Android Cloud
to Device
Messaging
Framework

How to keep data fresh?

Polling
Simple to implement
Ask for data periodically
Radio draws a lot of power

Pushing
Harder to implement
Less battery consumption
Only uses network when needed
Constant connection

Components
Intent
Description of an action to perform
BroadcastReceiver
Recieve intents
IntentService
Handles asynchronous request, started when needed
WakeLock
Wake up the device at desired level
AlarmManager
Schedule the app to run in the future
Synchronize polls with INTERVAL_*

Some interesting projects
CouchOne Mobile for Android
http://www.couch.io/android
Ericsson Labs - Mobile Push
https://labs.ericsson.com/apis/mobile-push/
Whoopingkof - Evented Sockets
http://github.com/voltron/whoopingkof/

"It allows third-party
application servers to
send lightweight messages
to their Android
applications"

"C2DM makes no
guarantees about delivery
or the order of messages"

"An application on an
Android device doesn’t
need to be running to
receive messages"

"It does not provide any
built-in user interface or
other handling for
message data"

"It requires devices
running Android 2.2 or
higher that also have the
Market application
installed"

"It uses an existing
connection for Google
services"

http://code.google.com/events/io/2010/sessions/push-applications-android.html

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

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

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);

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.
}
}

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

<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>

<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>

Send Message

Send message
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.
http://code.google.com/android/c2dm/index.html#lifecycle
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.
From the documentation:

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

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
ac2dm authorization token
SID=DQAAA...
LSID=DQAAA...
Auth=DQAAA...
Service
name for
C2DM
Authorizatio
n token

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);
}

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>"
Send message
Response codes
200 OK
- id=<MESSAGE ID> of sent message, success
- Error=<ERROR CODE>, failed to send
401 Not Authorized
503 Service Unavailable, retry

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

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

Receive a message

Receive a message
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
}
}
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

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);

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.
}
}

Problems
Can't send message to device if logged in with the role
account
401

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/

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