Touch a file whenever defn is called. Useful with LiveReload

(def reload-trigger-file "reload.trigger")

(defn- touch-reload-trigger
  "Called on defn invocation; updates the mod time on the reload trigger
   file so LiveReload will reload the web page."
  []
  (with-open [w (io/writer (var-get #'reload-trigger-file))]
    (.write w "x")))

(defn- trigger-on-defn [f & args]
  (touch-reload-trigger)
  (apply f args))

(defn start-reloading-on-defn []
  (add-hook #'defn #'trigger-on-defn))