Develop Reference

Displaying Fragments with data in a ViewPager on screen rotation

So in my MainActivity OnCreate I create 3 Fragments, each filled with different data. I then add these Fragments to my ViewPagerAdapter.
ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
MealsFragment exploreFragment = null;
MealsFragment favoriteFragment = null;
MealsFragment localFragment = null;
//if there is no saved instance, create fragments with passed data as args and add them to the viewpageradapter
if(savedInstanceState == null){
exploreFragment = fetchExploreMealsDataAndCreateFragment();
favoriteFragment = fetchFavoriteMealsDataAndCreateFragment();
localFragment = fetchLocalMealsDataAndCreateFragment();
}
//add fragments to the adapter
viewPagerAdapter.addFragment(exploreFragment, "EXPLORE");
viewPagerAdapter.addFragment(favoriteFragment, "FAVORITES");
viewPagerAdapter.addFragment(localFragment, "LOCAL");
//set adapter to the viewpager and link it with the different tabs
mviewPager.setAdapter(viewPagerAdapter);
mtabLayout.setupWithViewPager(mviewPager);
The 3 different fetch methods get the data from an API or DB, so I don't want to call these methods every time I rotate my screen and go through my lifecycle. That's why I firstly check if the savedInstanceState is null. But what happens now is that if my savedInstanceState is not null, the Fragments will be null since i initialised them that way.
Apparently this is not a problem since when I rotate the screen, the fragments remain the same. I was wondering what is going on here behind the scenes as I don't think this is the correct way of handling my situation. Any suggestions of improvement are appreciated aswell.
Thanks in advance!
EDIT:
Forgot to mention that ViewPagerAdapter is my own implementation of FragmentPageAdapter
public class ViewPagerAdapter extends FragmentPagerAdapter {
private final List<Fragment> fragmentList = new ArrayList<>();
private final List<String> fragmentTitleList = new ArrayList<>();
public ViewPagerAdapter(FragmentManager fm) {
super(fm);
}
#Override
public Fragment getItem(int i) {
return fragmentList.get(i);
}
#Override
public int getCount() {
return fragmentList.size();
}
#Nullable
#Override
public CharSequence getPageTitle(int position) {
return fragmentTitleList.get(position);
}
public void addFragment(Fragment fragment, String title){
fragmentList.add(fragment);
fragmentTitleList.add(title);
}
}

I was wondering what is going on here behind the scenes
Alright, so, this is going to involve looking at the source for FragmentPagerAdapter. What's relevant is the implementation of the instantiateItem() method... though really just a portion of it.
#NonNull
#Override
public Object instantiateItem(#NonNull ViewGroup container, int position) {
[...]
// Do we already have this fragment?
String name = makeFragmentName(container.getId(), itemId);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
mCurTransaction.attach(fragment);
} else {
fragment = getItem(position);
if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
mCurTransaction.add(container.getId(), fragment,
makeFragmentName(container.getId(), itemId));
}
[...]
}
Essentially, the getItem() method from your ViewPagerAdapter is only called once for each position for the lifetime of the Activity (even across configuration changes). Every time other than the first, the Fragment object is retrieved directly from the FragmentManager as opposed to the Adapter.
So yes, for all re-creations of your Activity, your Adapter is holding a List<Fragment> that is just null, null, null... but it doesn't matter, because this list is not accessed again.
HOWEVER
The above statements assume that every Fragment in your adapter was constructed and added to the FragmentManager before your configuration change, and this is not necessarily guaranteed.
By default, the off-screen page limit for the ViewPager is 1. That means that your third page (your localFragment), is not necessarily added to the ViewPager, and therefore the FragmentManager, on first launch. If you scroll over to the next page even one time, it will be, but this is not necessarily true.
Or perhaps you have manually set the off-screen page limit to be 2, in which case all the pages/Fragments will be added immediately.
Probably the best thing to do is to change how you're using FragmentPagerAdapter altogether. I'd put this inside your Activity as an inner class:
private class ExampleAdapter extends FragmentPagerAdapter {
public ExampleAdapter() {
super(getSupportFragmentManager());
}
#Override
public Fragment getItem(int position) {
switch (position) {
case 0: return fetchExploreMealsDataAndCreateFragment();
case 1: return fetchFavoriteMealsDataAndCreateFragment();
case 2: return fetchLocalMealsDataAndCreateFragment();
default: throw new IllegalArgumentException("unexpected position: " + position);
}
}
#Nullable
#Override
public CharSequence getPageTitle(int position) {
switch (position) {
case 0: return "EXPLORE";
case 1: return "FAVORITES";
case 2: return "LOCAL";
default: throw new IllegalArgumentException("unexpected position: " + position);
}
}
#Override
public int getCount() {
return 3;
}
}
And then your onCreate() could be changed to this:
FragmentPagerAdapter viewPagerAdapter = new ExampleAdapter();
mviewPager.setAdapter(viewPagerAdapter);
mtabLayout.setupWithViewPager(mviewPager);
The advantage of doing it this way is that you only call the "fetch and create" methods on demand, but still have the ability to call them after a configuration change in situations where they weren't loaded before the configuration change.

FragmentPagerAdapter strange behavior

So i have the next problem with FragmentPagerAdapter. I have a TabLayout with 3 tabs representing 3 fragments that i can switch. So when i switch to third fragment, for some reason the first one disappears (or its view). Does anyone know how to fix this problem? Thanks in advance.
public class SectionsPagerAdapter extends FragmentPagerAdapter {
private static final int FRAGMENT_COUNT = 3;
private final List<Fragment> listOfFragments = new ArrayList<>();
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
#Override
public Fragment getItem(int position) {
return listOfFragments.get(position);
}
#Override
public int getCount() {
return FRAGMENT_COUNT;
}
public void addFragment(Fragment fragment) {
listOfFragments.add(fragment);
}
}
This is the code for FragmentPagerAdapter.

And i think i fixed this :) Only thing i did is to override the FragmentPagerAdapter destroyItem() method, with empty body (no super).

I don't know what is your problem. but if in your app only 3 fragments are there than you can try setOffscreenPageLimit method of ViewPager
ViewPager.setOffscreenPageLimit(3);

One of the problems here is a slightly confusing API.
In FragmentPagerAdapter, getItem(int position) actually means "create item". In other words, you shouldn't try to manually cache the Fragments inside a list in your Adapter.
#Override
public Fragment getItem(int position) {
return listOfFragments.get(position); //No! don't do this
}
Even though it may see counter-intuitive, you should actually create a new instance of the Fragment you want inside getItem, sticking very closely to the official Google example:
#Override
public Fragment getItem(int position) {
switch (position) {
case 0:
return FirstFragment.getInstance();
case 1:
return SecondFragment.getInstance();
}
}
Otherwise you will run into problems where the FragmentManager's cache and your own List<Fragment> cache are out-of-sync. In short, caching of Fragments is handled by the FragmentManager and you don't need to roll-your-own caching.

I don't know really what happen but this is my code, hope this help you
public class AdapterFragmentViewPager extends FragmentPagerAdapter {
private List<Fragment> fragmentList = new ArrayList<>();
public AdapterFragmentViewPager(FragmentManager fm) {
super(fm);
}
public void addFragment(Fragment fragment) {
fragmentList.add(fragment);
}
#Override
public Fragment getItem(int position) {
return fragmentList.get(position);
}
#Override
public int getCount() {
return fragmentList.size();
}
}
and this is my Tablayout:
private AdapterFragmentViewPager adapterFragmentViewPager = new AdapterFragmentViewPager(getSupportFragmentManager());
TabLayout.Tab home = tabLayout.newTab();
tabLayout.addTab(home);
TabLayout.Tab newest = tabLayout.newTab();
tabLayout.addTab(newest);
TabLayout.Tab author = tabLayout.newTab();
tabLayout.addTab(author);
TabLayout.Tab category = tabLayout.newTab();
tabLayout.addTab(category);
TabLayout.Tab saved = tabLayout.newTab();
tabLayout.addTab(saved);
adapterFragmentViewPager.addFragment(FragmentHome.newInstance());
adapterFragmentViewPager.addFragment(FragmentQuote.newInstance());
adapterFragmentViewPager.addFragment(FragmentAuthor.newInstance());
adapterFragmentViewPager.addFragment(FragmentCategory.newInstance());
adapterFragmentViewPager.addFragment(FragmentFavorites.newInstance());
viewPager.setAdapter(adapterFragmentViewPager);
tabLayout.setupWithViewPager(viewPager);

Using viewpager with specific fragments

I am creating an app that is using a viewpager to slide between 4 specific fragments.
All the examples of viewpager I have read so far, create new fragments each time the getPosition method of FragmentPagerAdapter is called. So it's something like:
return FragmentA.newInstance();
What I have done is the following:
In the main activity
public static final int FRAGMENTS = 4;
public static final String FRAGMENT_LIST ="LIST";
public static final String FRAGMENT_SETTINGS = "SETTINGS";
public static final String FRAGMENT_MAP = "MAP";
public static final String FRAGMENT_TICKET = "TICKET";
MainAdapter _adapter;
ViewPager _pager;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getSupportFragmentManager()
.beginTransaction()
.add(new FragmentMap(), FRAGMENT_MAP)
.add(new FragmentList(), FRAGMENT_LIST)
.add(new FragmentTicket(), FRAGMENT_TICKET)
.add(new FragmentSettings(), FRAGMENT_SETTINGS)
.commit();
}
_adapter = new MainAdapter(getSupportFragmentManager());
_pager = (ViewPager) findViewById(R.id.pager);
_pager.setAdapter(_adapter);
}
and in the adapter:
public class MainAdapter extends FragmentPagerAdapter {
FragmentManager _manager;
public MainAdapter(FragmentManager fm) {
super(fm);
_manager = fm;
}
#Override
public Fragment getItem(int position) {
return _manager.getFragments().get(position);
}
#Override
public int getCount() {
return ActivityMain.FRAGMENTS;
}
}
This raises an exception because the adapter is trying to change the tag of each fragment in getItem
My questions are:
a) Is it incorrect to always use the same fragment every time? I have seen no example that uses the above method or a similar one, they always create a new instance in the getItem method
b) If I wish for fragments to have some persistence, then does that mean that I should store the data that should be held by each fragment in static variables and always create new instances that use those variables?

a) You must create a new instance in the getItem() method, this method is not called every time you switch fragment from your viewpager.
I recommend you to use your adapter like
public class MainAdapter extends FragmentPagerAdapter {
public MainAdapter(FragmentManager fm) {
super(fm);
}
#Override
public Fragment getItem(int position) {
case 0 :
return new FragmentMap();
case 1 :
return new FragmentList();
case 2 :
return new FragmentTicket();
case 3 :
return new FragmentSettings();
default :
return null;
}
#Override
public int getCount() {
return ActivityMain.FRAGMENTS;
}
}
b) Fragments in FragmentPagerAdapter are persistents, and they will be recreate only if you switch several fragment in your ViewPager. You can set the refresh limit by _pager.setOffscreenPageLimit(3); for exemple if you never want to recreate your fragments in your case.
For more information : http://developer.android.com/reference/android/support/v4/view/ViewPager.html#setOffscreenPageLimit(int)

a) I think it's perfectly ok. They always create ne instance because it is easier and in many cases (like image gallery) it is better.
b) Static variables get lost if you send the app to backround and then return to it. Shared prefferences or bundle should do the work.

You should use FragmentStatePagerAdapter. If you fragmentstatepager adapter, you can remove or add fragment dynamically.

How to add Fragment to first position in FragmentPagerAdapter

I am using FragmentPagerAdapter and a ViewPager to add custom Fragments EDIT: from my MainActivity (also sending a bunch of extra data based on a JSON response via bundle) and using swiping motions to move to the next Fragments in the List.
public class MyPagerAdapter extends FragmentPagerAdapter implements Serializable {
public List<Fragment> fragments;
public FragmentManager fm;
public MyPagerAdapter(FragmentManager fm) {
super(fm);
this.fm = fm;
this.fragments = new ArrayList<Fragment>();
}
#Override
public Fragment getItem(int position) {
return fragments.get(position);
}
#Override
public int getCount() {
return fragments.size();
}
}
Everything is working fine as long as I'm adding new Fragments by
using
MyPagerAdapter pageAdapter = new MyPagerAdapter(getSupportFragmentManager());
pager = (ViewPager)findViewById(R.id.myViewPager);
pageAdapter.fragments.add(new CustomFragment());
pager.setAdapter(pageAdapter);
But I can't find a proper way to add Fragments to the beginning of the List and swipe back.
I've tried both
pageAdapter.fragments.add(0, new CustomFragment());
as well as changing the FragmentPagerAdapters List to LinkedList and using
pageAdapter.fragments.addFirst(new CustomFragment());
and then refreshing the adapter by using
pageAdapter.notifyDataSetChanged();
and i keep getting the following exception:
java.lang.IllegalStateException: Can't change tag of fragment CustomFragment{2ead9520 #10 id=0x7f0a0002 android:switcher:2131361794:10}: was android:switcher:2131361794:10 now android:switcher:2131361794:11

The key methods no one has talked about yet is public int getItemPosition(Object) which is used to remap fragments to pages after they move and public long getItemId(int position) which must be overridden by a pager adapter that reorders fragments. The default implementation uses the position of the page as the id, so reordering confuses the FragmentPagerAdapter.
(I am leaving out the Serializable interface as it is irrelevant for the purposes of answering the question - How to reorder fragments in a FragmentPagerAdapter).
public class MyPagerAdapter extends FragmentPagerAdapter {
public List<Fragment> fragments;
public FragmentManager fm;
public MyPagerAdapter(FragmentManager fm) {
super(fm);
this.fm = fm;
this.fragments = new ArrayList<Fragment>();
}
void addFragmentAtPosition(int position, Fragment f) {
if(position == fragments.size())
fragments.add(f);
else
fragments.add(position, f);
notifyDataSetChanged();
}
void removeFragmentAtPosition(int position) {
Fragment f = fragments.remove(position);
if(f != null)
fm.beginTransaction().remove(f).commit();
notifyDataSetChanged();
}
#Override
public Fragment getItem(int position) {
return fragments.get(position);
}
#Override
public int getCount() {
return fragments.size();
}
#Override
public int getItemPosition(Object object){
/*
* called when the fragments are reordered to get the
* changes.
*/
int idx = fragments.indexOf(object);
return idx < 0 ? POSITION_NONE : idx;
}
#Override
public long getItemId(int position) {
/*
* map to a position independent ID, because this
* adapter reorders fragments
*/
return System.identityHashCode(fragments.get(position));
}
}
The key additions are the overrides of public int getItemPosition(Object) and public long getItemId(int). These allow the FragmentPagerAdapter to reposition the existing fragments and to identify the existing active fragments in the FragmentManager cache correctly.

You should not create and add fragments this way. Instead just instantiate the fragments in getItem and the adapter will take care of using them. just do this:
#Override
public Fragment getItem(int position) {
Fragment fragment = new CustomFragment();
fragments.add(fragment)
return fragment
}

I would suggest you don't keep a list of references to fragments since it is not necessary and you risk to create memory leaks.
What i would do is create the fragment only when required like this :
#Override
public Fragment getItem(int position) {
Fragment fragment = new MyFragment();
return fragment;
}
To solve your problem you should create the fragment based on your needs, for example if you have fragments of different class instances like for example one instance of MyFragment another one of YourFragment and so on, just keep a list which says which kind of fragment occupy that position.
For example:
myListMap = new HashMap<Integer, Integer>();
myListMap.put(position, type);
and then create the fragment on the fly:
#Override
public Fragment getItem(int position) {
Fragment fragment = null;
int type = ...find fragment type in that position ....
if(type == MYFRAGMENTTYPE) {
fragment = new MyFragment();
}
return fragment;
}

Don't know if you still need the answer, but I was trying to do something similar and just found the solution :)
You are receiving that exception because of your getItem function. You are returning the fragment in the position that the function receives, and this position is always the last position of the array because that would correspond to the last added fragment in a "typical" usage.
In your case, you want to add a new Fragment in the first position, so your getItem will return twice the same fragment and throw the exception.
To avoid this you need to create a public function into your Adapter and the index of the fragment you are adding, and then return this specific fragment.
PS.: I'm developing only in Kotlin for about 6 months now, so it can have some typos.
public int newFragmentIndex = 0;
public List<Fragment> fragments;
...
public void addFragmentAt(int index, Fragment fragment) {
newFragmentIndex = index;
pageAdapter.fragments.add(index, fragment);
notifyDataSetChanged();
}
public void addFragment(Fragment fragment) {
newFragmentIndex = fragments.size();
pageAdapter.fragments.add(fragment);
notifyDataSetChanged();
}
...
#Override
public Fragment getItem(int position) {
return fragments.get(newFragmentIndex);
}
To call this from your Activity, change your pageAdapter.fragments.add(new CustomFragment());
to
pageAdapter.fragments.addFragment(new CustomFragment());
And
pageAdapter.fragments.add(0, new CustomFragment());
to
pageAdapter.fragments.addFragmentAt(0, new CustomFragment());
Hope this helps you with your problem!

Fragment constantly updated

I've got a FragmentActivity when I instantiate three different (n, n+1, n+2) Fragments.
I need to keep each Fragment updated when user swipes to it, so I used ViewPager.SimpleOnPageChangeListener.onPageSelected in the Fragment Activity, so when user swipes to n+1 or n+2 Fragment and again to n that function update the content.
Without using this workaround if I'm in the Fragment n+1, both n and n+2 are already loaded! I'd like instead that the Fragment load when the user swipes to it, without "pre-load".
This workaround works fine for me but it has a problem: the n Fragment that is the first in the list at start up of the app doesn't load its content. To load its content I have to swipe to n+1 then go back to n.
I know that the content of the Fragment should be setted on the class called at the moment of instantiate the fragment and that extends Fragment class, but in this way I don't know how to keep up to date each Fragment, as I do using onPageSelected.
Any suggestions?
EDIT 1:
I istantiate my fragments in this way in onCreate():
for(int x = 0; x < 3; x++) {
Bundle b = new Bundle();
b.putString( "id" , x );
Fragment myFrag = Fragment.instantiate( myContext , Mm_FragmentPage.class.getName() );
myFrag.setArguments( b );
fragments.add(myFrag);
}
Then I set the adapter in the ViewPager:
mPagerAdapter = new PagerAdapter( super.getSupportFragmentManager() , fragments );
mPager.setAdapter( mPagerAdapter );
Then I use the adapter in the TitlePageIndicator
titleIndicator = (TitlePageIndicator) findViewById( R.id.titleFragments );
titleIndicator.setViewPager( mPager );
titleIndicator.setOnPageChangeListener( new myPageChangeListener() );
And, at the end, the class PagerAdapter:
public class PagerAdapter extends FragmentPagerAdapter
{
// fragments to instantiate in the viewpager
private List<Fragment> fragments;
// constructor
public PagerAdapter(FragmentManager fm, List<Fragment> fragments)
{
super(fm);
this.fragments = fragments;
}
// return access to fragment from position, required override
#Override
public Fragment getItem(int position)
{
return this.fragments.get(position);
}
// number of fragments in list, required override
#Override
public int getCount()
{
return this.fragments.size();
}
#Override
public CharSequence getPageTitle(int position)
{
return getResources().getStringArray( R.array.tab_header_name )[ position ];
}
}

OK, so first thing you need to set OnPageChangeListener on the ViewPager and implement method onPageSelected(int i) and call the adapter's notifyDataSetChanged(), like so:
mPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageScrolled(int i, float v, int i2) {
}
#Override
public void onPageSelected(int i) {
//Tell the adapter that the content was changed
mPager.getAdapter().notifyDataSetChanged();
}
#Override
public void onPageScrollStateChanged(int i) {
}
});
In order to keep the fragments updated, you need to extends FragmentStatePagerAdapter and not FragmentPagerAdapter like what you did. The difference is that with FragmentPagerAdapter the ViewPager will never re-create the fragments, while in FragmentStatePagerAdapter it will.
Then on getItem(..) make sure to return a new instance of the fragment with the new content by passing the content to its arguments via setArguments(). Then override also getItemPosition(..) to tell the adapter that the fragment is not found, and therefore it must re-create it.
public class MyPagerAdapter extends FragmentStatePagerAdapter {
//List to hold the fragments to be shown
//NOTE: It's a list of Fragment classes, not a list of Fragment instances!
private List<Class<? extends Fragment> fragments;
public MyPagerAdapter(FragmentManager fm) {
super(fm);
fragments.add(SomeFragment.class);
fragments.add(AnotherFragment.class);
fragments.add(MoreFragment.class);
}
#Override
public Fragment getItem(int i) {
try {
//Creates a new instance of the fragment
Fragment instance = fragments.get(i).newInstance();
//Put the new content by passing Bundle with new content
instance.setArguments(args);
return instance;
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
return null;
}
#Override
public int getItemPosition(Object object) {
//NOTE: you might want to put better logic here
return POSITION_NONE;
}
#Override
public int getCount() {
return pages.size();
}
}
Every time you slide from one fragment to another, onPageSelected() will be fired calling notifyDataSetChanged() which will force the adapter to check also if the position of the fragment has changed. Since we return POSITION_NONE in getItemPosition(..), the adapter thinks that the position changed and will then call getItem(i). In getItem(i) we return a new instance (optionally, passing new arguments). Problem solved! :)
I just tested it by myself, created a small app that have a counter which increases everytime the user slides the page and it works!
This way you can drop the ViewPager.SimpleOnPageChangeListener.onPageSelected.
Learn more about ViewPager.