[xwiki-devs] New Observation component: Usage scenarios

Hi, How do you feel about the next usage scenarios for the observation component? A. Simple observing of changes This is the most common usage of the old notification mechanism. It allows a listener to see when something gets changed. It receives as the source the changed document, from which it can get the original document (there's a doc.getOriginalDocument() method for this). For the moment, the data argument is the context, as it is still a vital object for most of the code, and it has to be passed around all the time. The event will hold the type of change, like adding a new document, deleting, or just saving a new version. In the future I'd like to increase the granularity, to make this "change" a set of basic changes, to be able to say that (object added, attachment removed, content changed) is what happened to the document (see http://jira.xwiki.org/jira/browse/XE-213 too). With this event, a listener can then update some data indexer, like the Lucene plugin does, or send the new documents/attachments to a repository, or send a mail with the new code, or ping on IRC. B. Simple observing of actions This is also implemented in the old notification mechanism. It allows a listener to monitor actions. It receives as the source the affected document, and as the additional data the same context, as it is needed. The event holds the action name, and I'd like to add the full URL, too, as other important information might be in there. I don't know if sending the complete request object is a good idea, but I think we should be able to obtain it somehow. With this event, a listener can update some statistics, refresh some caches or cleanup some indexes, send an attachment to another repository (like SVN), or, based on the URL, it can be used to take some action, like if the action was "createspace" and the "notify=admins" parameter was given in the request, then the observer can send the notifications, without polluting the application code with the checks and mail sending calls. C. Precondition/Postcondition enforcing The previous two use cases allow a simple ping that something has already happened. (Actually, the listeners can also change the passed document, as the actual object is sent.) But I'd like to be able to write a listener that can say "this is not allowed, please stop this action", or "these changes are not OK, don't save this document". So, a precondition can be that "/save/" cannot be performed between 6PM and 7AM, which would be a needed feature in some companies, or that "/view/" cannot be performed on the "SectionA" space from the IP addresses like 192.168.2.*, which belongs to the SectionB users (these are action events). And a postcondition can be that the XWiki.XWikiPreferences can only be updated by changing XWikiGlobalRights objects, other types of changes can only be performed by a superadmin only, or that adding some content that looks like spam is not allowed (these are document events). So, on top of the previous use cases, I'd like to add a "phase" to the events, which indicates if the event is sent before or after the change (or we can just add new events), and a new type of exception thrown by the listeners, which is not ignored. Right now, if a listener throws an exception, it is ignored. This exception, when received by the code that sends the notification, should stop the normal processing and display "action not allowed" + ex.getMessage to the user. D. Rendering hooks While working on the SkinExtensions and trying to write a better #toc macro, I was faced with the problem that there's no way a plugin can change the rendered content at the end. Yes, it can alter it line by line, and in theory there is an endRenderingHook sent with the content, but that is actually buggy, as an empty string is sent instead of the content, and the result is discarded. Still, even if fixed, this is sent only for *rendered* content, not *parsed* content. My initial idea was to change the PluginInterface by adding two new methods that will be called before and after the content was generated, but that is a bad idea, since the PluginInterface is already too big and has too many types of methods. And since the new rendering component is not yet available, I thought that we can also use events for this. So, instead of writing new methods in the PluginInterface, we can add other type of events, which gets sent in the parsing/rendering process. That way, we don't need to have a plugin that implements all these methods, and adding a new feature will require just sending a new type of event, instead of changing an interface (which is dangerous). The source would be an object holding the string, which can be modified by the listeners. Other information could be the content document, if there is one, the rendering/parsing context, anything else? E. Other generic events Like FlushCacheEvent, StartupEvent, ShutdownEvent, VirtualInitEvent, VirtualShutdownEvent (sent when a wiki is deleted by the WikiManagerEvent), Application[Added/Enabled/Disabled/Removed]Event (sent by the ApplicationManager), Space[Added/Removed]Event (sent by the SpaceManagerPlugin), etc. Comments, please? -- Sergiu Dumitriu http://purl.org/net/sergiu/