Question

When using Reagent, how do I use React's refs?

Answer

Credit: this entry is entirely based on Paulus Esterhazy's Reagent Mysteries series

We'll start with a code fragment, because it is worth a 1000 words:

(defn video-ui []
  (let [!video (clojure.core/atom nil)]    ;; stores the
    (fn [{:keys [src]}]
      [:div
       [:div
        [:video {:src src
                 :style {:width 400}
                 :ref (fn [el]
                        (reset! !video el))}]]
       [:div
        [:button {:on-click (fn []
                              (when-let [video @!video] ;; not nil?
                                (if (.-paused video)
                                  (.play video)
                                  (.pause video))))}
         "Toogle"]]])))

Notes:

1. This example uses a Form-2 component, which allows us to retain state outside of the renderer fn. The same technique would work with a Form-3 component.
2. We capture state in !video. In this example, the state we capture is a reference to a video HTML element.
3. !video is a clojure.core/atom and not a reaagent.core/atom. We don't use a normal atom because refs never change during the lifecycle of a component and if we used a reagent atom, it would cause an unnecessary re-render when the ref callback mutates the atom.
4. On the :video component there's a :ref callback function which establishes the state in !video. You can attach a ref callback to any of the Hiccup elements.
5. Thereafter, @!video is used with the :button's :on-click to manipulate the video DOM methods.
6. For full notes read Paulus' blog post
7. For more background on callback refs, see React's documentation

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