Read more about the implementation of GCM support on the Android client

They have already written about GCM. What is this article for?

True, they wrote. Literally this week, the GCM article , a new push notification service from Google, was published on Habré (if you are not familiar with Google Cloud Messaging for Android, then I advise you to read it before reading this article, especially in my article the process of creating a project is not described with GCM). I do not know if the author of GCM used it in a real application or not, but I had to. That is why I want to describe something that had no place in the previous article, or that was not explained. Add this all with a comment in a previous article, I fear, an impossible task.

Required Permissions

• <uses-permission android:name="android.permission.INTERNET" /> 

  Everything is clear, we don’t need GCM without Internet access.
  

•  <uses-permission android:name="android.permission.GET_ACCOUNTS" /> 

  GCM requires access to Google account
  

   <uses-permission android:name="android.permission.WAKE_LOCK"/> 

  On this occasion, there was even a dispute in the previous topic, but none of the participants decided to look at the source code. Documentation is silent on this moment, and only says that you may want to grab PowerManager.WakeLock . So, if you use the standard GCM library, then you will have to add such permission.
  
  In short, the mechanism of operation is as follows: our application subscribes to receive broadcast requests. When we receive a request, we set the received Intent to the class name ( setClassName() ) in the name of our service extending GCMBaseIntentService , then we capture WakeLock with the PowerManager.PARTIAL_WAKE_LOCK flag (we don’t let only the CPU fall asleep, the screen and so on sleep peacefully), start Intent as a service, on exit from the onHandleIntent service, release WakeLock .
  ')
  They didn’t believe and didn’t add this permission, and as a result we get the following exception:
  

   java.lang.SecurityException: Neither user 10110 nor current process has android.permission.WAKE_LOCK. 

  
  

•  <permission android:name="{  }.permission.C2D_MESSAGE" android:protectionLevel="signature" /> <uses-permission android:name="{  }.permission.C2D_MESSAGE" /> 

  We create our own permission and request it ourselves. We do this so that no one except us can receive our messages.
  
  Note: if you set minSdkVersion to 16 or higher (Jelly Bean and later versions), then you don’t need this permission (after 2 years, I hope, you can omit it).

•  <uses-permission android:name="com.google.android.c2dm.permission.RECEIVE" /> 

  Actually permission to register in GCM and receive messages.

Does the registration code change (registationId)?

Consider the code from the example application:

 final String regId = GCMRegistrar.getRegistrationId(this); if (regId.equals("")) { // Automatically registers application on startup. GCMRegistrar.register(this, SENDER_ID); } 

There seems to be no other conditions. So, does not change? If you follow this link: http://developer.android.com/intl/ru/guide/google/gcm/adv.html#reg-state , you can find out what can change. There are two such cases:

1. Program update
2. Backup and restore from it

To check for a software update, I wrote a small assistant class. Maybe someone will come in handy:

 import android.content.Context; import android.content.pm.PackageInfo; import android.content.pm.PackageManager; import android.content.pm.PackageManager.NameNotFoundException; import android.preference.PreferenceManager; public final class ApplicationVersionHelper { public static final String APP_VERSION_PREFS = "application_version"; public static boolean isApplicationVersionCodeEqualsSavedApplicationVersionCode(Context context) { return getApplicationVersionCode(context) == getApplicationVersionCodeFromPreferences(context); } public static int getApplicationVersionCode(Context context) { PackageManager pm = context.getPackageManager(); PackageInfo packageInfo; int applicationVersion = 1; try { packageInfo = pm.getPackageInfo(context.getPackageName(), 0); applicationVersion = packageInfo.versionCode; } catch (NameNotFoundException ignored) { } return applicationVersion; } public static int getApplicationVersionCodeFromPreferences(Context context) { return context.getSharedPreferences(APP_VERSION_PREFS, Context.MODE_PRIVATE).getInt("application_version_code", 0); } public static void putCurrentPackageVersionInPreferences(Context context) { context.getSharedPreferences(APP_VERSION_PREFS, Context.MODE_PRIVATE).edit().putInt("application_version_code", getApplicationVersionCode(context)).commit(); } } 

Note that the settings are not obtained through the PreferenceManager.getDefaultSharedPreferences , but through the named settings file. For what this is done, I will explain later.
Now we need to call putCurrentPackageVersionInPreferences after successful registration in GCM and on our service, and the registration verification code turns into:

 final String regId = GCMRegistrar.getRegistrationId(this); if (regId.equals("") || !isApplicationVersionCodeEqualsSavedApplicationVersionCode(this)) { // Automatically registers application on startup. GCMRegistrar.register(this, SENDER_ID); } 

To handle the creation of a backup (not everyone knows about this feature at all. If it became interesting, then read here - http://developer.android.com/intl/ru/guide/topics/data/backup.html ) I propose the following solution : just do not save the settings with the name from the ApplicationVersionHelper.APP_VERSION_PREFS constant when backing up. So the named settings file came in handy :) Then isApplicationVersionCodeEqualsSavedApplicationVersionCode will return false when recovering the data and we will send a request for registration.

Handlers in GCMIntentService

In GCMIntentService (class inherited from GCMBaseIntentService )
we have to override several methods. Briefly about them:

•  protected void onRegistered(Context context, String registrationId) 

  this method is called after successful registration in GCM, from here we need to transfer registrationId to our server

•  protected void onUnregistered(Context context, String registrationId) 

  This method is called after a successful cancellation of registration in GCM, we also transfer registrationId to our server for exclusion from mailing (many applications will never use this feature)

•  protected void onMessage(Context context, Intent intent) 

  receiving a message from GCM, if there is a payload (payload), then the data lies in the intent

•  protected void onDeletedMessages(Context context, int total) 

  Receive notification from GCM about deleted messages, what it is and with what they are, see here - http://developer.android.com/intl/ru/guide/google/gcm/adv.html#payload

•  public void onError(Context context, String errorId) 

  unrecoverable error while retrieving data, errorId error code

•  protected boolean onRecoverableError(Context context, String errorId) 

  recoverable error when receiving data in errorId error code. If we return true , then let us make another attempt, if false , then we will stop trying. I recommend returning super.onRecoverableError(context, errorId); in this method super.onRecoverableError(context, errorId);

Clean for yourself!

Remember to cancel the registration process if it is running, and call GCMRegistrar.onDestroy in the onDestroy method of your main Activity. Here is how I do it:

 @Override protected void onDestroy() { if (registerTask != null) { registerTask.cancel(true); } try { CMRegistrar.onDestroy(this); } catch(Exception ignored) { } super.onDestroy(); } 

registerTask here - asynchronous task ( AsyncTask ).

Conclusion

I advise you to read http://developer.android.com/intl/ru/guide/google/gcm/index.html (and there are 5 points) before using GCM in your application, and if you have questions (how about WAKE_LOCK permissions), then Do not be afraid to get into the source code.