Renders list of descriptors for all the flows.

def Handle(self, args, token=None):
    """Renders list of descriptors for all the flows."""

    if data_store.RelationalDBEnabled():
      flow_iterator = iteritems(registry.FlowRegistry.FLOW_REGISTRY)
    else:
      flow_iterator = iteritems(registry.AFF4FlowRegistry.FLOW_REGISTRY)

    result = []
    for name, cls in sorted(flow_iterator):

      # Flows without a category do not show up in the GUI.
      if not getattr(cls, "category", None):
        continue

      # Only show flows that the user is allowed to start.
      try:
        if self.access_check_fn:
          self.access_check_fn(token.username, name)
      except access_control.UnauthorizedAccess:
        continue

      result.append(ApiFlowDescriptor().InitFromFlowClass(cls, token=token))

    return ApiListFlowDescriptorsResult(items=result)