eclipse中，继承ViewPart绘制ScrollingGraphicalViewer，怎么设置可以时用户在拖动ViewPart边框时ScrollingGraphicalViewer的content显示始终居中

要使ScrollingGraphicalViewer的content始终居中，你需要在ViewPart的createPartControl方法中设置GraphicalViewer的RootEditPart。RootEditPart是图形编辑器的根部件，你可以通过设置它的Viewport的位置来控制content的显示位置。

以下是示例代码：

public class YourViewPart extends ViewPart {
    private ScrollingGraphicalViewer viewer;

    @Override
    public void createPartControl(Composite parent) {
        viewer = new ScrollingGraphicalViewer();
        viewer.createControl(parent);

        // 设置RootEditPart
        viewer.setRootEditPart(new ScalableRootEditPart() {
            @Override
            public void setScale(double zoom) {
                super.setScale(zoom);
                // 缩放时居中显示
                centerViewport();
            }

            @Override
            protected void refreshViewport() {
                super.refreshViewport();
                // 刷新时居中显示
                centerViewport();
            }

            private void centerViewport() {
                IFigure figure = getLayer(LayerConstants.PRIMARY_LAYER);
                if (figure instanceof ScalableLayeredPane) {
                    ScalableLayeredPane layeredPane = (ScalableLayeredPane) figure;
                    Rectangle bounds = layeredPane.getBounds().getCopy();
                    layeredPane.translateFromParent(bounds);
                    layeredPane.getClientArea(Rectangle.SINGLETON);
                    bounds.translate(-Rectangle.SINGLETON.x, -Rectangle.SINGLETON.y);
                    Point viewLocation = new Point(-bounds.x, -bounds.y);
                    Point viewportLocation = layeredPane.getViewport().getViewLocation();
                    Dimension viewportSize = layeredPane.getViewport().getClientArea().getSize();
                    Point centerOffset = new Point((viewportSize.width - bounds.width) / 2,
                            (viewportSize.height - bounds.height) / 2);
                    Point newViewportLocation = new Point(centerOffset.x - viewLocation.x,
                            centerOffset.y - viewLocation.y);
                    if (!viewportLocation.equals(newViewportLocation)) {
                        layeredPane.getViewport().setViewLocation(newViewportLocation);
                    }
                }
            }
        });

        // 设置内容
        viewer.setContents(createContents());
    }

    private IFigure createContents() {
        // 创建RootFigure
        IFigure root = new FreeformLayer();
        root.setLayoutManager(new FreeformLayout());

        // 创建子Figure
        RectangleFigure rect = new RectangleFigure();
        rect.setBackgroundColor(ColorConstants.lightGray);
        rect.setBounds(new Rectangle(100, 100, 200, 100));
        root.add(rect);

        return root;
    }

    @Override
    public void setFocus() {
        viewer.getControl().setFocus();
    }
}

在上面的代码中，我们重写了ScalableRootEditPart的setScale和refreshViewport方法，在缩放和刷新时都调用了centerViewport方法，该方法会计算出content居中显示时Viewport的位置，并设置给ScalableLayeredPane的Viewport。注意，我们在创建RootEditPart时使用了FreeformLayout，这样可以自由布局子Figure。

这样，当用户拖动ViewPart边框时，GraphicalViewer的content会始终居中显示。