Add new FAQ

CHANGES.md

@@ -1,3 +1,10 @@
+
+## Unreleased 0.NN.N  (2017.NN.NN)
+
+#### New Docs
+
+  - added [a new FAQ entry](/docs/FAQs/PollADatabaseEvery60.md) How do I turn on/off polling a database every 60 secs  (hint: effects)
+
 ## 0.10.1  (2017.08.17)
 
   - fix a showstopper bug introduced at the last minute in v0.10.0

docs/FAQs/PollADatabaseEvery60.md

@@ -0,0 +1,68 @@
+### Question
+
+When the user switches to a certain panel, I'd like to kickoff a regular poll of my 
+backend database - say every 60 seconds.
+ 
+And, then later, when the user switches away from that panel, I want to stop that polling.
+
+How do i do it?
+
+
+### Before I Answer - An Architectural Note 
+
+The boarder React community often uses the "load data from server on Component mount" 
+model of architecture. They'll often collocate queries with view components and perform a 
+GET (to the server) within the View's `componentDidMount`. 
+
+This is not idiomatic re-frame. Views are not imperative in nature (they don't issue 
+database queries), they simply provide a rendering of the current application state. 
+For this and 
+more browse [PurelyFunctional.tv's writeup](https://purelyfunctional.tv/article/react-vs-re-frame/)
+
+Instead, re-frame is CQRS in nature - "stuff" only happens 
+because of an `event`.  When the user clicks on a panel-changing widget,  
+an event is dispatched, and it is the handler for this event which knows 
+that X, Y and Z needs to happen  (assume one of these letters is "load data from server"). 
+
+So, having got that out the way ... 
+ 
+### An Answer 
+
+We'll create an effect. It will general in nature. 
+
+It will schedule and unschedule the regular dispatch of an event.  That 
+event could do the server poll, or ... it could do other things. So  
+this is effect could be used to achieve more than what was asked for 
+in this FAQ. 
+
+We must first design our `effect` - a data format (micro DSL) returned by an 
+event handler, which specifies the scheduled/regular dispatch. To `:start` 
+a regular dispatch, an event handler would return data in this format:
+```clj
+{:interval  {:action    :start
+             :id        :panel-1-query     ;; my id for this (so I cancel later)
+             :frequency 60000              ;; how many millisecsbetween dispatches 
+             :event     [:panel-query 1]}} ;; what to dispatch 
+```
+
+And to later cancel the regular dispatch, an event handler would do this:
+```clj
+{:interval  {:action    :cancel
+             :id        :panel-1-query}}   ;; my id for this (provided to :start)  
+```
+
+Now, register an effect handler for `:interval`:
+```clj
+re-frame.core/(reg-fx 
+  :interval 
+  (let [live-intervals (atom {})] 
+    (fn [{:keys [action id frequency event]}]
+      (if (= action :start) 
+        (swap! live-intervals assoc id (js/setInterval #(dispatch event) frequency))) 
+        (do (js/clearInterval (get live-intervals id)) 
+            (swap! live-intervals dissoc id))))
+```
+
+You'd probably want a bit more error checking, but that's the (untested) sketch.
+
+

docs/FAQs/README.md

@@ -10,6 +10,7 @@
 6. [How can I detect exceptions in Event Handlers?](CatchingEventExceptions.md)
 7. [How do I store normalised data in app-db?](DB_Normalisation.md)
 8. [How do I register a global interceptor](GlobalInterceptors.md)
+9. [How do I turn on/off polling a database every 60 secs](PollADatabaseEvery60.md)
 
 
 ## Want To Add An FAQ?