android - FLAG_CANCEL_CURRENT or FLAG_UPDATE_CURRENT

Question

My app sets a repeating alarm on user interaction, it might change the interval time set for the broadcast with Alarm Manager.
There is not much in the way of extras.
Is the update or cancel flag better in this case?

Thanks

Answer 1

Never use FLAG_CANCEL_CURRENT with PendingIntents that you use when setting alarms. If you want to reschedule the alarm for a different time you don't need any flag at all; just create a duplicate PendingIntent with flags of zero and then use it to set() an alarm: this will implicitly cancel the existing alarm and then set it for the newly-specified time. If you used FLAG_CANCEL_CURRENT when you created the new PendingIntent, though, it breaks the Alarm Manager's ability to recognize that it's "the same" as the now-canceled PendingIntent, and you wind up with the old one hanging around, undeliverable, taking up memory and CPU. I've seen apps with this bug rack up literally hundreds of stale alarms in the system, enough to be a noticeable performance and memory-usage hit.

If you just want to change the extras without actually rescheduling the existing alarm, that is what FLAG_UPDATE_CURRENT is for. If you want to reschedule, don't use any flags at all.