On Nov 24, 2011, at 4:55 PM, Denis Gervalle wrote: > On Thu, Nov 24, 2011 at 16:25, Vincent Massol <vinc...@massol.net> wrote: > >> >> On Nov 24, 2011, at 4:06 PM, Denis Gervalle wrote: >> >>> On Thu, Nov 24, 2011 at 13:58, Vincent Massol <vinc...@massol.net> >> wrote: >>> >>>> Hi devs, >>>> >>>> Summary: >>>> ======== >>>> >>>> I'd like to add the notion of Priority to Event Listeners. The reason is >>>> that in some cases it's important that some listeners execute before >> others. >>>> >>>> The problem at hand: >>>> ================= >>>> >>>> Here's a typical use case: When receiving the ApplicationStartedEvent, >> we >>>> have lot of code that needs to initialize. Initialization order is >>>> important (you can compare it to run levels in OS): for example some >> init >>>> must happen after the Database initialization has happened. >>>> >>>> Note that another solution exists for this use case: some >> initializations >>>> could introduce their own events (such as a DatabaseStartedEvent) and >> other >>>> init could listen on those events instead of the generic >>>> ApplicationStartedEvent. However I can see several drawbacks to this: >>>> * it's less generic than the priority solution >>>> * it means creating more events >>>> * but more importantly it means that modules will have strong >> dependencies >>>> (at maven level) on each other whereas it's not necessary and shouldn't >> be >>>> the case. In our example use case: it means that inits that must happen >>>> after database is started will need to depend on oldcore (which is >> where DB >>>> is started ATM) >>>> >>>> Proposal: >>>> ======== >>>> >>>> * Don't break backward compat in Observation module >>>> * Introduce a PrioritizedEventListener interface that adds a >> getPriority() >>>> method >>>> * Modify ObservationManager implementation to take into account >> priorities >>>> * In order to make it simple I propose to have only a single priority >> per >>>> Listener and not a priority per event supported by a given listener >>>> >>>> General Context >>>> ============= >>>> >>>> To give some context here's what I'd like to do on the medium term: >>>> >>>> * Step 1: Introduce notion of priority in EventListeners >>>> * Step 2: Refactor XWiki init to use an EventListener on AppStarted with >>>> low priority >>>> * Step 3: Refactor wiki macros to use an EventListener on AppStarted >> with >>>> priority value lower than at step2 >>>> * Step 4: Write an EventListener for the new UI Extensions with a >> priority >>>> value higher than the one of step2 <-- this is the initial goal that >> led >>>> me to make this proposal ;) >>>> >>>> WDYT? >>>> >>> >>> Sounds good if not overkill for the goal. >>> An simpler alternative would be to have more than a single AppStarted >>> event, like there is more than one starting level in Linux. >>> Let say one level before XWiki, the one during, the one after ? is there >> so >>> many other use case ? >> >> > Well, I have said +1 anyway, but... > > >> Yes this is very close to the other solution I explained above. >> > > Surely. > > >> But it's far less generic and introduces knowing stuff you don't really >> need to know. For me it's a "poorman" implementation of priorities. >> > > Well, it depends on the pursued goal. What I mostly dislike in priority > systems, it the management of priorities. When you say that you are > listening on a let says DatabaseStarted event, you clearly explain what you > really need. On the other hand, when you say that you want to be run at > priority 100 of the AppStartedEvent, without another document saying that > starting at 100, database is ready, and ensuring this in the database > module, you do not really know what it means really.
Yes it's static dependency vs loose dependency. One is statically typed the other is documentation. But that's the only way I know to not have modules depend on each other. I like the static way of course but what you proposed while it's good for this specific use case doesn't solve all other use cases, which is why I'm ambivalent about it and why I think the generic solution is a bit better (I could be convinced otherwise with enough arguing and if enough devs prefer the non generic way ;)). Thanks -Vincent >> I don't know about other use cases right now but I can imagine a lot of >> them (for example, some extensions that needs open office to be >> initialized, etc). We need to remember that we're developing a platform and >> not just for XE's needs. The extra amount of work is negligible IMO not to >> do it (btw I have a working version locally already, I just need to fine >> tune it a bit for improved performance maybe). >> > > > >> >> Thanks >> -Vincent >> >> _______________________________________________ >> devs mailing list >> devs@xwiki.org >> http://lists.xwiki.org/mailman/listinfo/devs >> > > > > -- > Denis Gervalle > SOFTEC sa - CEO > eGuilde sarl - CTO > _______________________________________________ > devs mailing list > devs@xwiki.org > http://lists.xwiki.org/mailman/listinfo/devs _______________________________________________ devs mailing list devs@xwiki.org http://lists.xwiki.org/mailman/listinfo/devs
