add-watch

function

since v0.0-927

clojure.core/add-watch

Edit

Details:

Examples:

(def a (atom {}))

(add-watch a :logger
  (fn [_key _atom old-state new-state]
    (println "old:" old-state)
    (println "new:" new-state)))

(swap! a assoc :foo "bar")
;;=> will print the following:
;; old: {}
;; new: {:foo "bar"}

See Also:

• remove-watch

Adds a watch function to an atom 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. Note that an atom's state
may have changed again prior to the fn call, so use old/new-state
rather than derefing the reference. 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.  Bear in
mind that regardless of the result or action of the watch fns the
atom's value will change.  Example:

    (def a (atom 0))
    (add-watch a :inc (fn [k r o n] (assert (== 0 n))))
    (swap! a inc)
    ;; Assertion Error
    (deref a)
    ;=> 1

(defn add-watch
  [iref key f]
  (-add-watch iref key f)
  iref)