Android. Is WorkManager running when app is closed

问题:

I want to schedule nightly database updates. So I use new Android WorkManager. My understanding is that once scheduled it will always run in the background independently from the app's lifecycle. Is that right? My first tests show that Work is only being performed when the app is running.

val locationWork = PeriodicWorkRequest.Builder(UpdateDatabaseWorker::class.java, 24, TimeUnit.HOURS)
                        .addTag("DATABASE_UPDATE_SERVICE")
                        .build()
WorkManager.getInstance().enqueue(locationWork)

回答1:

Based on various issues reported on the WorkManager bugtracker, their documentation is not completely precise about the exact behavior of the WorkManager in such edge cases.

On certain devices, apps are force stopped when the app is cleared from task manager, so that part is expected. ... source

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Unfortunately, some devices implement killing the app from the recents menu as a force stop. Stock Android does not do this. When an app is force stopped, it cannot execute jobs, receive alarms or broadcasts, etc. So unfortunately, it's infeasible for us to address it - the problem lies in the OS and there is no workaround. source

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The only issue that we have come across is the case where some Chinese OEMs treat swipe to dismiss from Recents as a force stop. When that happens, WorkManager will reschedule all pending jobs, next time the app starts up. Given that this is a CDD violation, there is not much more that WorkManager can do given its a client library. source

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To add to this, if a device manufacturer has decided to modify stock Android to force-stop the app, WorkManager will stop working (as will JobScheduler, alarms, broadcast receivers, etc.). There is no way to work around this. Some device manufacturers do this, unfortunately, so in those cases WorkManager will stop working until the next time the app is launched. source

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With intense testing of a OneTimeWorkRequest (without constraints) on a Pixel 2 XL with stock android the behavior is the following:

• Task manager close:
  • Work continues (after a bit)
• Reboot device (work running):
  • Work continues after reboot done
• App info "Force stop":
  • Work stops, will only continue when app is started again
• Reboot device (work was "Force Stopped"):
  • Work does not continue until the app is started again

回答2:

My understanding is that once scheduled it will always run in the background independently from the app's lifecycle. Is that right?

Yes. Based on the documentation

The task is still guaranteed to run, even if your app is force-quit or the device is rebooted.

WorkManager chooses the appropriate way to run your task based on factors such as the device API level and the app state. If WorkManager executes one of your tasks while the app is running, WorkManager can run your task in a new thread in your app's process. If your app is not running, WorkManager chooses an appropriate way to schedule a background task--depending on the device API level.

WorkManager might use JobScheduler, Firebase JobDispatcher, or AlarmManager depending on the API level. It will repect the Doze and conaider all other constraints before executing the Work. You can expect some delay in Doze mode since it could wait for maintenance window.

Note:

WorkManager is intended for tasks that require a guarantee that the system will run them even if the app exits, like uploading app data to a server. It is not intended for in-process background work that can safely be terminated if the app process goes away; for situations like that, we recommend using ThreadPools.

回答3:

This is what documentation is saying:

Note: WorkManager is intended for tasks that require a guarantee that the system will run them even if the app exits, like uploading app data to a server. It is not intended for in-process background work that can safely be terminated if the app process goes away; for situations like that, we recommend using ThreadPools.

But there must be some condition. if that condition meet then WorkManager will run the task (this is important). Conditions like "only while device is charging and online"

Read this carefully, The WorkManager attempts to run your task at the interval you request, subject to the constraints you impose and its other requirements.

Here I found a good tutorial about how to use WorkManager for scheduling tasks : https://android.jlelse.eu/how-scheduling-work-with-new-android-jetpack-component-workmanager-852163f4825b