I use a viewpager with a tablayout in which I add some fragments with an adapter.
In my fragments oncreateview and onviewcreated are called.
My question is : is a fragment created again when the user scrolls to the next fragment of the tablayout and comes back to the first fragment? Or the fragments are created at the initialization of the viewpager and tablayout and then don't change?
I ask because I want to change a value from a tablayout's fragment and that change must entail a change in another fragment of the tablayout.
If my question is not clear, tell me and I will try to explain better.
There is a property called setOffscreenPageLimit in ViewPager.
You can set the number of pages that should be retained to either side of the current page in the view hierarchy in an idle state. Pages beyond this limit will be recreated from the adapter when needed.
viewpager.setOffscreenPageLimit(X)
it means it will create X number of pageson either side of your current page. And once it it is out of your page range it will recreate it.
By default 1 page beside the current page is kept "in memory" and not recreated. You can change that through the following line:
viewPager.setOffscreenPageLimit(YOUR_LIMIT);
It is suggested that you keep this LIMIT low, you can read about it here.
The Fragments that are within your specified limit will be kept "in memory" and will not be recreated. However Fragments that are beyond this limit will be recreated.
Related
I know there's setUserVisibleHint I can utilize but this isn't what I'm looking for.
For example, I have 9 pages in ViewPager. I want to keep the state of those pages, so I call setOffscreenPageLimit(8) which does the job, but I only want the current and the next Fragment gets instantiated which means only max 2 onActivityCreated() would be called at a time. Is it possible?
I am trying to read RSS feed and show the content of it in card view which is inside ViewPager with tab layout. It shows fragment with data initially, but when swiped back from another tab, the whole card layout with data,disappears.
I had to handle the same situation. I had TabLayout in ViewPager.
I have used FragmentPagerAdapter with off screen page limit set to (NO_OF_TABS - 1). This is to hold all the fragments in memory and they wont be destroyed and recreated as you swipe back and forth. As my fragments are small in size meaning not many views every fragment has so it was okay for me. But this way fragments always holds the latest data. Say, if user had entered text in EditText and swipe to end and come back, then fragment will still have the text that user had entered.
In your case, if you have not set the page limit the default is 1 so the fragment gets destroyed when you swipe away so you lose the data.
IN ADDITION TO ABOVE:
If you can not keep all fragments in memory, you may consider using FragmentStatePagerAdapter. This adapter saves the states of fragments that are being destroyed and it will use the same state to represent in fragment when user swipe back to this fragment. But there are issues around StaePagerAdapter while it destroys adn recreate the fragments, there are indexing issues so you will easily get IndexOutOfBoundException if you swipe. This is with page screen limit set to less than the total fragments.
I am using a ViewPager inside a TabLayout. The ViewPager has 3 pages where each page is a fragment.
When the activity containing the TabLayout starts, the fragment callback methods upto onResume() are called for fragment1, which is as expected and fragment1 is displayed on the screen.
However, logcat shows that the callback methods upto onResume are also called for fragment2, although fragment2 is currently not being displayed on the screen. This looks odd to me since onResume should only be called when a fragment is about to become visible.
When I select fragment2, onResume is called for fragment3, although fragment3 is not being displayed. So there seems to be a pattern to this behaviour.
What could be the cause for this?
Update: The ViewPager I am using is a subclass of FragmentPagerAdapter.
If is default behavior of ViewPager to call next Fragment when ViewPager is initilized.
You need to use setOffscreenPageLimit()
setOffscreenPageLimit() sets the number of pages that should be retained to either side of the current page
Default value is 1 so next Fragment to the left and to the right will stay in memory.
To prevent from calling next Fragment you need to set 0 value to OffscreenPageLimit
viewPager.setOffscreenPageLimit(0);
on the ViewPager object.
EDIT:
But i have checked by setting limit to 0.
setOffscreenPageLimit(0) will not work now. when you set limit to 0 you can see below warning in LogCat:
Requested offscreen page limit 0 too small; defaulting to 1
So i advise you to either call setOffscreenPageLimit(2) which will keep all your 3 Fragments in memory otherwise don't call setOffscreenPageLimit(int limit) on ViewPager.
Two things contribute to this behavior:
Fragments are not required to have a UI, in which case it is never "visible" on screen. Basically, if a Fragment is attached to an Activity, its lifecycle methods will be called as appropriate.
ViewPager loads items to either side of the item that is currently displayed. This is in order to have content to show as the user scrolls the page with his finger. By default the off screen page limit is 1, which means that when your first fragment is the current item, the second fragment is loaded off screen in preparation for scrolling.
You can always check which item is displayed by the ViewPager using getCurrentItem(). If you are having problems because the fragment are destroyed and recreated needlessly when they go beyond the off screen page limit, you can increase the page limit with setOffscreenPageLimit().
I have an Activity with ViewPager and three Fragments with-in. One Fragment contains ListView with custom CursorAdapter which loads data from database.
I've noticed that my cursor adapter loads data every time I switch Fragments in ViewPager. I think that it's normal and is due to the fact that every Fragment has its own lifecycle.
Regarding this it will be great pleasure for me if the users of the stackoverflow explain about their experience or best practices at all.
Thanks!
ViewPagers maintain the state of only certain number of fragments, which defaults to 1 for both sides of the current selected fragment. For example, if you have 3 fragments, when the first is selected, only first two fragments will be instantiated and have the listview data loaded. Alternatively, if you have the second fragment selected, the first and the third will be instantiated. If you switch to the third fragment, the second fragment will be retained, but the first one will be lost. However, you can set the number of fragments to be retained with calling setOffscreenPageLimit method on viewPager with any number of fragments to retain you need. Though you should remember, that setting the number too high may cause your app to consume too much memory.
For example, if you want your fragment to not reload listview content from db while switching fragments and you have 3 framents in your viewPager, you may write the following code:
ViewPager mViewPager;
mViewPager.setOffscreenPageLimit(2);
I am trying to implement swipe views with 2 tabs. For that, I am using view pager with 2 fragments. Now, the problem is that as soon as the main activity is opened (that contains those two tabs), onCreateView function is called for both the fragments. Please help me as how can I avoid calling of onCreateView of second fragment when one is in use.
Thanks,
Arpit
ViewPager retains the fragment to the left and to the right of the current view by default. This is to reduce a choppy user experience - that way you can begin swiping left or right and immediately see what is there without delay.
It is possible to disable (or increase the number of fragments to be retained) with setOffscreenPageLimit(0), but seriously consider if this is the right approach.