On Saturday, March 28, 2015 at 8:22:30 AM UTC+11, Colin Yates wrote: > I see, so a subscription is more than just a projection of the db, it > has a life-time where as an event handler is about a snapshot in time > (guaranteed to be now?). > > I wasn't asking about the mechanical reasons why they weren't > appropriate, more the concept, which you explained - thanks. > > On 27 March 2015 at 21:15, Mike Thompson <[email protected]> wrote: > > On Saturday, March 28, 2015 at 7:52:13 AM UTC+11, Colin Yates wrote: > >> Hi Mike, yep, that is what I meant by "> I can work around this - the > >> subscription code typically delegates to a 'plain' defn so I can > >> retrieve the data from the db and call the same defn, but sometimes > >> that jars a bit." :). > >> > >> > >> On 27 March 2015 at 20:47, Mike Thompson <[email protected]> wrote: > >> > On Saturday, March 28, 2015 at 7:36:26 AM UTC+11, Colin Yates wrote: > >> >> I have various chunks of reference data, say a tree or a list of _all_ > >> >> (i.e. active and historical) entities. I then have various > >> >> subscriptions which refine that view, for example: > >> >> - only active > >> >> - only active but including a given id > >> >> > >> >> In the handler I sometimes need access to this data, but according to > >> >> the re-frame doc: > >> >> > >> >> "Rules: > >> >> > >> >> components never source data directly from app-db, and instead, they > >> >> use a subscription. > >> >> subscriptions are only ever used by components (they are never used in, > >> >> say, event handlers)." > >> >> > >> >> I can work around this - the subscription code typically delegates to a > >> >> 'plain' defn so I can retrieve the data from the db and call the same > >> >> defn, but sometimes that jars a bit. > >> >> > >> >> The fact it is useful for me to do something not only discouraged but > >> >> actively against the rules makes me question my design somewhat; what > >> >> is the rationale for not allowing an event handler to view a > >> >> subscription? Am I wrong in viewing a subscription as merely a view on > >> >> the data in which case I don't see the danger... > >> >> > >> >> I get that components should be divorced from the structure of the DB > >> >> and event handlers necessarily need to know the structure but I see a > >> >> subscriptions as more than just structure - it sometimes applies > >> >> transformations that I would want to re-use. > > > > I'll expand ... > > > > Think about a subscription handler as: > > 1. A query function (db) -> val ... > > 2. some reaction wrapping around the outside > > > > The reaction wrapping is very useful for when you need "a stream" of > > updates over time. Components need to get a told when "app-db" changes. > > > > But event handlers don't need a stream. They need a one-off value, based > > off the db param they have been supplied. > > > > If you do try to use a subscription in an event handler, you'll get a > > memory leak. The reaction won't get properly "disposed" (it is > > automatically done for you when a Signal chain feeds through into a > > component). > > > > So, repeating myself: event handlers don't need a constant stream of > > updates. They don't need a subscription. All they need to do is call a > > function on "db" to get a value, so factor that function out of the > > subscription and make it available. > >
This strikes me as an FAQ, so I've started a page: https://github.com/Day8/re-frame/wiki/FAQ Please feel free to edit for clarity. I wonder what else should be going in the FAQ? -- Mike -- Note that posts from new members are moderated - please be patient with your first post. --- You received this message because you are subscribed to the Google Groups "ClojureScript" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/clojurescript.
