How a notification manager and broadcast receiver works under android?

A broadcast receiver [receiver] is an Android component which allows you to register for system or application events. All registered receivers for an event are notified by the Android runtime once this event happens.

For example, applications can register for the ACTION_BOOT_COMPLETED system event which is fired once the Android system has completed the boot process.

A receiver can be registered via the AndroidManifest.xml file.

Alternatively to this static registration, you can also register a receiver dynamically via the Context.registerReceiver[] method.

The implementing class for a receiver extends the BroadcastReceiver class.

If the event for which the broadcast receiver has registered happens, the onReceive[] method of the receiver is called by the Android system.

After the onReceive[] of the receiver class has finished, the Android system is allowed to recycle the receiver.

Before API level 11, you could not perform any asynchronous operation in the onReceive[] method, because once the onReceive[] method had been finished, the Android system was allowed to recycle that component. If you have potentially long running operations, you should trigger a service instead.

Since Android API 11 you can call the goAsync[] method. This method returns an object of the PendingResult type. The Android system considers the receiver as alive until you call the PendingResult.finish[] on this object. With this option you can trigger asynchronous processing in a receiver. As soon as that thread has completed, its task calls finish[] to indicate to the Android system that this component can be recycled.

As of Android 3.1 the Android system excludes all receiver from receiving intents by default if the corresponding application has never been started by the user or if the user explicitly stopped the application via the Android menu [in Manage Application ].

This is an additional security feature as the user can be sure that only the applications he started will receive broadcast intents.

You can use the following command from the adb command line tool. The class name and package names which are targeted via the command line tool need to be as defined in the manifest. You should send the intent you generated to your specific component, for example if you send a general ACTION_BOOT_COMPLETED broadcast, this will trigger a lot of things in an Android system.

A pending intent is a token that you give to another application. For example, the notification manager, alarm manager or other 3rd party applications]. This allows the other application to restore the permissions of your application to execute a predefined piece of code.

To perform a broadcast via a pending intent, get a PendingIntent via the getBroadcast[] method of the PendingIntent class. To perform an activity via a pending intent, you receive the activity via PendingIntent.getActivity[].

In this exercise you define a broadcast receiver which listens to telephone state changes. If the phone receives a phone call, then our receiver will be notified and log a message.

Create a new project called de.vogella.android.receiver.phone. Also create an activity.

TIP:Remember that your receiver is only called if the user started it once. This requires an activity.

Create the MyPhoneReceiver class.

Add the android.permission.READ_PHONE_STATE permission to your manifest file which allows you to listen to state changes in your receiver. Also Register your receiver in your manifest file. The resulting manifest should be similar to the following listing.

Install your application and simulate a phone call via the emulator controls. Validate that your receiver is called and logs a message to the LogCat view.

In this chapter we will schedule a receiver via the Android alert manager system service. Once called, it uses the Android vibrator manager and a popup message [Toast] to notify the user.

Create a new project called de.vogella.android.alarm with the activity called AlarmActivity.

Create the following layout.

Create the following broadcast receiver class. This class will get the vibrator service.

Register this class as a broadcast receiver in AndroidManifest.xml and request authorization to vibrate the phone.

Change the code of your AlarmActivity class to the following. This activity creates an intent to start the receiver and register this intent with the alarm manager service.

Run your application on the device. Set your time and start the alarm. After the defined number of seconds a Toast should be displayed. Keep in mind that the vibration alarm does not work on the Android emulator.

Receiver can be registered via the Android manifest file. You can also register and unregister a receiver at runtime via the Context.registerReceiver[] and Context.unregisterReceiver[] methods.

You can use the PackageManager class to enable or disable receivers registered in your AndroidManifest.xml file.

An intent to trigger a receiver [ broadcast intent ] is not available anymore after it was sent and processed by the system. If you use the sendStickyBroadcast[Intent] method, the corresponding intent is sticky, meaning the intent you are sending stays around after the broadcast is complete.

The Android system uses sticky broadcast for certain system information. For example, the battery status is send as sticky intent and can get received at any time. The following example demonstrates that.

You can retrieve that data through the return value of registerReceiver[BroadcastReceiver, IntentFilter] `. This also works for a null `BroadcastReceiver.

In all other ways, this behaves just as sendBroadcast[Intent].

Sticky broadcast intents typically require special permissions.

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