store/[org.clojure/clojure "1.8.0"] clj::clojure.core/add-watch

Official Documentation

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Arities
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   [reference key fn]

Docstring
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  Adds a watch function to an agent/atom/var/ref reference. The watch
  fn must be a fn of 4 args: a key, the reference, its old-state, its
  new-state. Whenever the reference's state might have been changed,
  any registered watches will have their functions called. The watch fn
  will be called synchronously, on the agent's thread if an agent,
  before any pending sends if agent or ref. Note that an atom's or
  ref's state may have changed again prior to the fn call, so use
  old/new-state rather than derefing the reference. Note also that watch
  fns may be called from multiple threads simultaneously. Var watchers
  are triggered only by root binding changes, not thread-local
  set!s. Keys must be unique per reference, and can be used to remove
  the watch with remove-watch, but are otherwise considered opaque by
  the watch mechanism.

Source

(defn add-watch
  "Adds a watch function to an agent/atom/var/ref reference. The watch
  fn must be a fn of 4 args: a key, the reference, its old-state, its
  new-state. Whenever the reference's state might have been changed,
  any registered watches will have their functions called. The watch fn
  will be called synchronously, on the agent's thread if an agent,
  before any pending sends if agent or ref. Note that an atom's or
  ref's state may have changed again prior to the fn call, so use
  old/new-state rather than derefing the reference. Note also that watch
  fns may be called from multiple threads simultaneously. Var watchers
  are triggered only by root binding changes, not thread-local
  set!s. Keys must be unique per reference, and can be used to remove
  the watch with remove-watch, but are otherwise considered opaque by
  the watch mechanism."
  {:added "1.0"
   :static true}
  [^clojure.lang.IRef reference key fn] (.addWatch reference key fn))

Example 1

Edit
;; timing of agent actions and watches using nrepl (ymmv)
(def a (agent 0))
;; => #'user/a
a
;; => #<Agent@2bd9c3e7: 0>
(add-watch a :key (fn [k r os ns] (print k r os ns)))
;; => #<Agent@2bd9c3e7: 0>
(send a inc)
;; => :key #<Agent@2bd9c3e7: 1> 0 1
;; => #<Agent@2bd9c3e7: 1>
a
;; => :key #<Agent@2bd9c3e7: 2> 1 2
;; => #<Agent@2bd9c3e7: 2>

Example 2

Edit
;; Add useful context to watcher function:
(defn watch-agent [_agent context]
  (let [watch-fn (fn [_context _key _ref old-value new-value]
                   ;; ...
)]
    (add-watch _agent nil (partial watch-fn context))))

Uses on crossclj