Activity or Notification via Ordered Broadcast

I’ve run into the following generalized question a lot recently:

I have an event that occurs in the background. I want to update my activity, if the activity is on the screen. Otherwise, I want to raise a Notification.

This seems perfectly reasonable. Every time I hand-waved my way around an answer, I felt like there should be a better solution for this. To quote Commodus, “It vexes me. I’m terribly vexed.”

A few days ago, a solution popped into mind, and based on preliminary experiments, it seems to work well. And, it lets us use an ordered broadcast.

Ordered broadcasts are like regular Intent broadcasts, except that they are ordered.

(yes, people pay $40/year for insights like these — you can too!)

Specifically, the broadcast is sent to one BroadcastReceiver at a time. Each receiver has the ability to abort the broadcast, meaning that no other receiver after it will get the broadcast. And, you can prioritize the receivers.

Hence, the recipe for the activity-or-Notification pattern is:

1. Define an action string you will use when the event occurs that you want to go to the activity or notification (e.g., com.commonsware.java.packages.are.fun.EVENT).

2. Dynamically register a BroadcastReceiever in your activity, with an IntentFilter set up for the aforementioned action string and with a positive priority (the default priority for a filter is 0). This receiver should then have the activity do whatever it needs to do to update the UI based on this event. The receiver should also call abortBroadcast() to prevent others from getting it. Be sure to register the receiver in onStart() or onResume() and unregister the receiver in the corresponding onStop or onPause() method.

3. Register in your manifest a BroadcastReceiver, with an <intent-filter> set up for the aforementioned action string. This receiver should raise the Notification.

4. In your service (e.g., an IntentService), when the event occurs, call sendOrderedBroadcast().

And that’s it. If the activity is on-screen, its receiver will be registered, so it will get the event, process it, and cancel the broadcast. If the activity is not on-screen, its receiver will not be registered, so the event will go to the default handler, in the form of your manifest-registered BroadcastReceiver, which will raise the Notification.

I have some code that demonstrates this, though it’s not published in any of my GitHub repos at this time. I’ll be adding it to the repo for The Busy Coder’s Guide to Advanced Android Development at some point, when I cover the subject. If somebody needs this code, or has any questions on the technique, please join the cw-android Google Group and ask!