From 146394f3ca0f226eb77ef3115364deb161951289 Mon Sep 17 00:00:00 2001
From: Andrey Antukh <niwi@niwi.nz>
Date: Fri, 5 Mar 2021 17:31:26 +0100
Subject: [PATCH] :tada: Add thumbnail caching.

Avoid unnecesary rendering unchanged pages.
---
 backend/src/app/rpc/queries/files.clj   |  1 -
 frontend/src/app/worker/thumbnails.cljs | 19 ++++++++++++++-----
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/backend/src/app/rpc/queries/files.clj b/backend/src/app/rpc/queries/files.clj
index d3e3bfd61c..b96903ddd0 100644
--- a/backend/src/app/rpc/queries/files.clj
+++ b/backend/src/app/rpc/queries/files.clj
@@ -166,7 +166,6 @@
     (let [file (retrieve-file conn file-id)]
       (get-in file [:data :pages-index id]))))
 
-
 ;; --- Query: Shared Library Files
 
 (def ^:private sql:shared-files
diff --git a/frontend/src/app/worker/thumbnails.cljs b/frontend/src/app/worker/thumbnails.cljs
index 4bcf3cc77d..d91d47f69c 100644
--- a/frontend/src/app/worker/thumbnails.cljs
+++ b/frontend/src/app/worker/thumbnails.cljs
@@ -45,13 +45,22 @@
                      (fn [error]
                        (reject error))))))))
 
+(defonce cache (atom {}))
+
+(defn render-page
+  [data ckey]
+  (let [prev (get @cache ckey)]
+    (if (= (:data prev) data)
+      (:result prev)
+      (let [elem   (mf/element exports/page-svg #js {:data data :width "290" :height "150"})
+            result (rds/renderToStaticMarkup elem)]
+        (swap! cache assoc ckey {:data data :result result})
+        result))))
+
 (defmethod impl/handler :thumbnails/generate
   [{:keys [file-id page-id] :as message}]
   (p/then
    (request-page file-id page-id)
    (fn [data]
-     (let [elem (mf/element exports/page-svg #js {:data data
-                                                  :width "290"
-                                                  :height "150"})]
-       {:svg (rds/renderToStaticMarkup elem)
-        :fonts @fonts/loaded}))))
+     {:svg (render-page data #{file-id page-id})
+      :fonts @fonts/loaded})))