Proper use of ITopicSubscriber and OSGi Events

[Jan Thielemann]

Hello everybody,

I recently created a tutorial on how to use the ServerPushTemplate, IServerPushCallback and ITopicSubscriber (http://wiki.idempiere.org/en/Developing_Plug-Ins_-_ServerPushTemplate,_IServerPushCallback_and_ITopicSubscriber).

Today a coworker found out a bug. If two users log in, they can send each other messages. If one of the users logs out and the other one tries to send him a message, the message is still caught in the onMessage() method from the ITopicSubscriber which results in a NPE thrown by template.executeAsynch(callback) because the desktop is null after the first user logged out.

I already tried to unsubscribe the DashboardPanel in the onPageDetached() method with the following code but it didn't work:

IMessageService service = Service.locator().locate(IMessageService.class).getService();

if (service != null) {

ITopic<Map<String, Integer>> intopic = service.getTopic(ON_USER_LOGGED_IN_TOPIC);

intopic.unsubscribe(this);
}

I wonder what i am doing wrong. What should i do to stop my DashboardPanel from receiving messages?

I saw that in DPRecentItems, there are also some other things i don't use yet: 

• org.zkoss.zk.ui.util.DesktopCleanup
  
• org.osgi.service.event.EventHandler
• org.osgi.service.event.Event
• java.lang.ref.WeakReference
  
• (custom) TopicSubscriber

Do i need to use all the classes above so that i don't get exceptions from "dead" DashboardPanels? Do i need to use it in the same way it is used in DPRecentItems or are there other ways of doing it?

I hope you can help me.

[Jan Thielemann]

Found out that if you store the ITopic you subscribe to in an instance variable, unsubscribe works but i can't imagine that thats the intention behind this. Any suggestions or explanations?

[Heng Sin Low]

1. DesktopCleanup is needed since you don't always get the onPageDetached event ( for e.g, user just close the browser tab )

2. Desktop reference should be keep using WeakReference to prevent memory leakage

It seems DPRecentItems doesn't  unsubscribe from topic so that could be a bug that needs fixing ( Potential performance and memory issue ).

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[Heng Sin Low]

That's an interesting finding, will take a look when I have time.

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[Jan Thielemann]

Hi Hengsin,

please keep in mind that when you call

IMessageService service = Service.locator().locate(IMessageService.class).getService();

it is possible that the BaseActivator will create a new ServiceTracker which results in an new ITopic. So unsubsribing the topic with 

IMessageService service = Service.locator().locate(IMessageService.class).getService();

if (service != null) {

ITopic<Map<String, Integer>> intopic = service.getTopic(ON_USER_LOGGED_IN_TOPIC);

intopic.unsubscribe(this);
}

does not work most of the time.