notes/docs/lectures/mdp/13_broadcasts.md

Broadcast Receivers

It is the 4th component in Android

1. Activity
   • User interface
2. Service
   • Performs a long-running background task
3. Content Provider
   • Storage and provision of data
4. Broadcast receivers

Components are entry points

Event-based

Respond to system-wide broadcast announcements

• The user has not necessarily done something, but the OS / phone / another application has
  • e.g. the screen has turned off, battery is low, phone booted, new text
• Can be sent by an application, or received by an application from the OS
• Create the process, instantiate a BroadcastReceiver

Intents are sent to specific activities

• Explicitly or implicitly via intent filters

Broadcasts are sent to anything that cares to listen

• System-wide intent broadcasts

Broadcasts

• Broadcasts are intents
  • Send an intent to start an activity or service
• Send an intent to trigger broadcast receivers that subscribe to that particular class of intent
  • Can define our own broadcast intents
  • Cannot send system intents (battery, screen, etc.), as the security model prevents it
  • Uses binder for messaging

Broadcast Receiver

• Must be registered

  • Either in the manifest or context-registered
  • Context-registered -> runtime
    • To receive events while running

• Declare in manifest

  <receiver android:name="MyReceiver">
      <intent-filter>
          <action android:name="com.example" />
      </intent-filter>
  </receiver>
  

• Specify broadcast intents we are interested in

  • Receiver is registered at boot-time / install-time / run-time (context)

Implementation

• Sub-class BroadcastReceiver
  • Specify which intents we are interested in
• Implement the onReceive() method
• Send a message to our application to change our behaviour
  • Reduce the volume when a notification comes through
• Start an activity
  • Do not start activities from broadcast receivers (jarring for user)
• Start a service
  • A gateway to another component, e.g. start polling for emails
• Show a notification
  • Alert the user that there is something that they need to interact with at some point

Normal and Ordered Broadcasts

sendBroadcast(intent) - intent can be implicit or explicit

sendOrderedBroadcast(intent, receiverPermission)

sendOrderedBroadcast(intent, receiverPermission, resultReceiver)

Receive data back from the broadcast, resultReceiver will be treated as a final receiver at the end of the broadcast.

Ordered broadcasts are received by the highest-priority receivers first

Preferred receivers are given the chance to consume the broadcast before it is delivered to less preferred receivers

Sticky Broadcast

Non-Sticky - if you weren’t listening at the time, you’ve missed it

Sticky intents are cached by Android

• These are so much slower than just implementing direct calls within your own app IPC for each receiver to register. They will also incur a massive IPC overhead

Sticky broadcasts are deprecated

Local Broadcasts

Data broadcast will not leave the application

Other apps cannot send broadcasts to our application

• More efficient than sending a global broadcast
  • No IPC

Now deprecated due to the MVVM architecture

Context-registered Receivers

Can no longer use a manifest-declared receiver for most implicit broadcasts

• Broadcasts that don’t target our app specifically

We can receive broadcasts as long as their registered context is valid

• If we register it in onCreate, we should unregister in onDestroy to prevent leaking the receiver

Broadcasts in practice

The application receiving the broadcasts must have been explicitly started / not explicitly stopped

• Applications cannot intercept broadcasts without the user having some awareness that they have given it permission

Either the sender or receiver of a broadcast can implement permissions

Lifecycle

Process is aggressively killed once onReceive() has returned

• Long-running code should be in a service instead
• goAsync()
  • Can be called in onReceive() to keep the broadcast active after returning from that function
  • ~10 seconds to do something