Event is the mechanism that the toolkit uses to communicate between a client, server, host, and between the various components in the toolkit.
Event notification consists of defined structures for sending a toolkit-specific event (signaled by an event notifier) and for receiving the event that processed by an event handler. The handler processes the event or propagates it to other handlers.
An event manager acts as the event controller that controls notifiers and handlers to manage both local and remote events. For remote event notifications between a client workstation and a server workstation, an event manager instance exists on the client workstation and another instance exists on the server workstation. The local event manager listens to arriving event and sends newly received event to objects which are interested in receiving the event. If the object is located in a remote workstation, the default behavior is to send the event to the remote workstation's event manager so that it can then propagate the event to its local handlers.
The event manager is implemented using the EventManager class. The EventManager class provides two event notification interfaces to perform two different functions:
▪ Firstly, register a workstation as a notifier, or a subscriber for an event coming from a specific remote workstation. A workstation can be a sender and receiver simultaneously and can be a client or server.
▪ Secondly, register a handler for a specific local or remote event.
The following guide shows how and when you can use these interfaces:
▪ During the application startup process, the client workstation or a server workstation can be registered as a handler for specific remote events signaled by specific notifiers. For remote events from server workstation, register the client event manager as a handler with the event manager on the server workstation. For local events to remote server workstation, register the client event manager as a handler so that it can pass the event to the event manager of the server workstation listening for this event. By doing this registration during the application startup process, the handling and notification for these events keeps active during the whole life cycle of the application.
Note At any time during the life of the application, the workstation can be registered as a handler of a remote event (though this does not happen frequently), or remove its handler identity.
▪ During a normal operation process, a client or server workstation can register a handler to manage an event from a notifier. If the notifier is local, the event manager adds the handler to the notifier's list of handlers. If the notifier is remote (assumed if the event manager can not find the notifier on the same workstation), the event manager adds the handler to its own list of handlers. To send events to the client, the server workstation registers itself as a handler and uses the dispatch method to send the event to the client workstation. Right after it sends the event, the server workstation can remove itself from the notifier's list of handlers and finish its processing.