On Sunday, August 9, 2015 at 10:50:57 AM UTC+3, Gary Verhaegen wrote:
> You deref if you want to get the current value. The argument to om/update!,
> however, must be a cursor, not a random value.
>
>
> In this particular case, the code uses om/update! because it does not need to
> use the old value to compute the new one. Since the code of submit does not
> seem to require the value at the cursor, there is no need to deref.
>
> On Sunday, 9 August 2015, Asher Coren <[email protected]> wrote:
> Hello,
>
>
>
> I'm trying to understand when we must deref a cursor to get its value.
>
>
>
> In the om cursors wiki (https://github.com/omcljs/om/wiki/Cursors) it states
> that
>
> event handlers are considered not part of the render phase, and therefore
> cursors in handlers should be drefed.
>
> Same is shown in the Basic-tutorial
> (https://github.com/omcljs/om/wiki/Basic-Tutorial):
>
>
>
> (defn contact-view [contact owner]
>
> (reify
>
> om/IRenderState
>
> (render-state [this {:keys [delete]}]
>
> (dom/li nil
>
> (dom/span nil (display-name contact))
>
> (dom/button #js {:onClick (fn [e] (put! delete @contact))}
> "Delete")))))
>
>
>
> But, in the TodoMVC code
> (https://github.com/swannodette/todomvc/blob/gh-pages/labs/architecture-examples/om/src/todomvc/item.cljs),
> the handlers (onclick, onchange...) use the cursor without drefing it:
>
>
>
> (defn submit [e todo owner comm]
>
> (when-let [edit-text (om/get-state owner :edit-text)]
>
> (if-not (string/blank? (.trim edit-text))
>
> (do
>
> (om/update! todo :title edit-text)
>
> (put! comm [:save todo]))
>
> (put! comm [:destroy todo])))
>
> false)
>
>
>
> So, what is the correct way?
>
>
>
> Thank you.
>
>
>
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Gary,
Thank you for your reply. I realize now that I pasted the wrong part of code.
Your explanation is correct with regards to `om/update!`. But, later in the
code we have:
(dom/button
#js {:className "destroy"
:onClick (fn [_] (put! comm [:destroy todo]))}))
Here again we send the cursor itself (`todo`) without derefing. When reading
from the channel, it is used like this:
(defn destroy-todo [app {:keys [id]}]
(om/transact! app :todos
(fn [todos] (into [] (remove #(= (:id %) id) todos)))
[:delete id]))
Here we require the value of the cursor ({:keys [id}) - but we sent it without
derefing!
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