Add a new audio extension event OnLevelChanged.

extensions/common/api/audio.idl

Index: extensions/common/api/audio.idl
diff --git a/extensions/common/api/audio.idl b/extensions/common/api/audio.idl
index 0f088276c5da0c3eac0f310a4b46c9fb94d8f364..9461cbd3cf04377d9a04ada630b88dbbdaaaa250 100644
--- a/extensions/common/api/audio.idl
+++ b/extensions/common/api/audio.idl
@@ -16,7 +16,7 @@ namespace audio {
     boolean isActive;
     // True if this is muted.
     boolean isMuted;
-    // The output volume ranging from 0.0 to 1.0.
+    // The output volume ranging from 0.0 to 100.0.
     double volume;
   };
 
@@ -29,7 +29,7 @@ namespace audio {
     boolean isActive;
     // True if this is muted.
     boolean isMuted;
-    // The input gain ranging from 0.0 to 1.0.
+    // The input gain ranging from 0.0 to 100.0.

[asargent_no_longer_on_chrome, 2015/03/17 16:21:15]

As discussed in chat, if this is a supported API on stable channel then we
cannot change the meaning of parameters like this since it breaks backwards
compatibility. (unless the documentation was just wrong, and the implementation
has always actually used 100 instead of 1 as the maximum value)

[jennyz, 2015/03/17 18:17:28]

This was documented incorrectly before, now it is corrected.

     double gain;
   };
 
@@ -50,10 +50,17 @@ namespace audio {
   callback SetPropertiesCallback = void();
 
   interface Functions {
-    // Get the information of all audio output and input devices.
+    // Gets the information of all audio output and input devices.
     static void getInfo(GetInfoCallback callback);
 
-    // Select a subset of audio devices as active.
+  // Sets the active nodes to the nodes specified by |ids|.
+  // It can pass in the "complete" active node id list of either input
+  // nodes, or output nodes, or both. If only input node ids are passed in,
+  // it will only change the input nodes' active status, output nodes will NOT
+  // be changed; similarly for the case if only output nodes are passed.
+  // If the nodes specified in |new_active_ids| are already active, they will
+  // remain active. Otherwise, the old active nodes will be de-activated before
+  // we activate the new nodes with the same type(input/output).
     static void setActiveDevices(DOMString[] ids,
                                  SetActiveDevicesCallback callback);
 
@@ -61,10 +68,15 @@ namespace audio {
     static void setProperties(DOMString id,
                               DeviceProperties properties,
                               SetPropertiesCallback callback);
-  };
+     };
 
   interface Events {
     // Fired when anything changes to the audio device configuration.
     static void onDeviceChanged();
+
+    // Fired when sound level changes for an active audio node.
+    // |id|: id of the audio node.
+    // |level|: new sound level of the node(volume for output, gain for input).

[asargent_no_longer_on_chrome, 2015/03/17 16:21:15]

it might be worth mentioning the scale of the |level| parameter here (eg 0 to
100)

Also, why is |leve|l a long here whereas we use a double for volume/gain in the
InputDeviceInfo/OutputDeviceInfo dictionaries above?

[jennyz, 2015/03/17 18:17:28]

We are in the process to transit the old audio extension APIs to the new APIs.
The InputDeviceInfo/OutputDeviceInfo will eventually be deprecated and become
legacy. In the old definition, the volume and gain are defined as double, but
actually they should be defined as int, because the data sent from cras audio
api are actually int32. The legacy may come some even older data migration
story, which does not exist anymore. Therefore, we would like to make it right
in the new APIs.

+    static void OnLevelChanged(DOMString id, long level);
   };
 };