[chrome://downloads] Pre-fetch foreground progress image to speed up changing the zoom level.

chrome/browser/resources/downloads/downloads.js

 // Copyright (c) 2012 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.
 
 // TODO(jhawkins): Use hidden instead of showInline* and display:none.
 
 /**
  * The type of the download object. The definition is based on
  * chrome/browser/ui/webui/downloads_dom_handler.cc:CreateDownloadItemValue()
  * @typedef {{by_ext_id: (string|undefined),
@@ skipping 83 matching lines @@
   this.summary_ = $('downloads-summary-text');
   this.searchText_ = '';
 
   // Keep track of the dates of the newest and oldest downloads so that we
   // know where to insert them.
   this.newestTime_ = -1;
 
   // Icon load request queue.
   this.iconLoadQueue_ = [];
   this.isIconLoading_ = false;
+
+  this.progressForeground1_ = new Image();
+  this.progressForeground1_.src =
+    'chrome://theme/IDR_DOWNLOAD_PROGRESS_FOREGROUND_32@1x';
+  this.progressForeground2_ = new Image();
+  this.progressForeground2_.src =
+    'chrome://theme/IDR_DOWNLOAD_PROGRESS_FOREGROUND_32@2x';
 }
 
 /**
  * Called when a download has been updated or added.
  * @param {BackendDownloadObject} download A backend download object
  */
 Downloads.prototype.updated = function(download) {
   var id = download.id;
   if (!!this.downloads_[id]) {
     this.downloads_[id].update(download);
@@ skipping 365 matching lines @@
 }
 computeDownloadProgress();
 
 // Listens for when device-pixel-ratio changes between any zoom level.
 [0.3, 0.4, 0.6, 0.7, 0.8, 0.95, 1.05, 1.2, 1.4, 1.6, 1.9, 2.2, 2.7, 3.5, 4.5
 ].forEach(function(scale) {
   var media = '(-webkit-min-device-pixel-ratio:' + scale + ')';
   window.matchMedia(media).addListener(computeDownloadProgress);
 });
 
-var ImageCache = {};
-function getCachedImage(src) {
-  if (!ImageCache[src]) {
-    ImageCache[src] = new Image();
-    ImageCache[src].src = src;
-  }
-  return ImageCache[src];
-}
-
 /**
  * Updates the download to reflect new data.
  * @param {BackendDownloadObject} download A backend download object
  */
 Download.prototype.update = function(download) {
   this.id_ = download.id;
   this.filePath_ = download.file_path;
   this.fileUrl_ = download.file_url;
   this.fileName_ = download.file_name;
   this.url_ = download.url;
@@ skipping 44 matching lines @@
     showInlineBlock(this.nodeFileName_,
                     this.state_ != Download.States.COMPLETE ||
                         this.fileExternallyRemoved_);
 
     if (this.state_ == Download.States.IN_PROGRESS) {
       this.nodeProgressForeground_.style.display = 'block';
       this.nodeProgressBackground_.style.display = 'block';
       this.nodeProgressForeground_.width = Download.Progress.width;
       this.nodeProgressForeground_.height = Download.Progress.height;
 
-      var foregroundImage = getCachedImage(
-          'chrome://theme/IDR_DOWNLOAD_PROGRESS_FOREGROUND_32@' +
-          window.devicePixelRatio + 'x');
+      var foregroundImage = (window.devicePixelRatio < 2) ?
+        downloads.progressForeground1_ : downloads.progressForeground2_;
 
       // Draw a pie-slice for the progress.
       this.canvasProgress_.globalCompositeOperation = 'copy';
       this.canvasProgress_.drawImage(
           foregroundImage,
           0, 0,  // sx, sy
           foregroundImage.width,
           foregroundImage.height,
           0, 0,  // x, y
           Download.Progress.width, Download.Progress.height);
@@ skipping 395 matching lines @@
       resultsTimeout = setTimeout(tryDownloadUpdatedPeriodically, 5);
       break;
     }
   }
 }
 
 // Add handlers to HTML elements.
 window.addEventListener('DOMContentLoaded', load);
 
 preventDefaultOnPoundLinkClicks();  // From util.js.