Don't start the hotword extension on Chrome startup if the user doesn't have hotwording enabled.

chrome/browser/resources/hotword/state_manager.js

Index: chrome/browser/resources/hotword/state_manager.js
diff --git a/chrome/browser/resources/hotword/state_manager.js b/chrome/browser/resources/hotword/state_manager.js
index ebe68355d80815250acd0df74ba2eaa7c479bb31..7c8e30a2c4e1d0bda7e1920058aa038e003cce20 100644
--- a/chrome/browser/resources/hotword/state_manager.js
+++ b/chrome/browser/resources/hotword/state_manager.js
@@ -101,6 +101,7 @@ cr.define('hotword', function() {
      * @private
      */
     this.idleStateChangedListener_ = this.handleIdleStateChanged_.bind(this);
+    this.startupListener_ = this.handleStartup_.bind(this);
 
     /**
      * Whether this user is locked.
@@ -141,6 +142,11 @@ cr.define('hotword', function() {
     this.chime_.src = chrome.extension.getURL(
         hotword.constants.SHARED_MODULE_ROOT + '/audio/chime.wav');
     document.body.appendChild(this.chime_);
+
+    // In order to remove this listener, it must first be added. This handles
+    // the case on first Chrome startup where this event is never registered,
+    // so can't be removed when it's determined that hotwording is disabled.
+    chrome.runtime.onStartup.addListener(this.startupListener_);

[Matt Giuca, 2015/05/15 06:54:42]

Why not just not remove it if it hasn't been added?

if (chrome.runtime.onStartup.hasListener(this.startupListener_))
  chrome.runtime.onStartup.removeListener(this.startupListener_);

[Anand Mistry (off Chromium), 2015/05/15 07:08:36]

Ah, undocumented and poorly understood behaviour of extension API events. Even
if there's no listener registered from the JS perspective, there could still be
a wake-up listener registered to re-start the event page. If order to clear
that, you first need to add one. Then when you remove it, JS sees the refcount
go to 0 and remove the wake-up event. So this forces one to be added so that the
JS part of the API implementation does the right thing. This pattern is used
heavily in this extension since it it's the only way to achieve deterministic
results.

[Matt Giuca, 2015/05/15 08:32:43]

Gross :(

Could you explain that a bit in the comment (otherwise it just begs the
question: "why not just check if it's removed?")

(Not the whole thing, but just enough to let readers know that there could be
one registered even if you can't see it.)

[Anand Mistry (off Chromium), 2015/05/18 01:24:22]

Done.

   }
 
   /**
@@ -279,12 +285,18 @@ cr.define('hotword', function() {
           chrome.idle.onStateChanged.addListener(
               this.idleStateChangedListener_);
         }
+        if (!chrome.runtime.onStartup.hasListener(this.startupListener_))
+          chrome.runtime.onStartup.addListener(this.startupListener_);
       } else {
         // Not enabled. Shut down if running.
         this.shutdownDetector_();
 
         chrome.idle.onStateChanged.removeListener(
             this.idleStateChangedListener_);
+        // If hotwording isn't enabled, don't start this component extension on
+        // Chrome startup. If a user enables hotwording, the status change
+        // event will be fired and the onStartup event will be registered.
+        chrome.runtime.onStartup.removeListener(this.startupListener_);
       }
     },
 
@@ -602,6 +614,14 @@ cr.define('hotword', function() {
 
       if (oldLocked != this.isLocked_)
         this.updateStateFromStatus_();
+    },
+
+    /**
+     * Handles a chrome.runtime.onStartup event.
+     * @private
+     */
+    handleStartup_: function() {
+      updateStatus();
     }
   };