android - How periodic work works with WorkManager?

Question

I tried to use WorkManager along with PeriodicWorkRequestBuilder, but I've got confused and I didn't find satisfying answer in documentation.

Here's a visualization from docs:

[     before flex     |     flex     ][     before flex     |     flex     ]...
[   cannot run work   | can run work ][   cannot run work   | can run work ]...
\____________________________________/\____________________________________/...
               interval 1                            interval 2             ...(repeat)

Now let's take an example. Let's say we have an app for restaurant menu and we want to download fresh menu everyday before the dinner (I don't know when you eat the dinner, imagine people in our restaurant eat dinner between 3:00 PM and 6:00PM). So we need to download the menu everyday before 3:00 PM so the user could see immediately what's today. E.g. we want to schedule DownloadMenuWorker everyday between 09:00 AM and 12:00 PM (menu to our server database is added everyday in the morning and we want 3 hours buffer before dinner time starts). Here's code that enqueue appropriate Worker:

val request = PeriodicWorkRequestBuilder<DownloadMenuWorker>(
    repeatInterval = Duration.ofHours(24),
    flexTimeInterval = Duration.ofHours(3)
).apply {
    setInitialDelay(calculateNext12PM())
}.build()
WorkManager.getInstance(context).enqueueUniquePeriodicWork(
    UNIQUE_WORK_NAME
    ExistingPeriodicWorkPolicy.KEEP,
    request
)

If I understand correctly, it should delay the first run to the next 12 PM, and after that it should run periodically everyday between 09:00 AM and 12:00 PM (because of repeatInterval = 24 hours and flexTimeInterval = 3 hours). Now my questions is, what if the x time run the real start of the Worker will be 12:24 PM because of the inexactness of periodic requests? The next 24 hours for the x+1 time run will count from 12:24 PM or from 12:00 PM? If it counts from 12:24 PM does it mean that x + 100 run can be completely different than original assumptions (between 9:00 AM and 12:00 PM) and our guests will have fresh menu when dinner time has finished?

In other words, which of the following visualizations is correct?

1.

2.

The second case will cause that the delay and inexactness will expand over many days.

question from:https://stackoverflow.com/questions/66059663/how-periodic-work-works-with-workmanager

Answer 1

Waitting for answers