/// <summary>
        /// Add new object to drawing canvas.
        /// Function is called when user left-clicks drawing canvas,
        /// and one of ToolObject-derived tools is active.
        /// </summary>
        protected static void AddNewObject(DrawingCanvas drawingCanvas, GraphicsBase o)
        {
            HelperFunctions.UnselectAll(drawingCanvas);

            o.IsSelected = true;
            drawingCanvas.SelectedObject = o;
            o.Clip = new RectangleGeometry(new Rect(0, 0, drawingCanvas.ActualWidth, drawingCanvas.ActualHeight));

            drawingCanvas.GraphicsList.Add(o);
            drawingCanvas.CaptureMouse();
        }
        /// <summary>
        /// Handle mouse down.
        /// Start moving, resizing or group selection.
        /// </summary>
        public override void OnMouseDown(DrawingCanvas drawingCanvas, MouseButtonEventArgs e)
        {
            commandChangeState = null;
            wasMove = false;

            Point point = e.GetPosition(drawingCanvas);

            selectMode = SelectionMode.None;

            GraphicsBase o;
            GraphicsBase movedObject = null;
            int handleNumber;

            // Test for resizing (only if control is selected, cursor is on the handle)
            for (int i = drawingCanvas.GraphicsList.Count - 1; i >= 0; i--)
            {
                o = drawingCanvas[i];

                if ( o.IsSelected )
                {
                    handleNumber = o.MakeHitTest(point);

                    if (handleNumber > 0)
                    {
                        selectMode = SelectionMode.Size;

                        // keep resized object in class member
                        resizedObject = o;
                        resizedObjectHandle = handleNumber;

                        // Since we want to resize only one object, unselect all other objects
                        HelperFunctions.UnselectAll(drawingCanvas);
                        o.IsSelected = true;

                        commandChangeState = new CommandChangeState(drawingCanvas);

                        break;
                    }
                }
            }

            // Test for move (cursor is on the object)
            if (selectMode == SelectionMode.None)
            {
                for (int i = drawingCanvas.GraphicsList.Count - 1; i >= 0; i--)
                {
                    o = drawingCanvas[i];

                    if (o.MakeHitTest(point) == 0)
                    {
                        movedObject = o;
                        break;
                    }
                }

                if (movedObject != null)
                {
                    selectMode = SelectionMode.Move;

                    // Unselect all if Ctrl is not pressed and clicked object is not selected yet
                    if (Keyboard.Modifiers != ModifierKeys.Control  && ! movedObject.IsSelected)
                    {
                        HelperFunctions.UnselectAll(drawingCanvas);
                    }

                    // Select clicked object
                    movedObject.IsSelected = true;
                    drawingCanvas.SelectedObject = movedObject;

                    // Set move cursor
                    drawingCanvas.Cursor = Cursors.SizeAll;

                    commandChangeState = new CommandChangeState(drawingCanvas);
                }
            }

            // Click on background
            if (selectMode == SelectionMode.None)
            {
                // Unselect all if Ctrl is not pressed
                if (Keyboard.Modifiers != ModifierKeys.Control)
                {
                    HelperFunctions.UnselectAll(drawingCanvas);
                }

                // Group selection. Create selection rectangle.
                GraphicsSelectionRectangle r = new GraphicsSelectionRectangle(
                    point.X, point.Y,
                    point.X + 1, point.Y + 1,
                    drawingCanvas.ActualScale);

                r.Clip = new RectangleGeometry(new Rect(0, 0, drawingCanvas.ActualWidth, drawingCanvas.ActualHeight));

                drawingCanvas.GraphicsList.Add(r);

                selectMode = SelectionMode.GroupSelection;
            }

            lastPoint = point;

            // Capture mouse until MouseUp event is received
            drawingCanvas.CaptureMouse();
        }