How the onPause() execution is very brief?

Question

According to the Android documentation in this link https://developer.android.com/guide/components/activities/activity-lifecycle.html#onpause:

onPause() execution is very brief, and does not necessarily afford enough time to perform save operations. For this reason, you should not use onPause() to save application or user data, make network calls, or execute database transactions; such work may not complete before the method completes. Instead, you should perform heavy-load shutdown operations during onStop().

What I don't understand is, how is it managed to make the execution of the onPause() method very brief ? As far as I know, when the onPause() method get called it won't be finished until it executes the lines of code within it. Or am I wrong?

Answer 1

The onPause() method call will finish executing when the user returns to the app.

Say saving your data takes 5 seconds. This begins when the user holds your app in memory. But lets say that the user now enters the app within the 5 seconds. This call stops the onPause() and begins the onResume(). You will not be sure that your data has been saved correctly :)

Answer 2

Like any other method, onPause() will finish executing all its (synchronous) instructions.

What I understand by 'brief' is a metaphor for the fact that the user briefly pauses the activity and will comeback to use it just shortly after.

For this purpose, docs state to not call time consuming functions like saving data or network calls because the user is still supposed to continue interacting with the activity: For example, you have a form of some type, you display an AlertDialog to confirm a choice, onPause() is called. Now imagine your saving the form data in the onPause(), this means every time you show that AlertDialog, the form is saved,... this is a bad practice. Now imagine you save it through an API to server, this even worse, specially with CREATE requests that may cause confusion (of duplicates) to server if called many times, or if you update the UI after the API response, this may create a lag or weird behavior if the user resumed the use of the activity faster than the async task response.

Generally, for good practices onPause() should be used for small tasks that consider a fast resume of the user interaction; like pausing sensors, small UI fixes. Other heavy tasks, data saving, etc... should be done in the onStop() or onDestroy()