Delay wallpaper load by 2 * average wallpaper load time.

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

+// 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_ = {};
+    this.wallpaperLoadStats_ = [];
+  };
+
+  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.
+    // This is actually associative array with key = username and value =
+    // start time. Associative array.
+    wallpaperLoadInProgress_: null,

[dzhioev (left Google), 2013/10/17 12:47:49]

We load only one wallpaper at time, so such dictionary is not needed. I propose
wallpaperLoadInProgress_ to be object with fields 'name' and 'startTime'.

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+
+    // 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.
+    wallpaperCurrent_: '',

[dzhioev (left Google), 2013/10/17 12:47:49]

currentWallpaper_

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+
+    // Array of times (in milliseconds) of last wallpaper load attempts.
+    // Length is limited by WALLPAPER_LOAD_STATS_MAX_LENGTH.
+    wallpaperLoadStats_: undefined,

[dzhioev (left Google), 2013/10/17 12:47:49]

Just loadStats_

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+
+    reset: function() {

[dzhioev (left Google), 2013/10/17 12:47:49]

You can delete 'wallpaperLoadPending_' in this method.

[dzhioev (left Google), 2013/10/17 12:47:49]

Please add comment for method.

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+      if (this.loadWallpaperTimeout_)

[dzhioev (left Google), 2013/10/17 12:47:49]

I think setTimeout can return 0 as a valid ID. So it's better to check
explicitly that 'loadWallpaperTimeout_' is not 'null' ( !== null )

[Alexander Alekseev, 2013/10/18 17:01:52]

probably if (loadWallpaperTimeout_ != null)  ?

+        clearTimeout(this.loadWallpaperTimeout_);

[dzhioev (left Google), 2013/10/17 12:47:49]

window.clearTimeout

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+      this.loadWallpaperTimeout_ = null;
+    },
+
+    /**
+     * Schedules wallpaper load.
+     */
+    scheduleLoad: function(email) {
+      if (this.wallpaperLoadPending_ && this.wallpaperLoadPending_ == email)
+        return;
+      delete this.wallpaperLoadPending_;
+      this.reset();
+      if (this.wallpaperLoadInProgress_.hasOwnProperty(email))
+        return;
+      if ((Object.getOwnPropertyNames(this.wallpaperLoadInProgress_).length ==
+          0) && (this.wallpaperCurrent_ == email)) {
+        return;
+      }
+      this.wallpaperLoadPending_ = email;
+      var now = new Date();
+      var timeout = WALLPAPER_LOAD_MIN_DELAY_MS;
+      if (this.wallpaperLoadTryNextAfter_ &&

[dzhioev (left Google), 2013/10/17 12:47:49]

I think this if-block can be simplified to:
if (this.wallpaperLoadTryNextAfter_)
  timeout = Math.max(timeout, this.wallpaperLoadTryNextAfter_ - Date.now());

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+          (now < this.wallpaperLoadTryNextAfter_) &&
+          (this.wallpaperLoadTryNextAfter_.getTime() -
+            now.getTime() > WALLPAPER_LOAD_MIN_DELAY_MS))
+        timeout = this.wallpaperLoadTryNextAfter_.getTime() -

[dzhioev (left Google), 2013/10/17 12:47:49]

You can drop getTime() in arithmetic calculations with Date, because Dete
implicitly converts to 'ms since epoch'.

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+            now.getTime();
+
+      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 (Object.getOwnPropertyNames(this.wallpaperLoadInProgress_).length != 0)
+        return;
+      var email = this.wallpaperLoadPending_;
+      delete this.wallpaperLoadPending_;
+      if (email == this.wallpaperCurrent_)
+        return;
+      this.wallpaperLoadInProgress_[email] = new Date();
+      chrome.send('loadWallpaper', [email]);
+    },
+
+    /**
+     * Calculates average wallpaper load time.
+     */
+    getWallpaperLoadDelay_: function() {
+      var avg = WALLPAPER_DEFAULT_LOAD_TIME_MS;
+      if (this.wallpaperLoadStats_.length) {
+        avg = this.wallpaperLoadStats_.reduce(

[dzhioev (left Google), 2013/10/17 12:47:49]

Maybe it should be moved to separate method. 

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+            function(previousValue, currentValue, index, array) {

[dzhioev (left Google), 2013/10/17 12:47:49]

You can drop 'index' and 'array'.

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+              return previousValue + currentValue;
+            }) / this.wallpaperLoadStats_.length;
+        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;
+        }

[dzhioev (left Google), 2013/10/17 12:47:49]

Brackets not needed for one line body.

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+      }
+      return avg;
+    },
+
+    /**
+     * Handles 'onWallpaperLoaded' event. Recalculates statistics and
+     * [re]schedules next wallpaper load.
+     */
+    onWallpaperLoaded: function(email) {
+      this.wallpaperCurrent_ = email;
+      var started = this.wallpaperLoadInProgress_[email];
+      delete this.wallpaperLoadInProgress_[email];
+      var finished = new Date();
+      var elapsed = started ? finished.getTime() - started.getTime() :
+          WALLPAPER_DEFAULT_LOAD_TIME_MS;
+      this.wallpaperLoadStats_.push(elapsed);
+      if (this.wallpaperLoadStats_.length >
+          WALLPAPER_LOAD_STATS_MAX_LENGTH)

[dzhioev (left Google), 2013/10/17 12:47:49]

No need to split condition that fits in one line.

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+        this.wallpaperLoadStats_.shift();
+
+      var nextAfter = new Date();
+      nextAfter.setTime(nextAfter.getTime() +
+          2 * this.getWallpaperLoadDelay_());
+      this.wallpaperLoadTryNextAfter_ = nextAfter;

[dzhioev (left Google), 2013/10/17 12:47:49]

I think we delay should be multiplied by 2 inside getWallpaperLoadDelay_ method.
Also previous 3 lines can be simplified to: 
this.wallpaperLoadTryNextAfter_ = new Date(Date.now() +
this.getWallpaperLoadDelay_());

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

I've renamed getWallpaperLoadDelay_ to getWallpaperLoadTime_ to clarify it's
purpose.

+      if (this.wallpaperLoadPending_) {
+        var newWallpaperEmail = this.wallpaperLoadPending_;
+        delete this.wallpaperLoadPending_;
+        if (newWallpaperEmail == email) {

[dzhioev (left Google), 2013/10/17 12:47:49]

How this can happen? I thought it's already checked in scheduleLoad.

[Alexander Alekseev, 2013/10/18 17:01:52]

Done.

+          this.reset();
+        } else {
+          this.scheduleLoad(newWallpaperEmail);
+        }
+      }
+    }
+  };
+
+  return {
+    WallpaperLoader: WallpaperLoader
+  };
+});