Delay wallpaper load by 2 * average wallpaper load time.

chrome/browser/resources/chromeos/login/wallpaper_loader.js

Index: chrome/browser/resources/chromeos/login/wallpaper_loader.js
diff --git a/chrome/browser/resources/chromeos/login/wallpaper_loader.js b/chrome/browser/resources/chromeos/login/wallpaper_loader.js
new file mode 100644
index 0000000000000000000000000000000000000000..01a3e0b2e2cad1fe40beef1e3dea717ebd8f984a
--- /dev/null
+++ b/chrome/browser/resources/chromeos/login/wallpaper_loader.js
@@ -0,0 +1,185 @@
+// Copyright (c) 2013 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.
+
+cr.define('login', function() {
+
+  /**
+   * Minimum wallpaper load delay in milliseconds.
+   * @type {number}
+   * @const
+   */
+  var WALLPAPER_LOAD_MIN_DELAY_MS = 100;
+
+  /**
+   * If last walpaper load time cannot be calculated, assume this value.
+   * @type {number}
+   * @const
+   */
+  var WALLPAPER_DEFAULT_LOAD_TIME_MS = 200;
+
+  /**
+   * Min and Max average wallpaper load time.
+   * Delay to next wallpaper load is 2 * <average load time>.
+   * @type {number}
+   * @const
+   */
+  var WALLPAPER_MIN_LOAD_TIME_MS = 50;
+  var WALLPAPER_MAX_LOAD_TIME_MS = 2000;
+
+  /**
+   * Number last wallpaper load times to remember.
+   * @type {number}
+   * @const
+   */
+  var WALLPAPER_LOAD_STATS_MAX_LENGTH = 4;
+
+
+  /**
+   * Creates a new pod row element.
+   * @constructor
+   * @extends {HTMLDivElement}
+   */
+  var WallpaperLoader = function() {
+    this.wallpaperLoadInProgress_ = {

[dzhioev (left Google), 2013/10/22 00:01:19]

Isn't it better to keep 'wallpaperLoadInProgress_' == null, if wallpaper is not
loading? 

[Alexander Alekseev, 2013/10/22 11:33:37]

This is to avoid double checks like:

if (wallpaperLoadInProgress_ && wallpaperLoadInProgress_.name == email) 

+        name: '',
+        date: undefined

[dzhioev (left Google), 2013/10/22 00:01:19]

startDate

+    };
+    this.loadStats_ = [];
+  };
+
+  WallpaperLoader.prototype = {
+    // When moving through users quickly at login screen, set a timeout to
+    // prevent loading intermediate wallpapers.
+    loadWallpaperTimeout_: null,
+
+    // When waiting for wallpaper load, remember load start time.
+    // wallpaperLoadInProgress_: { name: '', date: undefined }
+    wallpaperLoadInProgress_: undefined,
+
+    // Wait a delay and then load this wallpaper. Value = username.
+    wallpaperLoadPending_: undefined,
+
+    // Wait untill this Date before loading next wallpaper.
+    wallpaperLoadTryNextAfter_: undefined,
+
+    // Username, owner of current wallpaper.
+    currentWallpaper_: '',
+
+    // Array of times (in milliseconds) of last wallpaper load attempts.
+    // Length is limited by WALLPAPER_LOAD_STATS_MAX_LENGTH.
+    loadStats_: undefined,
+
+    /**
+     * Stop load timer. Clear pending record.
+     */
+    reset: function() {
+      delete this.wallpaperLoadPending_;
+      if (this.loadWallpaperTimeout_ != null)
+        window.clearTimeout(this.loadWallpaperTimeout_);
+      this.loadWallpaperTimeout_ = null;
+    },
+
+    /**
+     * Schedules wallpaper load.
+     */
+    scheduleLoad: function(email) {
+      if (this.wallpaperLoadPending_ && this.wallpaperLoadPending_ == email)
+        return;
+      this.reset();
+      if (this.wallpaperLoadInProgress_.name == email)
+        return;
+      if ((this.wallpaperLoadInProgress_.name == '') &&
+          (this.currentWallpaper_ == email))
+        return;
+
+      this.wallpaperLoadPending_ = email;
+      var now = new Date();
+      var timeout = WALLPAPER_LOAD_MIN_DELAY_MS;
+      if (this.wallpaperLoadTryNextAfter_)
+        timeout = Math.max(timeout, this.wallpaperLoadTryNextAfter_ - now);
+
+      this.loadWallpaperTimeout_ = window.setTimeout(
+        this.loadWallpaper_.bind(this), timeout);
+     },
+
+
+    /**
+     * Loads pending wallpaper, if any.
+     * @private
+     */
+    loadWallpaper_: function() {
+      this.loadWallpaperTimeout_ = null;
+      if (!this.wallpaperLoadPending_)
+        return;
+      if (this.wallpaperLoadInProgress_.name != '')
+        return;
+      var email = this.wallpaperLoadPending_;
+      delete this.wallpaperLoadPending_;
+      if (email == this.currentWallpaper_)
+        return;
+      this.wallpaperLoadInProgress_.name = email;
+      this.wallpaperLoadInProgress_.date = new Date();
+      chrome.send('loadWallpaper', [email]);
+    },
+
+    /**
+     * Calculates average wallpaper load time.
+     */
+    calcLoadStatsAvg: function() {
+        return this.loadStats_.reduce(
+            function(previousValue, currentValue) {
+              return previousValue + currentValue;
+            }) / this.loadStats_.length;
+    },
+
+    /**
+     * Calculates average next wallpaper load delay time.
+     */
+    getWallpaperLoadTime_: function() {
+      var avg = WALLPAPER_DEFAULT_LOAD_TIME_MS;
+
+      if (this.loadStats_.length == 0)
+        return avg;
+
+      avg = this.calcLoadStatsAvg();
+      if (avg < WALLPAPER_MIN_LOAD_TIME_MS)
+        avg = WALLPAPER_MIN_LOAD_TIME_MS;
+
+      if (avg > WALLPAPER_MAX_LOAD_TIME_MS)
+        avg = WALLPAPER_MAX_LOAD_TIME_MS;
+
+      return avg;
+    },
+
+    /**
+     * Handles 'onWallpaperLoaded' event. Recalculates statistics and
+     * [re]schedules next wallpaper load.
+     */
+    onWallpaperLoaded: function(email) {
+      this.currentWallpaper_ = email;
+      if (email != this.wallpaperLoadInProgress_.name)
+          return;
+      this.wallpaperLoadInProgress_.name = '';
+      var started = this.wallpaperLoadInProgress_.date;
+      var finished = new Date();
+      var elapsed = started ? finished - started :
+          WALLPAPER_DEFAULT_LOAD_TIME_MS;
+      this.loadStats_.push(elapsed);
+      if (this.loadStats_.length > WALLPAPER_LOAD_STATS_MAX_LENGTH)
+        this.loadStats_.shift();
+
+      var nextAfter = new Date(Date.now() + 2 * this.getWallpaperLoadTime_());
+      this.wallpaperLoadTryNextAfter_ = nextAfter;

[dzhioev (left Google), 2013/10/22 00:01:19]

You can assign 'wallpaperLoadTryNextAfter_' directly, without using temporarily
variable.

[Alexander Alekseev, 2013/10/22 11:33:37]

Done.

+      if (this.wallpaperLoadPending_) {
+        var newWallpaperEmail = this.wallpaperLoadPending_;
+        this.reset();
+        this.scheduleLoad(newWallpaperEmail);
+      }
+    }
+  };
+
+  return {
+    WallpaperLoader: WallpaperLoader
+  };
+});