I have seen something called upcasting and downcasting (as contrasted with broadcasting). Mostly, I have seen it in UI layers.
e.g. a leaf node receives a change notification, decides it isn't interested in handling the notification, and so passes it up to its parent, etc. Downcasting is similar but in the opposite direction. e.g. a window receives an event and asks its children to handle it, and they do the same. In this case especially, the one that handles the event needs to mark it as handled so that no one else is asked to handle it. On April 27, 2021 5:49:56 AM HST, Tim Mackinnon <tim@testit.works> wrote: >Hi guys - yes that fire and forget was always how I had viewed >announcements, and potentially my scenario is misusing "announcements" >which is why I was interested in views here. > >However - as announcements are typically a mechanism for farming out >processing to others - how does one handle the scenario that you might >only want one object to handle the announcement (and not others)? > >in my specific example, with MVP - a view, totally decoupled in a web >browser, wants to pass on a button click to a presenter - however it >not clear what the best mechanism to bubble up that handling should be? >If the immediate presenter of the view can handle it - great - but if >not, how would you continue to broadcast wider to see if someone higher >up the parent component chain can handle it? > >If you go fire and forget (which is how I had viewed announcements >prior to this specific case) - then you have the problem a different >way - when someone processes an announcement it might already have been >handled earlier and so the consumer either has to check first, or their >work is simply ignored as the announcement payload could just filter a >response out. > >In my head, I'm sort of thinking this is akin to CPU interrupt chaining >- where an interrupt can cascade up a stack if not handled OR like >object inheritance where an object can choose to override a message and >stop it from cascading up the inheritance chain. > >Maybe announcements aren't intended for this at all - although it seems >an elegant way to handle it - with just 1 method addition to Announcer >(but it does slightly expose things - albeit in a structured way - and >similar to the similar #hasSubscriber:). > >I'm sure it "depends" - but interested in other observations from the >field, as I'm sure I'm not the first person to hit something like this. > >Tim > >On Tue, 27 Apr 2021, at 3:51 PM, Sven Van Caekenberghe wrote: >> The whole idea is to decouple producers and consumers, like in >> messaging systems. >> >> You should not care if there are other listening, just like the >> listeners should not care if there is someone posting data. >> >> Asking for subscribers is introducing a coupling. >> >> The announcement mechanism will/should deal with this in an efficient >way. >> >> > On 27 Apr 2021, at 16:03, Tim Mackinnon <tim@testit.works> wrote: >> > >> > From my rather long ramble below - I am still curious if its >distasteful to have a method on Announcer >> > >> > hasSubscriptionsHandling: anAnnouncement >> > "Answer true if I have any subscribers to anAnnouncement" >> > >> > ^(registry subscriptionsHandling: anAnnouncement ) notEmpty >> > >> > Tim >> > >> > On Thu, 22 Apr 2021, at 11:34 PM, Tim Mackinnon wrote: >> >> Hi everyone - I’ve always thought the article on announcements >many >> >> years ago was very cool - and we don’t seem to use them as much as >we >> >> could (but equally they aren’t a panacea to be overused everywhere > >> >> either - and they do get used in Pharo to some extent). >> >> >> >> Anyway, I’ve been playing around with CodeParadise (CP is a very >cool >> >> project, and Erik is very supportive and thinking about how to >write >> >> web apps a different way… I’m fascinated), >> >> >> >> And - CP uses announcements as mechanism to send events from the >View >> >> Client (in a web browser) to a Presenter on the server (which >makes >> >> total sense). >> >> >> >> In taking things for a spin, I hit an interesting problem on how >in a >> >> web component world, you should display a spelling test of words - > >> >> >> >> e.g. SpellingTest — has many —> SpellingWord(s). >> >> >> >> >> >> Initially I bunged it all in a single presenter with its >associated >> >> view, and it was a bit messy, and Erik guided me down a route >(that CP >> >> nicely supports) - that my SpellingTest view should have the >name/date >> >> of the test as well as an add word input field, but the list of >current >> >> Words (which I had bunged into a table) - were actually more >elegant as >> >> sub-components - hence a WordView - which renders a single word in >a >> >> DIV, and for the edit screen I was creating, a Delete button next >to >> >> the word (so you could delete it). So a 1 to many relationship >> >> essentials. >> >> >> >> This is where the announcements kick in (and lead to my ultimate >question). >> >> >> >> When you click the Delete button, if I use a sub component - my >view >> >> will generate a DeleteWordAnnouncement - which gets fed to my >> >> SpellingWordPresenter - however words in this sense don’t >naturally >> >> know their parent (the SpellingTest) - and its the parent test >that has >> >> a #deleteWord: method. >> >> >> >> I’ve been taking with Erik, on different ways to elegantly handle >this. >> >> >> >> a) you could change the model so words know their parent (in my >case, >> >> I’m using a 3rd party model for Flashcards, and they just don’t >know >> >> this - and adapting them would be a nuisance >> >> b) my TestPresenter could listen to announcements on the >WordPresenter >> >> - and I could get some communications between presenters (although > >> >> normally the Presenters just get events from Views, and pure >domain >> >> models - so it feels a bit abnormal to consider another Presenter >as a >> >> sort of model - but I could live with this >> >> c) given the composable nature of views/presenters (and CP is base >on a >> >> WebComponent model) - you could bubble up Announcements, so that >if an >> >> event isn’t handled by a view’s immediate presenter, you could >re-route >> >> it to the parent of the View (the component owner) and see if it’s > >> >> presenter could do something. >> >> >> >> >> >> I think (c) has a certain expectation to it - in fact when I >converted >> >> my initial one-presenter attempt into components, I still had >listener >> >> code in my TestPresenter that was expecting to get a deleteWord >> >> announcement and I was initially surprised that I wasn’t getting >it (as >> >> it was now just going to the Word component I had refactored out). > >> >> >> >> So I wonder if others here would expect things to work this way >too >> >> (and are there other examples in the wild that lead you here - or >scare >> >> you away from this?). >> >> >> >> Back to my Announcement question - if C is a good idea - why >doesn’t >> >> the Announcer class let you check if if will handle a particular >> >> announcement? The API has #hasSubscriber: and >#hasSubscriberClass: , >> >> but its missing: >> >> >> >> hasSubscriptionsHandling: anAnnouncement >> >> "Answer true if I have any subcribers to anAnnouncement" >> >> >> >> ^(registry subscriptionsHandling: anAnnouncement ) notEmpty >> >> >> >> >> >> And I am wondering if this is because it's a bad thing to expect >to be >> >> able to check? In my case above, I would want to do this to know >if CP >> >> should instead try announcing a message to a parent presenter >because >> >> the current presenter won’t handle it. In my example above, my >> >> WordComponentView will broadcast that the delete button was >clicked, >> >> but its actually a parent view which would reasonably want to >listen to >> >> this kind of event and process the delete. And in a many words >> >> scenario (the Test has many words), its unrealistic for the parent >to >> >> register to listen to each word component individually (in fact CP >sort >> >> of hides this from you), however if you could listen to an event >in >> >> your TestView, it seems to come out quite nicely - and looks a >bit >> >> like this: >> >> >> >> viewCreated >> >> super viewCreated. >> >> >> >> self view >> >> when: CpNavigationAnnouncement >> >> do: [ :action | self model goto: action location ]; >> >> >> >> when: CpAddWordAnnouncement do: [ :action | self addWord: action >data >> >> ]; >> >> when: CpDeleteWordAnnouncement do: [ :action | self deleteWord: >> >> action data ]. <— this is the one I’m talking about >> >> >> >> >> >> So I’m curious on the overall thought process here, but in >particular >> >> whether I should even submit a PR on Announcer for >> >> #hasSubscriptionsHandling: ? >> >> >> >> Tim >> >> >> >> >>
