preload images and identifications

sas/static/sas/js/viewer.js

@@ -4,23 +4,78 @@
  * @property {UserProfile} user The identified user
  */
 
+/**
+ * A container for a picture with the users identified on it
+ * able to prefetch its data.
+ */
+class PictureWithIdentifications {
+  identifications = null;
+  image_loading = false;
+  identifications_loading = false;
+
+  /**
+   * @param {Picture} picture
+   */
+  constructor(picture) {
+    Object.assign(this, picture);
+  }
+  /**
+   * @param {Picture} picture
+   */
+  static from_picture(picture) {
+    return new PictureWithIdentifications(picture);
+  }
+
+  /**
+   * If not already done, fetch the users identified on this picture and
+   * populate the identifications field
+   * @param {?Object=} options
+   * @return {Promise<void>}
+   */
+  async load_identifications(options) {
+    if (this.identifications_loading) {
+      return; // The users are already being fetched.
+    }
+    if (!!this.identifications && !options?.force_reload) {
+      // The users are already fetched
+      // and the user does not want to force the reload
+      return;
+    }
+    this.identifications_loading = true;
+    const url = `/api/sas/picture/${this.id}/identified`;
+    this.identifications = await (await fetch(url)).json();
+    this.identifications_loading = false;
+  }
+
+  /**
+   * Preload the photo and the identifications
+   * @return {Promise<void>}
+   */
+  async preload() {
+    const img = new Image();
+    img.src = this.compressed_url;
+    if (!img.complete) {
+      this.image_loading = true;
+      img.addEventListener("load", () => {
+        this.image_loading = false;
+      });
+    }
+    await this.load_identifications();
+  }
+}
+
 document.addEventListener("alpine:init", () => {
   Alpine.data("picture_viewer", () => ({
     /**
      * All the pictures that can be displayed on this picture viewer
-     * @type Picture[]
+     * @type PictureWithIdentifications[]
      **/
     pictures: [],
-    /**
-     * The users identified on the currently displayed picture
-     * @type PictureIdentification[]
-     **/
-    identifications: [],
     /**
      * The currently displayed picture
      * Default dummy data are pre-loaded to avoid javascript error
      * when loading the page at the beginning
-     * @type Picture
+     * @type PictureWithIdentifications
      **/
     current_picture: {
       is_moderated: true,
@@ -32,15 +87,16 @@ document.addEventListener("alpine:init", () => {
       full_size_url: "",
       owner: "",
       date: new Date(),
+      identifications: [],
     },
     /**
      * The picture which will be displayed next if the user press the "next" button
-     * @type ?Picture
+     * @type ?PictureWithIdentifications
      **/
     next_picture: null,
     /**
      * The picture which will be displayed next if the user press the "previous" button
-     * @type ?Picture
+     * @type ?PictureWithIdentifications
      **/
     previous_picture: null,
     /**
@@ -50,7 +106,6 @@ document.addEventListener("alpine:init", () => {
     /**
      * true if the page is in a loading state, else false
      **/
-    loading: true,
     /**
      * Error message when a moderation operation fails
      * @type string
@@ -64,11 +119,17 @@ document.addEventListener("alpine:init", () => {
     pushstate: History.PUSH,
 
     async init() {
-      this.pictures = await fetch_paginated(picture_endpoint);
+      this.pictures = (await fetch_paginated(picture_endpoint)).map(
+        PictureWithIdentifications.from_picture,
+      );
       this.selector = sithSelect2({
         element: $(this.$refs.search),
         data_source: remote_data_source("/api/user/search", {
-          excluded: () => [...this.identifications.map((i) => i.user.id)],
+          excluded: () => [
+            ...(this.current_picture.identifications || []).map(
+              (i) => i.user.id,
+            ),
+          ],
           result_converter: (obj) => Object({ ...obj, text: obj.display_name }),
         }),
         picture_getter: (user) => user.profile_pict,
@@ -99,8 +160,6 @@ document.addEventListener("alpine:init", () => {
      * the list of identified users are updated.
      */
     async update_picture() {
-      this.loading = true;
-
       const update_args = [
         { sas_picture_id: this.current_picture.id },
         "",
@@ -117,10 +176,13 @@ document.addEventListener("alpine:init", () => {
       const index = this.pictures.indexOf(this.current_picture);
       this.previous_picture = this.pictures[index - 1] || null;
       this.next_picture = this.pictures[index + 1] || null;
-      this.identifications = await (
-        await fetch(`/api/sas/picture/${this.current_picture.id}/identified`)
-      ).json();
-      this.loading = false;
+      await this.current_picture.load_identifications();
+      this.$refs.main_picture?.addEventListener("load", () => {
+        // once the current picture is loaded,
+        // start preloading the next and previous pictures
+        this.next_picture?.preload();
+        this.previous_picture?.preload();
+      });
     },
 
     async moderate_picture() {
@@ -131,8 +193,7 @@ document.addEventListener("alpine:init", () => {
         },
       );
       if (!res.ok) {
-        this.moderation_error =
-          gettext("Couldn't moderate picture") + " : " + res.statusText;
+        this.moderation_error = `${gettext("Couldn't moderate picture")} : ${res.statusText}`;
         return;
       }
       this.current_picture.is_moderated = true;
@@ -161,16 +222,14 @@ document.addEventListener("alpine:init", () => {
      * Send the identification request and update the list of identified users.
      */
     async submit_identification() {
-      this.loading = true;
       const url = `/api/sas/picture/${this.current_picture.id}/identified`;
       await fetch(url, {
         method: "PUT",
         body: JSON.stringify(this.selector.val().map((i) => parseInt(i))),
       });
       // refresh the identified users list
-      this.identifications = await (await fetch(url)).json();
+      await this.current_picture.load_identifications({ force_reload: true });
       this.selector.empty().trigger("change");
-      this.loading = false;
     },
 
     /**
@@ -187,16 +246,15 @@ document.addEventListener("alpine:init", () => {
      * @param {PictureIdentification} identification
      */
     async remove_identification(identification) {
-      this.loading = true;
       const res = await fetch(`/api/sas/relation/${identification.id}`, {
         method: "DELETE",
       });
-      if (res.ok) {
-        this.identifications = this.identifications.filter(
-          (i) => i.id !== identification.id,
-        );
+      if (res.ok && Array.isArray(this.current_picture.identifications)) {
+        this.current_picture.identifications =
+          this.current_picture.identifications.filter(
+            (i) => i.id !== identification.id,
+          );
       }
-      this.loading = false;
     },
   }));
 });