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Broadcast Receivers in Xamarin.Android

  • Artikkeli

This section discusses how to use a Broadcast Receiver.

Broadcast Receiver Overview

A broadcast receiver is an Android component that allows an application to respond to messages (an Android Intent) that are broadcast by the Android operating system or by an application. Broadcasts follow a publish-subscribe model – an event causes a broadcast to be published and received by those components that are interested in the event.

Android identifies two types of broadcasts:

  • Explicit broadcast – These types of broadcasts target a specific application. The most common use of an explicit broadcast is to start an Activity. An example of an explicit broadcast when an app needs to dial a phone number; it will dispatch an Intent that targets the Phone app on Android and pass along the phone number to be dialed. Android will then route the intent to the Phone app.
  • Implicit broadcast – These broadcasts are dispatched to all apps on the device. An example of an implicit broadcast is the ACTION_POWER_CONNECTED intent. This intent is published each time Android detects that the battery on the device is charging. Android will route this intent to all apps that have registered for this event.

The broadcast receiver is a subclass of the BroadcastReceiver type and it must override the OnReceive method. Android will execute OnReceive on the main thread, so this method should be designed to execute quickly. Care should be taken when spawning threads in OnReceive because Android may terminate the process when the method finishes. If a broadcast receiver must perform long running work then it is recommended to schedule a job using the JobScheduler or the Firebase Job Dispatcher. Scheduling work with a job will be discussed in a separate guide.

An intent filter is used to register a broadcast receiver so that Android can properly route messages. The intent filter can be specified at runtime (this is sometimes referred to as a context-registered receiver or as dynamic registration) or it can be statically defined in the Android Manifest (a manifest-registered receiver). Xamarin.Android provides a C# attribute, IntentFilterAttribute, that will statically register the intent filter (this will be discussed in more detail later in this guide). Starting in Android 8.0, it is not possible for an application to statically register for an implicit broadcast.

The primary difference between the manifest-registered receiver and the context-registered receiver is that a context-registered receiver will only respond to broadcasts while an application is running, while a manifest-registered receiver can respond to broadcasts even though the app may not be running.

There are two sets of APIs for managing a broadcast receiver and sending broadcasts:

  1. Context – The Android.Content.Context class can be used to register a broadcast receiver that will respond to system-wide events. The Context is also used to publish system-wide broadcasts.
  2. LocalBroadcastManager – This is an API that is available through the Xamarin Support Library v4 NuGet package. This class is used to keep broadcasts and broadcast receivers isolated in the context of the application that is using them. This class can be useful for preventing other applications from responding to application-only broadcasts or sending messages to private receivers.

A broadcast receiver may not display dialogs, and it is strongly discouraged to start an activity from within a broadcast receiver. If a broadcast receiver must notify the user, then it should publish a notification.

It is not possible to bind to or start a service from within a broadcast receiver.

This guide will cover how to create a broadcast receiver and how to register it so that it may receive broadcasts.

Creating a Broadcast Receiver

To create a broadcast receiver in Xamarin.Android, an application should subclass the BroadcastReceiver class, adorn it with the BroadcastReceiverAttribute, and override the OnReceive method:

[BroadcastReceiver(Enabled = true, Exported = false)]
public class SampleReceiver : BroadcastReceiver
{
    public override void OnReceive(Context context, Intent intent)
    {
        // Do stuff here.

        String value = intent.GetStringExtra("key");
    }
}

When Xamarin.Android compiles the class, it will also update the AndroidManifest with the necessary meta-data to register the receiver. For a statically-registered broadcast receiver, the Enabled properly must be set to true, otherwise Android will not be able to create an instance of the receiver.

The Exported property controls whether the broadcast receiver can receive messages from outside the application. If the property is not explicitly set, the default value of the property is determined by Android based on if there are any intent-filters associated with the broadcast receiver. If there is at least one intent-filter for the broadcast receiver then Android will assume that the Exported property is true. If there are no intent-filters associated with the broadcast receiver, then Android will assume that the value is false.

The OnReceive method receives a reference to the Intent that was dispatched to the broadcast receiver. This makes it possible for the sender of the intent to pass values to the broadcast receiver.

Statically registering a Broadcast Receiver with an Intent Filter

When a BroadcastReceiver is decorated with the IntentFilterAttribute, Xamarin.Android will add the necessary <intent-filter> element to the Android manifest at compile time. The following snippet is an example of a broadcast receiver that will run when a device has finished booting (if the appropriate Android permissions were granted by the user):

[BroadcastReceiver(Enabled = true)]
[IntentFilter(new[] { Android.Content.Intent.ActionBootCompleted })]
public class MyBootReceiver : BroadcastReceiver
{
    public override void OnReceive(Context context, Intent intent)
    {
        // Work that should be done when the device boots.     
    }
}

Note

In Android 8.0 (API 26 and above), Google placed limitations on what apps can do while users aren't directly interacting with them. These limitations affect background services and implicit broadcast receivers such as Android.Content.Intent.ActionBootCompleted. Because of these limitations, you might have difficulties registering a Boot Completed broadcast receiver on newer versions of Android. If this is the case, note that these restrictions do not apply to foreground services, which can be called from your broadcast receiver.

It is also possible to create an intent filter that will respond to custom intents. Consider the following example:

[BroadcastReceiver(Enabled = true)]
[IntentFilter(new[] { "com.xamarin.example.TEST" })]
public class MySampleBroadcastReceiver : BroadcastReceiver
{
    public override void OnReceive(Context context, Intent intent)
    {
        // Do stuff here
    }
}

Apps that target Android 8.0 (API level 26) or higher may not statically register for an implicit broadcast. Apps may still statically register for an explicit broadcast. There is a small list of implicit broadcasts that are exempt from this restriction. These exceptions are described in the Implicit Broadcast Exceptions guide in the Android documentation. Apps that are interested in implicit broadcasts must do so dynamically using the RegisterReceiver method. This is described next.

Context-Registering a Broadcast Receiver

Context-registration (also referred to as dynamic registration) of a receiver is performed by invoking the RegisterReceiver method, and the broadcast receiver must be unregistered with a call to the UnregisterReceiver method. To prevent leaking resources, it is important to unregister the receiver when it is no longer relevant for the context (the Activity or service). For example, a service may broadcast an intent to inform an Activity that updates are available to be displayed to the user. When the Activity starts, it would register for those Intents. When the Activity is moved into the background and no longer visible to the user, it should unregister the receiver because the UI for displaying the updates is no longer visible. The following code snippet is an example of how to register and unregister a broadcast receiver in the context of an Activity:

[Activity(Label = "MainActivity", MainLauncher = true, Icon = "@mipmap/icon")]
public class MainActivity: Activity
{
    MySampleBroadcastReceiver receiver;

    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        receiver = new MySampleBroadcastReceiver();

        // Code omitted for clarity
    }

    protected override void OnResume()
    {
        base.OnResume();
        RegisterReceiver(receiver, new IntentFilter("com.xamarin.example.TEST"));
        // Code omitted for clarity
    }

    protected override void OnPause()
    {
        UnregisterReceiver(receiver);
        // Code omitted for clarity
        base.OnPause();
    }
}

In the previous example, when the Activity comes into the foreground, it will register a broadcast receiver that will listen for a custom intent by using the OnResume lifecycle method. As the Activity moves into the background, the OnPause() method will unregister the receiver.

Publishing a Broadcast

A broadcast may be published to all apps installed on the device creating an Intent object and dispatching it with the SendBroadcast or the SendOrderedBroadcast method.

  1. Context.SendBroadcast methods – There are several implementations of this method. These methods will broadcast the intent to the entire system. Broadcast receivers that will receive the intent in an indeterminate order. This provides a great deal of flexibility but means that it is possible for other applications to register and receive the intent. This can pose a potential security risk. Applications may need to implement addition security to prevent unauthorized access. One possible solution is to use the LocalBroadcastManager which will only dispatch messages within the private space of the app. This code snippet is one example of how to dispatch an intent using one of the SendBroadcast methods:

    Intent message = new Intent("com.xamarin.example.TEST");
// If desired, pass some values to the broadcast receiver.
message.PutExtra("key", "value");
SendBroadcast(message);

    This snippet is another example of sending a broadcast by using the Intent.SetAction method to identify the action:

    Intent intent = new Intent();
intent.SetAction("com.xamarin.example.TEST");
intent.PutExtra("key", "value");
SendBroadcast(intent);

  2. Context.SendOrderedBroadcast – This is method is very similar to Context.SendBroadcast, with the difference being that the intent will be published one at time to receivers, in the order that the receivers were registered.

LocalBroadcastManager

The Xamarin Support Library v4 provides a helper class called LocalBroadcastManager. The LocalBroadcastManager is intended for apps that do not want to send or receive broadcasts from other apps on the device. The LocalBroadcastManager will only publish messages within the context of the application, and only to those broadcast receivers that are registered with the LocalBroadcastManager. This code snippet is an example of registering a broadcast receiver with LocalBroadcastManager:

Android.Support.V4.Content.LocalBroadcastManager.GetInstance(this). RegisterReceiver(receiver, new IntentFilter("com.xamarin.example.TEST"));

Other apps on the device cannot receive the messages that are published with the LocalBroadcastManager. This code snippet shows how to dispatch an Intent using the LocalBroadcastManager:

Intent message = new Intent("com.xamarin.example.TEST");
// If desired, pass some values to the broadcast receiver.
message.PutExtra("key", "value");
Android.Support.V4.Content.LocalBroadcastManager.GetInstance(this).SendBroadcast(message);