@Override
    public Topology currentTopology() {
        Iterable<Device> devices = manager.getVirtualDevices(networkId())
                .stream()
                .collect(Collectors.toSet());
        Iterable<Link> links = manager.getVirtualLinks(networkId())
                .stream()
                .collect(Collectors.toSet());

        DefaultGraphDescription graph =
                new DefaultGraphDescription(System.nanoTime(),
                                            System.currentTimeMillis(),
                                            devices, links);
        return new DefaultTopology(PID, graph);
    }
@Test
    public void testCurrentTopology() {
        VirtualNetwork virtualNetwork = setupVirtualNetworkTopology();

        TopologyService topologyService = manager.get(virtualNetwork.id(), TopologyService.class);
        Topology topology = topologyService.currentTopology();
        assertNotNull("The topology should not be null.", topology);
    }