BroadcastReceiver

Suppose you're creating an application for travelers and want to display a Toast every time Airplane mode is changed on the user's phone. You can use BroadcastReceiver for this purpose — it enables you to track phone events like timezone and Airplane mode changes.

Read on to learn more about BroadcastReceiver and find out how to use it in your apps.

What is BroadcastReceiver?

BroadcastReceiver is a component that responds to broadcast messages throughout the system. These can be messages from both the system itself and from any application (including yours). Examples of system messages include a change of date or time (DATE/TIME_CHANGED), turning on Airplane mode (AIRPLANE_MODE_CHANGED), a change in battery status (BATTERY_CHANGED/LOW/OKAY), and so on.

How is BroadcastReceiver used?

You start by creating your BroadcastReceiver:

class YourBroadcastReceiver : BroadcastReceiver() {
    
}

If you don't want to do this manually, Android Studio enables you to create a BroadcastReceiver using a ready-made template:

The receiver must have an onReceive() method. This method is called when the BroadcastReceiver is receiving an Intent broadcast:

override fun onReceive(context: Context, intent: Intent) {
    //Your code here
}

Receivers can either be static or dynamic. The static option is when the BroadcastReceiver subclass is added to the manifest file, and the dynamic option is when the BroadcastReceiver instance is registered during the application lifetime.

The main difference between these options is that a static receiver will work whether the application is running or not — if your app isn't already running the system will launch it when the broadcast is sent. By contrast, dynamic receivers are only triggered when the application is running. So, all dynamic receivers are unregistered at the point of application death.

First, let's look at how to use a static receiver. With this option, you only need to declare the receiver in the AndroidManifest.xml file inside application block:

<receiver
    android:name=".YourBroadcastReceiver"
    android:enabled="true"
    android:exported="true">
    <intent-filter>
        <action android:name="android.intent.action.AIRPLANE_MODE" />
    </intent-filter>
</receiver>

It's as simple as that. Now, when you execute the specified action, the onReceive() method will also be triggered.

You can see an example of a BroadcastReceiver that sends a Toast message when Airplane mode is changed below:

Dynamic receivers are created differently. You don't need to use the manifest file because the receiver will be instantiated and registered directly in the code. Instead, you need to create the receiver and a filter to check for the action you have selected. This is done in the following way:

val receiver: BroadcastReceiver = YourBroadcastReceiver()
val filter = IntentFilter(Intent.ACTION_AIRPLANE_MODE_CHANGED)
applicationContext.registerReceiver(receiver, filter)

As you can see above, the final step is to register the receiver. Now, when you execute the specified action, the onReceive() method will fire, too.

If you want to turn off your receiver, just add this line at the point where you wish to unregister it:

applicationContext.unregisterReceiver(receiver)

Following this, the receiver will no longer be available.

Multiple actions

You may need your application to respond to more than one type of message. Both AIRPLANE_MODE_CHANGED and DATE_CHANGED, for example. So, what's the best way to implement this behavior? Should you add a separate filter for each action or use one filter for them all? The second option is a better way to go. You can do everything with a single filter, so there's no need to create several.

You can see how to add multiple actions using the static approach below:

<receiver
    android:name=".YourBroadcastReceiver"
    android:enabled="true"
    android:exported="true">
    <intent-filter>
        <action android:name="android.intent.action.AIRPLANE_MODE" />
        <action android:name="android.intent.action.DATE_CHANGED" />
    </intent-filter>
</receiver>

And the following example demonstrates how to add more than one action dynamically:

val filter = IntentFilter().apply {
    addAction(Intent.ACTION_AIRPLANE_MODE_CHANGED)
    addAction(Intent.ACTION_DATE_CHANGED)
}

If you need a different result for each action, you can use a when expression in your onReceive() method:

when (intent.action) {
    Intent.ACTION_AIRPLANE_MODE_CHANGED ->
        Toast.makeText(context, "Airplane mode was changed", LENGTH_LONG).show()
    Intent.ACTION_DATE_CHANGED ->
        Toast.makeText(context, "The date was changed", LENGTH_LONG).show()
}

Now the user will see a different Toast message when each action is performed.

Bear in mind that you can add several actions to your filter and create unique messages for them all. The examples in this section are only limited to two for the sake of simplicity.

Conclusion

BroadcastReceiver is a very useful tool that can respond to system and application messages. Before using it, you need to decide whether you want to create your receiver statically or dynamically. Don't forget that you can add multiple actions to your receiver using a single filter.