Warn user when calling subscribe outside of a reactive context

src/re_frame/interop.clj

@@ -91,3 +91,7 @@
   "Doesn't make sense in a Clojure context currently."
   [reactive-val]
   "rx-clj")
+
+(defn reactive?
+  []
+  true)

src/re_frame/interop.cljs

@@ -72,3 +72,7 @@
            reagent.ratom/Track "tr"
            "other")
          (hash reactive-val))))
+
+(defn reactive?
+  []
+  (reagent.ratom/reactive?))

src/re_frame/subs.cljc

@@ -1,7 +1,7 @@
 (ns re-frame.subs
  (:require
    [re-frame.db        :refer [app-db]]
-   [re-frame.interop   :refer [add-on-dispose! debug-enabled? make-reaction ratom? deref? dispose! reagent-id]]
+   [re-frame.interop   :refer [add-on-dispose! debug-enabled? make-reaction ratom? deref? dispose! reagent-id reactive?]]
    [re-frame.loggers   :refer [console]]
    [re-frame.utils     :refer [first-in-vector]]
    [re-frame.registrar :refer [get-handler clear-handlers register-handler]]
@@ -61,11 +61,19 @@
   ([query-v dyn-v]
    (get @query->reaction [query-v dyn-v])))
 
-
 ;; -- subscribe ---------------------------------------------------------------
 
+(defn warn-when-not-reactive
+  []
+  (when (and debug-enabled? (not (reactive?)))
+    (console :warn
+             "re-frame: Subscribe was called outside of a reactive context.\n"
+             "See: https://day8.github.io/re-frame/FAQs/UseASubscriptionInAJsEvent/\n"
+             "https://day8.github.io/re-frame/FAQs/UseASubscriptionInAnEventHandler/")))
+
 (defn subscribe
   ([query]
+   (warn-when-not-reactive)
    (trace/with-trace {:operation (first-in-vector query)
                       :op-type   :sub/create
                       :tags      {:query-v query}}
@@ -84,6 +92,7 @@
            (cache-and-return query [] (handler-fn app-db query)))))))
 
   ([query dynv]
+   (warn-when-not-reactive)
    (trace/with-trace {:operation (first-in-vector query)
                       :op-type   :sub/create
                       :tags      {:query-v query