LightweightSystem.setContents代码解读

LightweightSystem.setContents(IFigure figure))  实际上是将figure放到LWS的RootFigure中，作为RootFigure的唯一孩子。
源代码如下：

/**
 * Sets the contents of the LightweightSystem to the passed figure. This figure should be
 * the top-level Figure in a Draw2d application.
 *
 * @param figure the new root figure
 * @since 2.0
 */
public void setContents(IFigure figure) {
 if (contents != null)
  root.remove(contents);
 contents = figure;
 root.add(contents);
}