Question

Does re-frame allow me to register global interceptors? Ones which are included for every event handler?

Answer (v1.0.0 onwards)

Yes, re-frame provides an API for registering global interceptors.

The following code creates a global interceptor to keep a track of all events:

;; We'll be recording events into this atom 
;; The most recent events will be at the front of the list. 
(def event-store (atom (list)))


(defn keep-last-20
  [existing new-one]
  (take 20 (conj existing new-one)))


;; this interceptor will collect events and add them to the atom above
(def event-collector
  (re-frame.core/->interceptor
    :id      :event-collector
    :before  (fn [context]
               (swap! event-store keep-last-20 (re-frame.core/get-coeffect context :event))
               context)))

;; register this global interceptor early in program's boot process,
;; using re-frame's API
(re-frame.core/reg-global-interceptor event-collector)

Answer (prior to v1.0.0)

Prior to v1.0.0, re-frame provided no API to directly support this feature, but there are ways of making it 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:

(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:

(my-reg-event-db 
   :event-id
   (fn [db v] 
      ...))

And, hey presto, you'd have your omni-ceptor "globally" injected.