouseCapture();
                }
                IsDragging = false;
            }
        }

        protected override void OnMouseLeftButtonUp(MouseButtonEventArgs e)
        {
            if (base.IsMouseCaptured && IsDragging)
            {
                e.Handled = true;
                ClearValue(IsDraggingPropertyKey);
                ReleaseMouseCapture();
                var delta = GetRelPos(e);
                RaiseEvent(new DragCompletedEventArgs(delta.X, delta.Y, canceled: false));
            }
        }
    }