Add a new FAQ

CHANGES.md

@@ -7,7 +7,8 @@
   - added [a new mental model](/docs/MentalModelOmnibus.md#on-dsls-and-machines)
   - added [a new FAQ entry](/docs/FAQs/When-Does-Dispatch-Happen.md) on dispatch processing 
   - added [a new FAQ entry](/docs/FAQs/DB_Normalisation.md) on representing normalised data in `app-db`. 
-
+  - added [a new FAQ entry](/docs/FAQs/GlobalInterceptors.md) on how to register a global interceptor.
+  
 #### Breaking
 
   - [#357](https://github.com/Day8/re-frame/pull/357)

docs/FAQs/GlobalInterceptors.md

@@ -0,0 +1,49 @@
+### Question
+
+Does re-frame allow me to register global interceptors? Ones which are included 
+for every event handler?
+
+### Short Answer 
+
+No, nothing direct.
+
+### Longer Answer 
+
+It's easy to make happen.
+
+Let's assume you have an interceptor called `omni-ceptor` which you want
+automatically added to all your event handlers.
+
+You'd write a replacement for both `reg-event-db` and `reg-event-fx`, and get
+these replacements to automatically add `omni-ceptor` to the interceptor 
+chain at registration time. 
+
+Here's how to write one of these auto-injecting replacements: 
+```clj
+(defn my-reg-event-db            ;; a replacement for reg-event-db
+
+   ;; 2-arity with no interceptors 
+   ([id handler] 
+     (my-reg-event-db id nil handler))
+     
+   ;; 3-arity with interceptors
+   ([id interceptors handler] 
+     (re-frame.core/reg-event-db   ;; which uses reg-event-db 
+       id
+       [omni-ceptor interceptors] ;; <-- inject `omni-ceptor`
+       handler)))
+``` 
+
+NB: did you know that interceptor chains are flattened and nils are removed?
+
+With this in place, you would always use `my-reg-event-db` 
+instead of the standard `reg-event-db`: 
+```clj
+(my-reg-event-db 
+   :event-id
+   (fn [db v] 
+      ...))
+```
+
+And, hey presto, you'd have your `omni-ceptor` "globally" injected.
+

docs/FAQs/README.md

@@ -9,7 +9,7 @@
 5. [Why do we need to clear the subscription cache when reloading with Figwheel?](Why-Clear-Sub-Cache.md)
 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)
 
 
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