Make sync interval more reliable

[le-cds]

As it is now, starting the app initiates a sync attempt. This should not be the case: starting the app should not interfere with the standard sync cycle.

[le-cds]

The reason for this is that the WorkManager API doesn't guarantee exact timings. Instead, our sync interval is treated as the minimum amound of time that needs to pass between subsequent invocations of WatchSyncWorker. If our sync interval is 15 minutes, it's perfectly possible that we'll end up syncing every 3 hours because Android tries not to drain the battery.

This is fine if we're concerned about phone battery life, but the way sync intervals can be selected suggests different behaviour. It may be a good idea to switch APIs.

This blog post has more details.

[le-cds]

It seems like alarm managers are the way to go.

[le-cds]

To sync as regularly as the app settings suggest, something like AlarmManager.setExactAndAllowWhileIdle(...) (unless further investigation indicates that this will drain the battery to an unreasonable extent. More care must be taken to schedule an alarm than was required for WorkManager. Given the possible battery drain implications, we should also be sure not to have an alarm scheduled unless necessary.

An alarm should be schedule if all of the following conditions apply:

• Garmin Connect is installed.
• We have calendar access permissions.
• At least one calendar is selected for syncing.

Rescheduling (which might entail cancelling an existing alarm altogether) should take place in these situations:

• We leave the calendar list fragment.
• The sync interval is changed on the preference page.
• The system has finished booting.

An alarm will need to be handled a bit more carefully as well since the system might go back into idling while we're sending our message. We may need to acquire an explicit wake lock which is only released once we're done syncing.

[le-cds]

Link collection so I don't forget:

• Schedule repeating alarms
• AlarmManager
• Optimize for Doze and App Standby