Generating events
-----------------
To make sense of the many things going on in a server, monitoring items can
be useful. Though in many cases, data change does not convey enough
information to be the optimal solution. Events can be generated at any time,
hold a lot of information and can be filtered so the client only receives the
specific attributes of interest.

Emitting events by calling methods
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The following example will be based on the server method tutorial. We will be
creating a method node which generates an event from the server node.

The event we want to generate should be very simple. Since the `BaseEventType` is abstract,
we will have to create our own event type. `EventTypes` are saved internally as `ObjectTypes`,
so add the type as you would a new `ObjectType`.

.. code-block:: c

   
   static UA_NodeId eventType;
   
   static UA_StatusCode
   addNewEventType(UA_Server *server) {
       UA_ObjectTypeAttributes attr = UA_ObjectTypeAttributes_default;
       attr.displayName = UA_LOCALIZEDTEXT("en-US", "SimpleEventType");
       attr.description = UA_LOCALIZEDTEXT("en-US", "The simple event type we created");
       return UA_Server_addObjectTypeNode(server, UA_NODEID_NULL,
                                          UA_NS0ID(BASEEVENTTYPE), UA_NS0ID(HASSUBTYPE),
                                          UA_QUALIFIEDNAME(0, "SimpleEventType"),
                                          attr, NULL, &eventType);
   }
   
Creating an event
^^^^^^^^^^^^^^^^^
In order to set up the event, we can first use ``UA_Server_createEvent`` to
give us a node representation of the event. All we need for this is our
`EventType`. Once we have our event node, which is saved internally as an
`ObjectNode`, we can define the attributes the event has the same way we
would define the attributes of an object node. It is not necessary to define
the attributes `EventId`, `ReceiveTime`, `SourceNode` or `EventType` since
these are set automatically by the server. In this example, we will be
setting the fields 'Message' and 'Severity' in addition to `Time` which is
needed to make the example UaExpert compliant.

.. code-block:: c

   
   static UA_StatusCode
   createEventMethodCallback(UA_Server *server,
                             const UA_NodeId *sessionId, void *sessionHandle,
                             const UA_NodeId *methodId, void *methodContext,
                             const UA_NodeId *objectId, void *objectContext,
                             size_t inputSize, const UA_Variant *input,
                             size_t outputSize, UA_Variant *output) {
       UA_LOG_INFO(UA_Log_Stdout, UA_LOGCATEGORY_APPLICATION, "Creating event");
   
       UA_UInt16 severity = 100;
       UA_LocalizedText message = UA_LOCALIZEDTEXT("en-US", "An event has been generated.");
       UA_StatusCode res = UA_Server_createEvent(server, UA_NS0ID(SERVER), eventType,
                                                 severity, message, NULL, NULL, NULL);
       if(res != UA_STATUSCODE_GOOD) {
           UA_LOG_WARNING(UA_Log_Stdout, UA_LOGCATEGORY_APPLICATION,
                          "Creating event failed. StatusCode %s",
                          UA_StatusCode_name(res));
       }
       return res;
   }
   
Now, all that is left to do is to create a method node which triggers the
event creation for the example.

.. code-block:: c

   
   int main(void) {
       UA_Server *server = UA_Server_new();
   
       addNewEventType(server);
   
       UA_MethodAttributes generateAttr = UA_MethodAttributes_default;
       generateAttr.description = UA_LOCALIZEDTEXT("en-US","Generate an event.");
       generateAttr.displayName = UA_LOCALIZEDTEXT("en-US","Generate Event");
       generateAttr.executable = true;
       generateAttr.userExecutable = true;
       UA_Server_addMethodNode(server, UA_NODEID_NUMERIC(1, 62541),
                               UA_NS0ID(OBJECTSFOLDER), UA_NS0ID(HASCOMPONENT),
                               UA_QUALIFIEDNAME(1, "Generate Event"),
                               generateAttr, &createEventMethodCallback,
                               0, NULL, 0, NULL, NULL, NULL);
   
       UA_Server_runUntilInterrupt(server);
       UA_Server_delete(server);
       return 0;
   }

