How does fragment's lifecycle works inside viewpager? Why onStop is not called on navigation change?

Question

I'm using ViewPager 2 from AndroidX with 4 instances of the same fragment. My question is pretty straight forward. When I'm navigating to some another fragment(using navigation drawer or even something else). OnStop() , OnDestroy(), OnDettach() of the fragments inside the viewpager does not gets triggered. So why is that? And If I want to remove the listeners I've started already, in one of these methods, how can I do that?

For example, I'm using GreenRobot's EventBus. And I'm registering the EvenBus inside OnStart:

override fun onStart() {
    super.onStart()
    EventBus.getDefault().register(this)
}

And Removing it from OnStop:

override fun onStop() {
    Log.e(TAG, "onStop: ")
    EventBus.getDefault().unregister(this)
    super.onStop()
}

But when I navigate away from the viewpager as I explained above, onStop does not trigger. I even checked it by logging.

So is the fragment lifecycle works differently with viewpager? And if yes, how can I overcome this problem(unregistering EvetBus).

Check this out: https://developer.android.com/reference/android/support/v4/view/ViewPager#setoffscreenpagelimit — grrigore, Nov 13 '19 at 20:28
I know about OffscreenPageLimit, but it is irrelevant here, suppose even if I setOffscreenPageLimit to 1 and navigate away from the viewpager, onStop still won't be called. And won't be able to unregister the listener. — Parag Pawar, Nov 13 '19 at 20:36
Are you using `FragmentPagerAdapter` or `FragmentStatePagerAdapter`? — grrigore, Nov 13 '19 at 20:44
Neither, as I'm using ViewPager 2 from AndroidX, I'm using FragmentStateAdapter — Parag Pawar, Nov 13 '19 at 20:49

score 0 · Accepted Answer · answered Nov 14 '19 at 07:15

0

Unfortunately, EventBus does not provide great usefulness when it comes to ViewPager and Fragments inside it.

Though I found a solution, using the more traditional approach: Interfaces

It does not directly answer the question Why onStop is not called of fragments inside ViewPager on navigation change?

But it does save you from multiple Event triggers when using EvenBus with ViewPager. With interfaces, As you don't have to explicitly unregister the interface. It does not matter if the onStop is called.

answered Nov 14 '19 at 07:15

Parag Pawar

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Parag, I am facing exactly the same issue with the fragment lifecycle using `ViewPager2` and I want to write the code on `onDestroy()`. Can you help? – Rajat Sangrame Feb 28 '20 at 12:52
@RajatSangrame Sure, can you describe your problem in more detail? What do you want to do on onDestroy? – Parag Pawar Feb 28 '20 at 13:02
@RajatSangrame You might want to add a new question if it is unique, so that other experienced developer can help you too.. – Parag Pawar Feb 28 '20 at 13:03
I am using `ExoPlayer` in every `fragment`. It consumes a lot of memory. I want to release the memory of all the unnecessary fragments. – Rajat Sangrame Feb 28 '20 at 13:40
Did you try releasing the ExoPlayer instance in fragment's onDestroy method? – Parag Pawar Feb 28 '20 at 13:46
Yes. None of then are executing unless I hit the back button. – Rajat Sangrame Feb 28 '20 at 13:54
Can you add this as your question and post all the code you've tried so far, and share the link, so that I can help you better – Parag Pawar Feb 28 '20 at 14:04
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/208709/discussion-between-rajat-sangrame-and-parag-pawar). – Rajat Sangrame Feb 28 '20 at 14:05

score -1 · Answer 2 · answered Nov 14 '19 at 01:02

-1

You can use setUserVisibleHint to check the fragment visibility.

  override fun setUserVisibleHint(isVisibleToUser: Boolean) {
    super.setUserVisibleHint(isVisibleToUser)

    if (isVisibleToUser) {

       //Fragment is visible

    } else {

       //Fragment is invisible
    }
}

Hope this helps.

answered Nov 14 '19 at 01:02

GHH

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2

Nope, I need it for ViewPager 2, this is for Normal ViewPager. Did you read the question? – Parag Pawar Nov 14 '19 at 04:22

How does fragment's lifecycle works inside viewpager? Why onStop is not called on navigation change?

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