Developers are fully encouraged (and encouraged) to use the same ID / API for all their applications.
When sending a message, you specify a registration identifier that is implicitly associated with the package name of the app + device where it was generated. Only the application that requested the registration identifier will receive your message.
Source: I am a member of the Android team at Google.
Trevor Johns Jul 20 '12 at 21:30 2012-07-20 21:30
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