62. Android Broadcast Intents and Broadcast Receivers

In addition to providing a mechanism for launching application activities, intents are also used as a way to broadcast system wide messages to other components on the system. This involves the implementation of Broadcast Intents and Broadcast Receivers, both of which are the topic of this chapter.

62.1 An Overview of Broadcast Intents

Broadcast intents are Intent objects that are broadcast via a call to the sendBroadcast(), sendStickyBroadcast() or sendOrderedBroadcast() methods of the Activity class (the latter being used when results are required from the broadcast). In addition to providing a messaging and event system between application components, broadcast intents are also used by the Android system to notify interested applications about key system events (such as the external power supply or headphones being connected or disconnected).

When a broadcast intent is created, it must include an action string in addition to optional data and a category string. As with standard intents, data is added to a broadcast intent using key-value pairs in conjunction with the putExtra() method of the intent object. The optional category string may be assigned to a broadcast intent via a call to the addCategory() method.

The action string, which identifies the broadcast event...

62.2 An Overview of Broadcast Receivers

An application listens for specific broadcast intents by registering a broadcast receiver. Broadcast receivers are implemented by extending the Android BroadcastReceiver class and overriding the onReceive() method. The broadcast receiver may then be registered, either within code (for example within an activity), or within a manifest file. Part of the registration implementation involves the creation of intent filters to indicate the specific broadcast intents the receiver is required to listen for. This is achieved by referencing the action string of the broadcast intent. When a matching broadcast is detected, the onReceive() method of the broadcast receiver is called, at which point the method has 5 seconds within which to perform any necessary tasks before returning. It is important to note that a broadcast receiver does not need to be running all the time. In the event that a matching intent is detected, the Android runtime system will automatically...

62.3 Obtaining Results from a Broadcast

When a broadcast intent is sent using the sendBroadcast() method, there is no way for the initiating activity to receive results from any broadcast receivers that pick up the broadcast. In the event that return results are required, it is necessary to use the sendOrderedBroadcast() method instead. When a broadcast intent is sent using this method, it is delivered in sequential order to each broadcast receiver with a registered interest.

The sendOrderedBroadcast() method is called with a number of arguments including a reference to another broadcast receiver (known as the result receiver) which is to be notified when all other broadcast receivers have handled the intent, together with a set of data references into which those receivers can place result data. When all broadcast receivers have been given the opportunity to handle the broadcast, the onReceive() method of the result receiver is called and passed the result data.

62.4 Sticky Broadcast Intents

By default, broadcast intents disappear once they have been sent and handled by any interested broadcast receivers. A broadcast intent can, however, be defined as being “sticky”. A sticky intent, and the data contained therein, remains present in the system after it has completed. The data stored within a sticky broadcast intent can be obtained via the return value of a call to the registerReceiver() method, using the usual arguments (references to the broadcast receiver and intent filter object). Many of the Android system broadcasts are sticky, a prime example being those broadcasts relating to battery level status.

A sticky broadcast may be removed at any time via a call to the removeStickyBroadcast() method, passing through as an argument a reference to the broadcast intent to be removed.

62.5 The Broadcast Intent Example

The remainder of this chapter will work through the creation of an Android Studio based example of broadcast intents in action. In the first instance, a simple application will be created for the purpose of issuing a custom broadcast intent. A corresponding broadcast receiver will then be created that will display a message on the display of the Android device when the broadcast is detected. Finally, the broadcast receiver will be modified to detect notification by the system that external power has been disconnected from the device.

62.6 Creating the Example Application

Select the Create New Project quick start option from the welcome screen and, within the resulting new project dialog, choose the Empty Activity template before clicking on the Next button.

Enter SendBroadcast into the Name field and specify com.ebookfrenzy.sendbroadcast as the package name. Before clicking on the Finish button, change the Minimum API level setting to API 26: Android 8.0 (Oreo) and the Language menu to Kotlin.

Once the new project has been created, locate and load the activity_main.xml layout file located in the Project tool window under app -> res -> layout and, with the Layout Editor tool in Design mode, replace the TextView object with a Button view and set the text property so that it reads “Send Broadcast”. Once the text value has been set, follow the usual steps to extract the string to a resource named send_broadcast.

With the button still selected in the layout, locate the onClick property in...

62.7 Creating and Sending the Broadcast Intent

Having created the framework for the SendBroadcast application, it is now time to implement the code to send the broadcast intent. This involves implementing the broadcastIntent() method specified previously as the onClick target of the Button view in the user interface. Locate and double-click on the MainActivity.kt file and modify it to add the code to create and send the broadcast intent. Once modified, the source code for this class should read as follows:

package com.ebookfrenzy.sendbroadcast

 

import androidx.appcompat.app.AppCompatActivity

import android.os.Bundle

import android.content.Intent

import android.view.View

 

class MainActivity : AppCompatActivity() {

 

    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)

        setContentView...

62.8 Creating the Broadcast Receiver

In order to create the broadcast receiver, a new class needs to be created which subclasses the BroadcastReceiver superclass. Within the Project tool window, navigate to app -> java and right-click on the package name. From the resulting menu, select the New -> Other -> Broadcast Receiver menu option, name the class MyReceiver and make sure the Exported and Enabled options are selected. These settings allow the Android system to launch the receiver when needed and ensure that the class can receive messages sent by other applications on the device. With the class configured, click on Finish.

Once created, Android Studio will automatically load the new MyReceiver.kt class file into the editor where it should read as follows:

package com.ebookfrenzy.sendbroadcast

import android.content.BroadcastReceiver

import android.content.Context

import android.content.Intent

class MyReceiver : BroadcastReceiver() {

   ...

62.9 Registering the Broadcast Receiver

The project needs to publicize the presence of the broadcast receiver and must include an intent filter to specify the broadcast intents in which the receiver is interested. When the BroadcastReceiver class was created in the previous section, Android Studio automatically added a <receiver> element to the manifest file. All that remains, therefore, is to add code within the MainActivity.kt file to create an intent filter and to register the receiver:

package com.ebookfrenzy.sendbroadcast

 

import androidx.appcompat.app.AppCompatActivity

import android.os.Bundle

import android.content.Intent

import android.view.View

import android.content.IntentFilter

import android.content.BroadcastReceiver

 

class MainActivity : AppCompatActivity() {

 

    var receiver: BroadcastReceiver? = null

 

    override fun onCreate(savedInstanceState: Bundle?)...

62.10 Testing the Broadcast Example

In order to test the broadcast sender and receiver, run the SendBroadcast app on a device or AVD and wait for it to appear on the display. Once running, touch the button, at which point the toast message reading “Broadcast Intent Detected.” should pop up for a few seconds before fading away.

62.11 Listening for System Broadcasts

The final stage of this example is to modify the intent filter for the broadcast receiver to listen also for the system intent that is broadcast when external power is disconnected from the device. That action is android.intent.action.ACTION_POWER_DISCONNECTED. Modify the configureReceiver() method in the MainActivity.kt file to add this additional filter:

private fun configureReceiver() {

    val filter = IntentFilter()

    filter.addAction("com.ebookfrenzy.sendbroadcast")

    filter.addAction("android.intent.action.ACTION_POWER_DISCONNECTED")

    receiver = MyReceiver()

    registerReceiver(receiver, filter)

}

Since the onReceive() method in the MyReceiver.kt file is now going to be listening for two types of broadcast intent, it is worthwhile to modify the code so that the action string of the current...

62.12 Summary

Broadcast intents are a mechanism by which an intent can be issued for consumption by multiple components on an Android system. Broadcasts are detected by registering a Broadcast Receiver which, in turn, is configured to listen for intents that match particular action strings. In general, broadcast receivers remain dormant until woken up by the system when a matching intent is detected. Broadcast intents are also used by the Android system to issue notifications of events such as a low battery warning or the connection or disconnection of external power to the device.

In addition to providing an overview of Broadcast intents and receivers, this chapter has also worked through an example of sending broadcast intents and the implementation of a broadcast receiver to listen for both custom and system broadcast intents.