There are following two important steps to make BroadcastReceiver works for the systen broadcasted intents:
Creating the Broadcast Receiver.
Registering Broadcast Receiver
There is one additional steps in case you are going to implement your custom intents then you will have to create and broadcast those intents.
Creating the Broadcast Receiver
A broadcast receiver is implemented as a subclass of BroadcastReceiver class and overriding the onReceive() method where each message is received as a Intent object parameter.
public class MyReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
Toast.makeText(context, "Intent Detected.", Toast.LENGTH_LONG).show();
}
}
Registering Broadcast Receiver
An application listens for specific broadcast intents by registering a broadcast receiver in AndroidManifest.xml file. Consider we are going to register MyReceiver for system generated event ACTION_BOOT_COMPLETED which is fired by the system once the Android system has completed the boot process.
<application
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<receiver android:name="MyReceiver">
<intent-filter>
<action android:name="android.intent.action.BOOT_COMPLETED">
</action>
</intent-filter>
</receiver>
</application>
Now whenever your Android device gets booted, it will be intercepted by BroadcastReceiver MyReceiver and implemented logic inside onReceive() will be executed.
There are several system generated events defined as final static fields in the Intent class. The following table lists a few important system events.
Event Constant Description
android.intent.action.BATTERY_CHANGED Sticky broadcast containing the charging state, level, and other information about the battery.
android.intent.action.BATTERY_LOW Indicates low battery condition on the device.
android.intent.action.BATTERY_OKAY Indicates the battery is now okay after being low.
android.intent.action.BOOT_COMPLETED This is broadcast once, after the system has finished booting.
android.intent.action.BUG_REPORT Show activity for reporting a bug.
android.intent.action.CALL Perform a call to someone specified by the data.
android.intent.action.CALL_BUTTON The user pressed the "call" button to go to the dialer or other appropriate UI for placing a call.
android.intent.action.DATE_CHANGED The date has changed.
android.intent.action.REBOOT Have the device reboot.
Broadcasting Custom Intents
If you want your application itself should generate and send custom intents then you will have to create and send those intents by using the sendBroadcast() method inside your activity class. If you use the sendStickyBroadcast(Intent) method, the Intent is sticky, meaning the Intent you are sending stays around after the broadcast is complete.
public void broadcastIntent(View view)
{
Intent intent = new Intent();
intent.setAction("com.tutorialspoint.CUSTOM_INTENT");
sendBroadcast(intent);
}
This intent com.tutorialspoint.CUSTOM_INTENT can also be regsitered in similar way as we have regsitered system generated intent.
<application
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<receiver android:name="MyReceiver">
<intent-filter>
<action android:name="com.tutorialspoint.CUSTOM_INTENT">
</action>
</intent-filter>
</receiver>
</application>
Example
This example will explain you how to create BroadcastReceiver to intercept custom intent. Once you are familiar with custom intent, then you can program your application to intercept system generated intents. So let's follow the following steps to modify the Android application we created in Hello World Example chapter:
Step Description
1 You will use Eclipse IDE to create an Android application and name it as HelloWorld under a package com.example.helloworld as explained in the Hello World Example chapter.
2 Modify main activity file MainActivity.java to add broadcastIntent() method.
3 Create a new java file called MyReceiver.java under the package com.example.helloworld to define a BroadcastReceiver.
4 An application can handle one or more custom and system intents without any restrictions. Every indent you want to intercept must be registered in your AndroidManifest.xml file using <receiver.../> tag
5 Modify the detault content of res/layout/activity_main.xml file to include a button to broadcast intent.
6 Define a constant broadcast_intent in res/values/strings.xml file
7 Run the application to launch Android emulator and verify the result of the changes done in the aplication.
Following is the content of the modified main activity file src/com.example.helloworld/MainActivity.java. This file can include each of the fundamental lifecycle methods. We have added broadcastIntent() method to broadcast a custom intent.
package com.example.helloworld;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.content.Intent;
import android.view.View;
public class MainActivity extends Activity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
getMenuInflater().inflate(R.menu.activity_main, menu);
return true;
}
// broadcast a custom intent.
public void broadcastIntent(View view)
{
Intent intent = new Intent();
intent.setAction("com.tutorialspoint.CUSTOM_INTENT");
sendBroadcast(intent);
}
}
Following is the content of src/com.example.helloworld/MyReceiver.java:
package com.example.helloworld;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.widget.Toast;
public class MyReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
Toast.makeText(context, "Intent Detected.", Toast.LENGTH_LONG).show();
}
}
Following will the modified content of AndroidManifest.xml file. Here we have added <service.../> tag to include our service:
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.helloworld"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="8"
android:targetSdkVersion="15" />
<application
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name=".MainActivity"
android:label="@string/title_activity_main" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER"/>
</intent-filter>
</activity>
<receiver android:name="MyReceiver">
<intent-filter>
<action android:name="com.tutorialspoint.CUSTOM_INTENT">
</action>
</intent-filter>
</receiver>
</application>
</manifest>
Following will be the content of res/layout/activity_main.xml file to include a button to broadcast our custom intent:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<Button android:id="@+id/btnStartService"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="@string/broadcast_intent"
android:onClick="broadcastIntent"/>
</LinearLayout>
Following will be the content of res/values/strings.xml to define two new constants:
<resources>
<string name="app_name">HelloWorld</string>
<string name="hello_world">Hello world!</string>
<string name="menu_settings">Settings</string>
<string name="title_activity_main">MainActivity</string>
<string name="broadcast_intent">Broadcast Intent</string>
</resources>
For More Details
For instance, a Broadcast receiver triggers battery Low notification that you see on your mobile screen.
Other instances caused by a Broadcast Receiver are new friend notifications, new friend feeds, new message etc. on your Facebook app.
In fact, you see broadcast receivers at work all the time. Notifications like incoming messages, WiFi Activated/Deactivated message etc. are all real-time announcements of what is happening in the Android system and the applications
Consider this:
You have an important social gathering to attend. Because of your shoddy memory, you have requested your friend to notify you a day before the event. Now, because you have ‘registered’ for the said friend’s help, you will get a reminder from him as discussed. This is roughly how the Broadcast Receiver works.
We have also discussed an example at the end of this Android Tutorial (in the example, a notification is generated once the system time is changed).
How important is it to implement Broadcast Receivers correctly?
If you wish to create a good Android application, this is of utmost importance. If the broadcast events do not perform their job (of sending notifications to support the application’s primary task) perfectly, the application would not be intuitive and user friendly.
Registration of Broadcast Receiver
There are two ways to register a Broadcast Receiver; one is Static and the other Dynamic.
1) Static: Use <receiver> tag in your Manifest file. (AndroidManifest.xml)
2) Dynamic: Use Context.registerReceiver () method to dynamically register an instance.
Classes of Broadcasts
The two major classes of broadcasts are:
1) Ordered Broadcasts: These broadcasts are synchronous, and therefore follow a specific order. The order is defined using android: priority attribute. The receivers with greater priority would receive the broadcast first. In case there are receivers with same priority levels, the broadcast would not follow an order. Each receiver (when it receives the broadcast) can either pass on the notification to the next one, or abort the broadcast completely. On abort, the notification would not be passed on to the receivers next in line.
2) Normal Broadcasts: Normal broadcasts are not orderly. Therefore, the registered receivers often run all at the same time. This is very efficient, but the Receivers are unable to utilize the results.
Sometimes to avoid system overload, the system delivers the broadcasts one at a time, even in case of normal broadcasts. However, the receivers still cannot use the results.
Difference between Activity Intent and Broadcasting Intent
You must remember that Broadcasting Intents are different from the Intents used to start an Activity or a Service (discussed in previous Android Tutorials). The intent used to start an Activity makes changes to an operation the user is interacting with, so the user is aware of the process. However, in case of broadcasting intent, the operation runs completely in the background, and is therefore invisible to the user.
Implementing the Broadcast Receiver
You need to follow these steps to implement a broadcast receiver:
1) Create a subclass of Android’s BroadcastReceiver
2) Implement the onReceive() method: In order for the notification to be sent, an onReceive() method has to be implemented. Whenever the event for which the receiver is registered occurs, onReceive() is called. For instance, in case of battery low notification, the receiver is registered to Intent.ACTION_BATTERY_LOW event. As soon as the battery level falls below the defined level, this onReceive() method is called.
Following are the two arguments of the onReceive() method:
Context: This is used to access additional information, or to start services or activities.
Intent: The Intent object is used to register the receiver.
Security
As the broadcast receivers have a global work-space, security is very important concern here. If you do not define the limitations and filters for the registered receivers, other applications can abuse them.
Here are a few limitations that might help:
Whenever you publish a receiver in your application’s manifest, make it unavailable to external applications by using android: exported=”false”. You might think that specifying Intent filters while publishing the receiver would do the task for you, when in reality they are not enough.
When you send a broadcast, it is possible for the external applications too to receive them. This can be prevented by specifying a few limitations.
Similarly, when you register your receiver using registerReceiver, any application may send it broadcasts. This can be prevented using permissions as well.
(PS: As of Android 3.1, the Android system will not receive any external Intent, so the system is comparatively secure now.)
Prolonged Operations
The Broadcast Receiver object is active only for the duration of onReceive (Context, Intent).
Therefore, if you need to allow an action after receiving the notification services should be triggered, and not broadcast receivers.
To show a dialogue, then you should use NotificationManager API
If you wish to send a broadcast intent that would stick around even after the broadcast is complete, you must use sendStickyBroadcast (Intent) method.
Broadcast Receiver Example
In this sample application, a notification is generated when you change the system time. The notification when clicked leads the user to the Contacts. This is how the application works:
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