GWT UiBinder:如何制作一个使用"at"元素的自定义AbsolutePanel？

import com.google.gwt.user.client.ui.AbsolutePanel;

public class CustomAbsolutePanel extends AbsolutePanel {
    private String at;

    public void setAt(String at) {
        this.at = at;
    }

    public String getAt() {
        return at;
    }
}

<!DOCTYPE ui:UiBinder SYSTEM "http://dl.google.com/gwt/DTD/xhtml.ent">
<ui:UiBinder xmlns:ui="urn:ui:com.google.gwt.uibinder"
    xmlns:g="urn:import:com.google.gwt.user.client.ui">
    <ui:style>
        .customAbsolutePanel {
            background-color: #f5f5f5;
            border: 1px solid #ddd;
            padding: 10px;
        }
    </ui:style>
    <g:HTMLPanel>
        <g:CustomAbsolutePanel addStyleNames="{style.customAbsolutePanel}" at="customAtValue">
            <!-- Add your widgets here -->
        </g:CustomAbsolutePanel>
    </g:HTMLPanel>
</ui:UiBinder>

CustomAbsolutePanel customAbsolutePanel = (CustomAbsolutePanel) yourUiBinder.createAndBindUi();
String atValue = customAbsolutePanel.getAt();