GraphRoundtripSupport grs = new GraphRoundtripSupport();
    
// create a mapper with the tag "edgeProviderKey" 
// which will write int data for edges into attributes named "edgeIndex"
grs.addMapper("edgeProviderKey", "edgeIndex", AttributeConstants.TYPE_INT, GraphMLConstants.SCOPE_EDGE);
    
// get the LayoutGraph created by the GraphRoundtripSupport
LayoutGraph graph = grs.createRoundtripGraph();
    
// get the DataProvider with the tag "edgeProviderKey"
DataMap dp = (DataMap) graph.getDataProvider("edgeProviderKey");
    
// fill the Map with edge related data (here as example the index of the edge)
for (EdgeCursor ec = graph.edges(); ec.ok(); ec.next()) {
  Edge edge = ec.edge();
  dp.setInt(edge, edge.index());
}
// send the graph to the client
grs.sendGraph(graph, httpServletResponse);

GraphRoundtripSupport roundtripSupport = createRoundtripSupport();

// create a mapper with the tag "edgeProviderKey" 
// that will write int data for edges into attributes named "edgeIndex"
roundtripSupport.AddMapper<IEdge,string>("edgeProviderKey", "edgeIndex");

// get the properly configured IGraph created by the GraphRoundtripSupport      
IGraph graph = roundtripSupport.CreateRoundtripGraph();
      
// retrieve the IMapper instance and register some data
IMapperRegistry registry = graph.Lookup(typeof(IMapperRegistry)) as IMapperRegistry;
if (null != registry) {
  IMapper<IEdge, string> edgeIndexMapper = registry.GetMapper<IEdge,string>("edgeProviderKey");
  if( null != edgeIndexMapper ) {
    int i = 0;
    foreach( IEdge edge in graph.Edges ) {
      edgeIndexMapper.SetValue( edge, i++.ToString() );
    }
  }
}

// send the graph to the client
roundtripSupport.SendGraph(graph,context.Response);

 xmlns:xalan="http://xml.apache.org/xslt"><graphml>
  ...
  <!-- The custom GraphML attribute -->
  <key id="d0" for="edge" attr.name="edgeIndex" attr.type="int"/>
  
  <graph id="G" edgedefault="directed">
    ...
    <!-- An edge that has a <data> element referring to the GraphML attribute "d0" -->
    <edge id="e0">
	...
      <data key="d0">0</data>	<!-- The transferred value is 0 -->
    </edge>
    ...
  </graph>
</graphml>

var graph:DefaultGraph = DefaultGraph(graphCanvas.graph);
			
// register the mapper for the attribute to the graph
graph.mapperRegistry.addMapper( "clientEdgeProviderKey", new DictionaryMapper() );
			
var roundtripHandler:RoundtripHandler = new RoundtripHandler(graphCanvas);
		
// register the mapper for the attribute to the RoundtripHandler
roundtripHandler.addMapperAttribute(
			"clientEdgeProviderKey",
			"edgeIndex", 
			GraphMLConstants.SCOPE_NODE, 
			GraphMLConstants.TYPE_INT);
		...
// let the roundtripHandler read the data here
		...
// get the mapper						
var dataMapper:IMapper = DefaultGraph( graphCanvas.graph ).mapperRegistry.getMapper( "clientEdgeProviderKey" );
		
// read the data from the mapper
var mappedEdgeData:int = dataMapper.lookupValue(anEdge);