Runtime components : Core components : Events : Tasks : Implementing event notification
  
Implementing event notification
To implement event notification, perform the following tasks:
1 Implement the com.ibm.btt.base.Notifier interface. For example, suppose a client application needs to know about all printer error messages from any printer in the network. To accomplish this purpose you need to create a DSENotifier subclass called SampleNotifier that instantiate on the server workstation for the printer services. This class also implements a printerError method called by any service that is accessing a printer when the printer fails. SampleNotifier signals a toolkit event with the following arguments:
The name of the error message passed as an argument of printerError method
Attributes identifying the printer address in the network
The document to be printed when the failure occurred
public class SampleNotifier extends DSENotifier {
public DSEEventObject printerEvent;
}
public SampleNotifier () {
super();
}
public SampleNotifier(String aName) {
super(aName);
}
public void printerError(String aPrt, String aDocName, String message) {
Hashtable eventAttributes = new Hashtable();
eventAttributes.put ("printerAddress", aPrt);
eventAttributes.put ("document", aDocName);
printerEvent = new DSEEventObject("mMessage",this,eventAttributes);
signalEvent(printerEvent);
}
You can integrate the standard Java events with the event management in the toolkit.
2 Implement the signalEvent method for each event that the notifier can send out.
Note The standard Java event notification mechanism will call any other local listeners for that event.
3 Create a handler for the event and implement a dispatchEvent method to provide the behavior to process the event. For example, the SampleNotifier uses the PrinterEventsHandler:
public class PrinterEventsHandler extends DSEHandler {
}
public Handler dispatchEvent(DSEEventObject anEvent) {
// The dispatch method displays a string in the system standard output
// containing the event information
System.out.println("The event"+ anEvent.getName()+ "has been signaled. Printer:"+
anEvent.getParameters().get("printerAddress")+" Document:"
+anEvent.getParamenters().get("document"));
// The handler enables further event propagation by returning this
return this;
}
In this case, the dispatchEvent method passes a message to the system standard output.
Note If you do not want to propagate this event to the next registered handler, the return value of dispatchEvent must be null.
4 Add the handler registration to the client operation that requires event notification as shown in the following example:
import com.ibm.dse.base.*;
import com.ibm.dse.clientserver.*;
public class myClientOperation extends DSEClientOperation {
public PrinterEventsHandler eventsHandler;
}
public void execute() throws Exception() {
// If the client workstation where the application is running has
// sessions with different server workstations, the application must
// know which CSClient instance identifies the session with the
// server that fires the event. This can be requested by name
// from the Context class. In this case, the name of the CSClient
// instance that establishes the session with a server
// is "CSSessionTID".
CSClientService clientServerSession = (CSClientService)getService("CSSessionTID");
EventManager.registerInterestInRemoteEvent("allEvents", "SampleNotifier",
clientServerSession);
// The previous statements can be run only once in the lifecycle
// of the application, possibly at application startup.
// At any time, the application can instantiate a handler and
// register it for events coming from the SampleNotifier object.
eventsHandler= new PrinterEventsHandler;
eventsHandler.handleEvent("allEvents", "SampleNotifier", getContext());
// The application can continue its operation flow. The handler
// processes the event as soon as it is fired.
}
When the SampleNotifier instance sends out an event, it calls the dispatchEvent method of the PrinterEventsHandler. The handler displays a message in the system standard output.
Create a DSEEventObject instance. DSEEventObject is a subclass of EventObject with the following attributes:
name - the name of the event to which a handler should be registered
sourceName - the name of the notifier that signals the event
parameters - a Hash table containing the event information required by the event handler
originWorkstation - the TID of the workstation generating the event
One of the signalEvent methods accepts the DSEEventObject argument.
If the application is already working with an EventObject instance, use the signalEvent method that accepts a string with the event name and, if required, a Hash table containing the event attributes as arguments. This method builds a DSEEventObject that the event manager sends to the listening remote workstation.
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