Google Cloud Messaging (GCM) is an awesome little service. It enabled "push" messages to be sent from Google to a specific Android phone and picked up by your application. The scope for these messages is massive but the intent is for the content to be fairly small. A great example (and a great name) is a tickle. This is very small message intended to instruct the application to go and perform an action.
For example, say your app needs data to be kept in sync with your server. This could be done by polling, but polling is resource intensive and sucks up precious bandwidth. A far better idea would be for your server to send out a message whenever the necessary data is updated and tell your app to come and re-fetch the data when possible. The message sent to your app is called a tickle!
I've created a simple tutorial to briefly describe GCM and show an example. My example does have a lot of moving parts to it but hopefully is simple enough to get the basic idea. For my backend server I'm using a CFML server which keeps track of any device registering for messages and allows me to send a message out too that device.
The sample app uses three components:
- GCM Connection Server - A Google Cloud project to send the message from you to the device.
- Application Server - Your own personal server that tracks device ids and issues messages. This can be cfml, php aspx, whatever, but it needs the capacity to store data, preferably in a db.
- Your Android app - The app you write and distribute as necessary.
Step 1 - The GCM Server
OK First we need to setup our Google Cloud Messaging service. To do this we log onto Google's cloud messaging panel and configure a new project. This project will now sit and wait for instructions and when told will relay messages from our server to our elected android device.
To set this up, follow the instructions here: http://developer.android.com/google/gcm/gs.html
Step 2 - The Application Server
As mentioned for this I'm using CFML here with a mysql db. I've not spent much time on this so it doesn't look pretty but I just wanted to illustrate the point. First the db, I've used one simple table:
CREATE TABLE gcmtest{
intID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
strName varchar(100) not null,
registrationid varchar(200) not null
}
That's it for db work! We have an ID column, a human readable name and a registration id which will contain the device id which is registered with Google.
Now the application server needs two files. One to respond to incoming registration requests and one to send messages, first the registration responder. Now a word of warning, this isn't production worthy, it's just a test bed. You should sanitise and protect your data much more thoroughly. As you can see it takes two values from the URL and inserts them into the db.
receive.cfm:
<cfif structKeyExists(url, "registrationid") AND structKeyExists(url, "name")>
<cfquery name="qInsertReg" datasource="local">
insert into gcmtest
(
registrationid,
strname
)
values
(
<cfqueryparam cfsqltype="CF_SQL_VARCHAR" value="#url.registrationid#" />,
<cfqueryparam cfsqltype="CF_SQL_VARCHAR" value="#url.name#" />
)
</cfquery>
<cfoutput>Done.</cfoutput>
</cfif>
Next you need the page which will send out messages, send.cfm:
<cfif structKeyExists(url, "regid")>
<cfif structKeyExists(url, "strmsg")>
<cfset strmymsg = url.strmsg />
<cfelse>
<cfset strmymsg = "all your base are belong to me" />
</cfif>
<cfset stFields = { "registration_ids": [ "#url.regid#" ], "data": {msg: "#strmymsg#"} }>
<cfhttp url="https://android.googleapis.com/gcm/send" method="post" result="httpResp" timeout="60">
<cfhttpparam type="header" name="Content-Type" value="application/json" />
<cfhttpparam type="header" name="Authorization" value="key=***yourkeyfromgoogle***" />
<cfhttpparam type="body" value="#serializeJSON(stFields)#">
</cfhttp>
<cfif httpResp.status_code eq 200>
<span style="color:green;font-weight:bold;">Message Sent</span><br />
</cfif>
</cfif>
<cfquery name="qGet" datasource="local">
select * from gcmtest
</cfquery>
<form action="send.cfm" method="GET">
<strong>Select Recipient:</strong>
<br />
<cfoutput query="qGet">
<input type="radio" name="regid" value="#registrationid#">#strname#<br>
</cfoutput>
<br />
<strong>Message:</strong><br />
<input name="strmsg" type="text" /><br /><br />
<input type="submit" value="submit" name="submit">
</form>
As you can see, we get everything from the gcmtest table and output it with radio buttons and a message box. When you hit send it self posts and the cfhttp takes over. For the cfhttp we use a json struct containing the registration id and the message. This we pass to the url https://android.googleapis.com/gcm/send obviously not forgetting to pass along our authorization key we created in step one.
That's it for the server side!
Step 3 - Android In Action
Now we need to have our Android app a) register with and b) receive messages from CGM. Now there is a lot of code here, so I'll post the project on gitHub but I'm hoping to cover the basics. There are obviously loads of things you can do with this message but for now I'm just going to use the basic Google demo method of posting a notification. To do this we register a WakefulBroadcastReceiver which will keep the device alive in case it hears a broadcast message. Then it will fire an intentService.
a) Manifest.xml
You need these permissions:
<!-- GCM connects to Google Services. -->
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.GET_ACCOUNTS" />
<uses-permission android:name="android.permission.WAKE_LOCK" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<permission android:name="com.google.android.gcm.demo.app.permission.C2D_MESSAGE" android:protectionLevel="signature" />
<uses-permission android:name="com.google.android.gcm.demo.app.permission.C2D_MESSAGE" />
<uses-permission android:name="com.google.android.c2dm.permission.RECEIVE" />
You also need to register the services:
<receiver
android:name=".GcmBroadcastReceiver"
android:permission="com.google.android.c2dm.permission.SEND" >
<intent-filter>
<!-- Receives the actual messages. -->
<action android:name="com.google.android.c2dm.intent.RECEIVE" />
<category android:name="com.google.android.gcm.demo.app" />
</intent-filter>
</receiver>
<service android:name=".GcmIntentService" />
Then what we're going to do in ActivityMain is check for the existence of a stored preference for the users name. If found we'll start a Fragment to welcome the user, if not we'll ask the user for their name.
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
String strName = getNameFromPrefs();
if(strName.length() > 0){
Bundle bundle = new Bundle();
bundle.putString(FragmentWelcome.TAG_NAME, strName);
FragmentWelcome frag = new FragmentWelcome();
frag.setArguments(bundle);
ft.replace(R.id.activity_default_fragment_container, frag, FragmentWelcome.class.getSimpleName());
}else{
ft.replace(R.id.activity_default_fragment_container, new FragmentName(), FragmentName.class.getSimpleName());
}
ft.commit();
Now FragmentName is really simple and I shall spare you the details. It is a simple layout with an input text box. It checks for internet and validates the input and if so it calls a listener which returns to the main activity. The Activity saves the user's name in the savedPreferences and opens FragmentWelcome.
FragmentWelcome covers the following steps:
- Get user's name via intent
- verify GooglePlayServices
- Gets your GCM Sender ID (created in step 1). Store this in strings.xml or somewhere sensible.
-
Using your GCM Sender ID it posts to Google Play Services and gets your device id.
private void registerInBackground() {
new AsyncTask<String, Void, String>() {
@Override
protected String doInBackground(String... params) {
String msg = "";
String name = "";
try {
if (gcm == null) {
gcm = GoogleCloudMessaging.getInstance(context);
}
regid = gcm.register(SENDER_ID);
- Gets the URL of the receieve.cfm file we created in step 2 (again store it in strings or prefs) and appends the user's name and the device id to the url.
- Call an AsyncTask to open DefaultHttpClient and hit the url we have now created.
private class UploadRegistrationID extends AsyncTask<String, Void, String> {
@Override
protected String doInBackground(String... urls) {
String response = "";
for (String url : urls) {
DefaultHttpClient client = new DefaultHttpClient();
HttpGet httpGet = new HttpGet(url);
try {
HttpResponse execute = client.execute(httpGet);
InputStream content = execute.getEntity().getContent();
BufferedReader buffer = new BufferedReader(new InputStreamReader(content));
String s = "ServerResponse:";
while ((s = buffer.readLine()) != null) {
response += s;
}
} catch (Exception e) {
e.printStackTrace();
}
}
return response;
}
That's it. Once you've run your Android app and entered your name you should be able to go to your send.cfm file in a web browser and send a message through to your device. The message will then pop up in your notification window!
Here's the Android project on github:
https://github.com/jamessolo12/gcmPushDemo
Magic ;)
