The document discusses the Android Cloud to Device Messaging (C2DM) framework. It allows application servers to send lightweight messages to Android apps. Key points: - C2DM notifies apps of new data to fetch rather than sending large amounts of content. It makes no guarantees about message delivery or order. - Apps can receive messages even when not running by using intent broadcasts. C2DM only passes raw message data. - Apps must register with C2DM to receive messages and can unregister. Registration requires Android 2.2+ with the Market app installed. - The app server sends messages via HTTP using a registration ID and authorization token. C2DM handles storing and delivering messages when

1. Android Cloud
to Device
Messaging
Framework

3. "It allows third-party application servers
to send lightweight messages to their
Android applications"
Not designed to send a lot of content via messages. Instead
use C2DM to tell the application that there is new data on the
server to fetch.

4. "C2DM makes no guarantees about
delivery or the order of messages"
Eg. not suitable for instant messaging, instead use C2DM to
notify the user that there is new messages.

5. "An application on an Android device
doesn’t need to be running to receive
messages"
Intent broadcast wake up the app

6. "It does not provide any built-in user
interface or other handling for message
data"
Passes raw message data. You are free to do whatever you
want with the data. Fire a notification, send SMS, start another
application etc.

7. "It requires devices running Android 2.2
or higher that also have the Market
application installed"
Market is needed for technical reasons. No need to distribute
via Market.

8. "It uses an existing connection for
Google services"
Requires the user to be logged in with a Google Account on the
device.

9. Establish connection
with conn servers
Send to conn
Send to servers
device
Stores message
App server sends
HTTP POST
Google
http://code.google.com/events/io/2010/sessions/push-applications-android.html

10. Lifecycle
Register device
App server send message
Device receives message
Unregister device

11. Register device
App fires off Intent to register with Google
com.google.android.c2dm.intent.REGISTER
App receives Intent with registration ID from Google
com.google.android.c2dm.intent.REGISTRATION
App sends registration ID to app server

12. Register device
// Intent to register
Intent regIntent = new
Intent("com.google.android.c2dm.intent.REGISTER");
// Identifies the app
regIntent.putExtra("app",
PendingIntent.getBroadcast(context, 0, new Intent(), 0));
// Identifies the role account, same as used when sending
regIntent.putExtra("sender", senderEmail);
// Start registration process
context.startService(regIntent);

13. Register device
public void onReceive(Context context, Intent intent) {
if (intent.getAction().equals("...REGISTRATION")) {
handleRegistration(context, intent);
}
}
private void handleRegistration(Context context, Intent intent) {
String registration = intent.getStringExtra("registration_id");
if (intent.getStringExtra("error") != null) {
// Registration failed, should try again later.
} else if (intent.getStringExtra("unregistered") != null) {
// unregistration done
} else if (registration != null) {
// Send the registration ID to app server
// This should be done in a separate thread.
// When done, remember that all registration is done.
}
}

14. Unregister device
App fires off a unregister Intent to register with Google com.
google.android.c2dm.intent.UNREGISTER
App tells app server to remove the stored registration ID

15. <manifest ...
<application ...
<service android:name=".C2DMReceiver" />
<receiver android:name="com.google.android.c2dm.C2DMBroadcastReceiv
<intent-filter>
<action android:name="com.google.android.c2dm.intent.RECEIVE" /
<category android:name="com.markupartist.sthlmtraveling" />
</intent-filter>
<intent-filter>
<action android:name="com.google.android.c2dm.intent.REGISTRATI
<category android:name="com.markupartist.sthlmtraveling" />
</intent-filter>
</receiver>
</application>
<uses-sdk android:minSdkVersion="3" android:targetSdkVersion="8" />
<permission android:name="com.markupartist.sthlmtraveling.permission.
android:protectionLevel="signature" />
<uses-permission android:name="com.markupartist.sthlmtraveling.permis
C2D_MESSAGE" />
<uses-permission android:name="com.google.android.c2dm.permission.REC
<uses-permission android:name="android.permission.INTERNET" />
</manifest>

16. <manifest ...
<application ...
<service android:name=".C2DMReceiver" />
<receiver android:name="com.google.android.c2dm.C2DMBroadcastReceive
<intent-filter>
<action android:name="com.google.android.c2dm.intent.RECEIVE" />
<category android:name=" com.markupartist.sthlmtraveling " />
</intent-filter>
<intent-filter>
<action android:name="com.google.android.c2dm.intent.REGISTRATIO
<category android:name=" com.markupartist.sthlmtraveling " />
</intent-filter>
</receiver>
</application>
<uses-sdk android:minSdkVersion="3" android:targetSdkVersion="8" />
<permission android:name=" com.markupartist.sthlmtraveling .permission.
android:protectionLevel="signature" />
<uses-permission android:name=" com.markupartist.sthlmtraveling .permis
C2D_MESSAGE" />
<uses-permission android:name="com.google.android.c2dm.permission.RECE
<uses-permission android:name="android.permission.INTERNET" />
</manifest>

17. Send message
From the documentation:
For an application server to send a message, the following
things must be in place:
The application has a registration ID that allows it to receive
messages for a particular device.
The third-party application server has stored the registration
ID.
There is one more thing that needs to be in place for the
application server to send messages: a ClientLogin
authorization token. This is something that the developer must
have already set up on the application server for the
application (for more discussion, see Role of the Third-Party
Application Server). Now it will get used to send messages to
the device.
http://code.google.com/android/c2dm/index.html#lifecycle

18. Send message
Retrieve ac2dm auth token (put on app server)
Send authenticated POST
- GoogleLogin auth=<TOKEN>
- URL Encoded parameters
registration_id
collapse_key
deplay_while_idle - optional
data.<KEY> - optional
Using cURL to interact with Google Data services:
http://code.google.com/apis/gdata/articles/using_cURL.html

19. ac2dm authorization token
curl https://www.google.com/accounts/ClientLogin
-d Email=<ACCOUNT_EMAIL> -d Passwd=<PASSWORD>
-d accountType=HOSTED_OR_GOOGLE
-d source=markupartist-sthlmtraveling-1
-d service=ac2dm
SID=DQAAA...
LSID=DQAAA...
Auth=DQAAA...

20. Token can change
URL url = new URL(serverConfig.getC2DMUrl());
HttpURLConnection conn =
(HttpURLConnection) url.openConnection();
...
// Check for updated token header
String updatedAuthToken =
conn.getHeaderField("Update-Client-Auth");
if (updatedAuthToken != null &&
!authToken.equals(updatedAuthToken)) {
serverConfig.updateToken(updatedAuthToken);
}

21. Send message
curl https://android.apis.google.com/c2dm/send
-d registration_id=<REGISTRATION ID>
-d collapse_key=foo
-d data.key1=bar
-d delay_while_idle=1
-H "Authorization: GoogleLogin auth=<AUTH TOKEN>"
Response codes
200 OK
- id=<MESSAGE ID> of sent message, success
- Error=<ERROR CODE>, failed to send
401 Not Authorized
503 Service Unavailable, retry

22. collapse_key
Only the latest message with the same key will be delivered
Avoids multiple messages of the same type to be delivered
to an offline device
State should be in app server and not in the message
Eg. could be the URL to fetch data from

23. delay_while_idle
Message is not immediately sent to the device if it is idle

24. Receive a message
Device receives the message and converts it to Intent
- com.google.android.c2dm.intent.RECEIVE
App wakes up to handle Intent
- data.<KEY> is translated to extras
curl https://android.apis.google.com/c2dm/send
...
-d data.key1=bar
...
protected void onReceive(Context context, Intent intent) {
if (intent.getAction().equals("...RECEIVE")) {
String key1 = intent.getExtras().getString("key1");
// If starting another intent here remember to set the
// flag FLAG_ACTIVITY_NEW_TASK
}
}

25. Chrome to phone
http://code.google.
com/p/chrometophone/source/browse/#svn/trunk/android/c2dm
/com/google/android/c2dm
C2DMessaging.register(context, "Role account
email");
C2DMessaging.unregister(context);

26. public class C2DMReceiver extends C2DMBaseReceiver {
public C2DMReceiver() {
super("Role account email");
}
@Override
public void onRegistrered(Context context, String registration) {
// Handle registrations
}
@Override
public void onUnregistered(Context context) {
// Handle unregistered
}
@Override
public void onError(Context context, String errorId) {
// Handle errors
}
@Override
public void onMessage(Context context, Intent intent) {
// Handle received message.
}
}

27. Problems
Can't send message to device if logged in with the role
account

28. References
http://code.google.com/android/c2dm/
http://code.google.com/events/io/2010/sessions/push-
applications-android.html
http://code.google.com/p/chrometophone/

29. Demo
http://screenr.com/QDn
http://screenr.com/ovn