On Mar 11, 11:20 am, linh <nguyenlinh.m...@gmail.com> wrote:
> According to the API doc, add-watch must have 4 args: a key, a
> reference,
> its old-state and its new state. What I'm missing here is an addtional
> arg that can be passed in some way to update-fn so that update-fn
> knows what entries in my-atom has changed.
> Is there any way do this?
well you could always add a key :last-update containing the key
changed in the last update. Then your add-watch update function just
reads this key and knows what's been modified. To accomplish this of
course, in your
(swap! my-atom (some-fn...
some-fn would have to include code that sets the :last-update value
when done. Here's a sketch:
;; this just makes a big map atom where integer keys are associated
with integer values
(def my-atom
(atom (assoc (apply hash-map
(take 1000
(interleave
(iterate inc 0)
(iterate inc 100))))
:last-update nil)))
;; this adds a watcher that knows what's been changed
;; by reading the :last-update key and prints it out
(add-watch my-atom :update-watcher
(fn [me my-atom old new]
(println "this key was updated" (:last-update new))))
;; this is the update-function that both changes the desired key
;; and val and also updates the :last-update key
(defn my-atom-swapper [the-atom key newval]
(let [changed-atom (assoc the-atom key newval)]
(assoc changed-atom :last-update key)))
;; a simple test
(defn test-update-watcher []
(swap! my-atom my-atom-swapper :10 "ten")
(:10 @my-atom))
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