Add unpack interceptor

See #644

src/re_frame/core.cljc

@@ -12,8 +12,8 @@
     [re-frame.registrar        :as registrar]
     [re-frame.interceptor      :as interceptor]
     [re-frame.std-interceptors :as std-interceptors :refer [db-handler->interceptor
-                                                             fx-handler->interceptor
-                                                             ctx-handler->interceptor]]
+                                                            fx-handler->interceptor
+                                                            ctx-handler->interceptor]]
     [re-frame.utils            :as utils]
     [clojure.set               :as set]))
 
@@ -674,6 +674,21 @@
   [f]
   (std-interceptors/enrich f))
 
+(def ^{:api-docs/heading "Interceptors"} unpack
+  "An interceptor which provides the second element of the event vector to the
+   event handler as the event, intended to be used with the second element as a
+   map.
+
+   Your event handlers will look like this:
+
+       (reg-event-fx
+         :event-id
+         [... unpack ...]                    ;; <-- added to the interceptors
+         (fn [{:keys [db]} {:keys [x y z]}]  ;; <-- instead of [_ {:keys [x y z]}]
+           ...)
+   "
+   std-interceptors/unpack)
+
 (def ^{:api-docs/heading "Interceptors"} trim-v
   "An interceptor which removes the first element of the event vector,
   before it is supplied to the event handler, allowing you to write more

src/re_frame/std_interceptors.cljc

@@ -36,16 +36,31 @@
                       (console :log "No app-db changes resulted from:" event))))
                 context))))
 
+(def unpack
+  (->interceptor
+    :id      :unpack
+    :before  (fn unpack-before
+               [context]
+               (let [[event-id payload :as event] (get-coeffect context :event)]
+                 (-> context
+                     (assoc-coeffect :event (with-meta payload {:event-id event-id}))
+                     (assoc-coeffect ::packed-event event))))
+    :after   (fn unpack-after
+               [context]
+               (-> context
+                   (utils/dissoc-in [:coeffects ::packed-event])
+                   (assoc-coeffect :event (get-coeffect context ::packed-event))))))
+
 
 (def trim-v
   (->interceptor
     :id      :trim-v
-    :before  (fn trimv-before
+    :before  (fn trim-v-before
                [context]
                (-> context
                    (update-coeffect :event subvec 1)
                    (assoc-coeffect ::untrimmed-event (get-coeffect context :event))))
-    :after   (fn trimv-after
+    :after   (fn trim-v-after
                [context]
                (-> context
                    (utils/dissoc-in [:coeffects ::untrimmed-event])