Use update-state! instead of set-state!

Split the logic into testable functions, and use update-state!

src/cljs/widgetshop/app/products.cljs

@@ -1,17 +1,28 @@
 (ns widgetshop.app.products
   "Controls product listing information."
-  (:require [widgetshop.app.state :refer [update-state! set-state!]]
+  (:require [widgetshop.app.state :as state]
             [widgetshop.server :as server]))
 
+(defn- products-by-category [app category products-by-category]
+  (assoc-in app [:products-by-category category] products-by-category))
+
+(defn- set-categories [app categories]
+  (assoc-in app [:categories] categories))
+
+(defn- load-products-by-category! [{:keys [categories] :as app} server-get-fn! category-id]
+  (let [category (some #(when (= (:id %) category-id) %) categories)]
+    (server-get-fn! category)
+    (-> app
+        (assoc :category category)
+        (assoc-in [:products-by-category category] :loading))))
+
 (defn select-category-by-id! [category-id]
-  (update-state!
-   (fn [{:keys [categories] :as app}]
-     (let [category (some #(when (= (:id %) category-id) %) categories)]
-       (server/get! (str "/products/" (:id category))
-                    {:on-success #(set-state! [:products-by-category category] %)})
-       (-> app
-           (assoc :category category)
-           (assoc-in [:products-by-category category] :loading))))))
+  (state/update-state!
+    load-products-by-category!
+    (fn [category]
+      (server/get! (str "/products/" (:id category))
+                   {:on-success #(state/update-state! products-by-category category %)}))
+    category-id))
 
 (defn load-product-categories! []
-  (server/get! "/categories" {:on-success #(set-state! [:categories] %)}))
+  (server/get! "/categories" {:on-success #(state/update-state! set-categories %)}))