Gallery: Store image caches in Gallery items.

ui/file_manager/gallery/js/gallery_item.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 'use strict';
 
 /**
  * Object representing an image item (a photo).
  *
  * @param {FileEntry} entry Image entry.
  * @param {function():Promise} fethcedMediaProvider Function to provide the
  *     fetchedMedia metadata.
  * @constructor
  */
 Gallery.Item = function(entry, metadata, metadataCache, original) {
   /**
    * @type {FileEntry}
    * @private
    */
   this.entry_ = entry;
 
   /**
    * @type {Object}
    * @private
    */
   this.metadata_ = Object.freeze(metadata);
 
   /**
    * @type {MetadataCache}
+   * @private
    */
   this.metadataCache_ = metadataCache;
 
   /**
+   * The content cache is used for prefetching the next image when going through
+   * the images sequentially. The real life photos can be large (18Mpix = 72Mb
+   * pixel array) so we want only the minimum amount of caching.
+   * @type {Canvas}
+   */
+  this.screenImage = null;
+
+  /**
+   * We reuse previously generated screen-scale images so that going back to a
+   * recently loaded image looks instant even if the image is not in the content
+   * cache any more. Screen-scale images are small (~1Mpix) so we can afford to
+   * cache more of them.
+   * @type {Canvas}
+   */
+  this.contentImage = null;
+
+  /**
+   * Last accessed date to be used for selecting items whose cache are evicted.
+   * @type {Date}
+   */
+  this.lastAccessed = Date.now();
+
+  /**
    * @type {boolean}
    * @private
    */
   this.original_ = original;
 
   Object.seal(this);
 };
 
 /**
  * @return {FileEntry} Image entry.
@@ skipping 236 matching lines @@
         return Promise.reject(str('GALLERY_FILE_EXISTS'));
       }, function() {
         return new Promise(
             this.entry_.moveTo.bind(this.entry_, parentDirectory, newFileName));
       }.bind(this));
     }.bind(this));
   }.bind(this)).then(function(entry) {
     this.entry_ = entry;
   }.bind(this));
 };