Many of the media players on Android use the convention of
broadcasting intents for various events such as track started,
completed, etc. so that other apps can react to these events.
Scrobbling is a common use case.

For example, lastfm, Amazon CloudPlayer, WinAmp, and the built in
Android music player declare the following intent actions that they
broadcast when a new track starts:

<action android:name="fm.last.android.metachanged" />
<action android:name="com.sec.android.app.music.metachanged" />
<action android:name="com.nullsoft.winamp.metachanged" />
<action android:name="com.amazon.mp3.metachanged" />
<action android:name="com.android.music.metachanged" />

Does Rdio broadcast a similar intent action? I've looked around and
cant seem to find any mention of it.