(defonce mypos (reagent/atom {}))

(defn getsize [comp]
   (let [ node (reagent/dom-node comp)
          x (.-x (.getBoundingClientRect node))
          y (.-y (.getBoundingClientRect node))]
    {x y}))

(defn my_component []
  (let [state (reagent/atom {})] ;; you can include state
    (reagent/create-class
      {:component-did-mount
         (fn [comp]
           (swap! mypos #(getsize comp)))

       :display-name "complex-component"

       ;; note the keyword for this method
       :reagent-render
        (fn [] [:div [:b "Initial position: " @mypos]])})))

(defn page []
  [:div [my_component]])