Asynchronously load wallpapers when user pod is selected.

chrome/browser/resources/chromeos/login/user_pod_row.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.
 
 /**
  * @fileoverview User pod row implementation.
  */
 
 cr.define('login', function() {
   // Pod width. 170px Pod + 10px padding + 10px margin on both sides.
@@ skipping 629 matching lines @@
           pod.reset(false);
         }
       }
 
       this.focusedPod_ = podToFocus;
       if (podToFocus) {
         podToFocus.classList.remove('faded');
         podToFocus.classList.add('focused');
         podToFocus.reset(true);  // Reset and give focus.
         this.scrollPodIntoView(podToFocus);
+        chrome.send('userSelected', [podToFocus.user.username]);

[Nikita (slow), 2012/04/25 16:47:48]

See http://crosbug.com/1124339 it mentions one specific issue when we quickly
move through users as sign in screen.

Imagine selecting 3rd user with keyboard. First two would get selected quickly
so we need some timeout after user pod gets focus.

1. User pod focused
2. Set timeout (issue mentions 800ms)
3. When timeout is fired, check that same user pod is still focused and call
chrome.send('userSelectedDelayed') if it is still focused.

[bshe, 2012/04/25 23:54:43]

Done. I cleared the previous timeout and setup a new timeout when focused pod
changes. It should do the same (prevent loading wallpapers in between).
Although, 800ms seems a little too long to me, which combined with the 1 second
wallpaper loading time, makes the wallpaper changing sluggish (around 2 secs to
show the wallpaper). 

With the new preemption strategy, We can probably start to load the wallpaper
immediately. It only shows the wallpaper loaded successfully and discard all in
between wallpapers. This way we can reduce the wallpaper changing response time.
What do you think?

[Nikita (slow), 2012/04/26 17:22:03]

I still think that we should have some timeout and maybe experiment and decrease
it. Issue is that most time consuming operation won't be cancelled.

wallpaper_ = ui::ResourceBundle::GetSharedInstance().GetImageNamed(
  GetWallpaperInfo(index_).id).ToSkBitmap();

[bshe, 2012/04/26 20:25:21]

Make sense. I will keep it this way and using 800 as a start. We can adjust it
later.

       }
     },
 
     /**
      * Returns the currently activated pod.
      * @type {UserPod}
      */
     get activatedPod() {
       return this.activatedPod_;
     },
@@ skipping 187 matching lines @@
       if (this.podsWithPendingImages_.length == 0) {
         chrome.send('userImagesLoaded');
       }
     }
   };
 
   return {
     PodRow: PodRow
   };
 });