Develop Reference

onPageSelected runs BEFORE onCreateView is called?

I'm using a ViewPager to cycle through a set of fragments, and I want to update each fragment after it slides onto the screen. Basically, I want the text to "fade in" after the fragment has settled.
I tried using the fragment's onStart and onResume methods, and while this works for most of the pages, it does NOT work for the second page, because for whatever dumb reason, the first page AND the second page have their onStart/onResume methods called at the same time (before the second page ever hits the screen).
Now I'm trying to get it to work with the onPageChangeListener's onPageSelected callback. That method looks like this:
#Override
public void onPageSelected(final int position) {
mCurrentPosition = position;
PageFragment fragment = (PageFragment) ((MainActivity.ScreenSlidePagerAdapter) mViewPager.getAdapter()).getItem(position);
fragment.onSelect();
}
And the onSelect method in the fragment looks like this:
public void onSelect(){
new android.os.Handler().postDelayed(
new Runnable() {
public void run() {
mSwitcher.setText("");
mNum = getArguments() != null ? getArguments().getInt("num") : 1;
Media currentMedia = slideshow.getMedia().get(mNum);
mSwitcher.setText(currentMedia.getDisplayName());
}
},
4000);
}
The problem with this way is that the line mSwitcher.setText(""); throws a NullPointerException
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextSwitcher.setText(java.lang.CharSequence)' on a null object reference
Which would suggest that the onCreateView method in that class has yet to run since that's where the mSwitcher variable is instantiated. Which seems bananas to me, since the view is already sliding onto the screen at this point.
Any ideas about how to solve this problem would be greatly appreciated. This is my first Android experience, and I've been trying to solve this stupid text-fade-in issue for a full week with no luck. At this point I'm almost ready to abandon mobile as a platform because of how painful every minor change has been so far.

ViewPager keeps the next page in memory & this is it's default behaviour. You could adjust it by calling like:
viewPager.setOffscreenPageLimit(2);
However this might not be useful as if you pass 0 in above method, viewPager will ignore it.
You are going in right direction. I believe now problem is in your ScreenSlidePagerAdapter. In getItem(int position) you might have something like
if(position == 1)
return new PageFragment();
instead change the adapter to something like following,
public class ScreenSlidePagerAdapter extends FragmentPagerAdapter {
private List<Fragment> mFragments = new ArrayList<>();
public ScreenSlidePagerAdapter(FragmentManager fm, List<Item> items) {
super(fm);
for (Item item : items) {
mFragments.add(new PageFragment());
}
}
#Override
public int getCount() {
return mFragments.size();
}
#Override
public Fragment getItem(int position) {
return mFragments.get(position); // Return from list instead of new PagerFragment()
}
}

I have the similar problem as yours, onPageSelected() is called before the fragments are initialized, but your description is not detailed enough, such as how you select the second page.
When adapter is fed with Fragments, or we say getCount() > 0, getItem() will whatever returns a Fragment, which is not null. But this doesn't mean it is initialized, at least it doesn't if you extend from FragmentStatePagerAdapter.
when adapter is fed with data and called notifyDataSetChange(), adapter will initialize the first two pages by default. If you call setCurrentItem() to move to other pages immediately after notifyDataSetChange() the issue might happen. During the runtime, setCurrentItem() -> onPageSelected() might be called before the fragments are initialized.
my solution is using view.post() when setCurrentItem(). e.g.
viewPager.post(() -> viewPager.setCurrentItem(index));

I've got a weird bug when using a ViewPager inside a Fragment

Okay i'll try and make this as clear as possible. I have a Fragment called CheckerManager which contains a ViewPager. This ViewPager will allow the user to swipe between 3 Fragments all of which are an instance of another Fragment called CheckerFragment. I'm using a FragmentPagerAdapter to handle paging. Here's how it looks
private class PagerAdapter extends FragmentPagerAdapter {
CharSequence mTabTitles[];
public PagerAdapter(FragmentManager fm, CharSequence tabTitles[]) {
super(fm);
mTabTitles = tabTitles;
}
#Override
public Fragment getItem(int position) {
switch(position) {
case 0:
return CheckerFragment.newInstance(MainFragment.DRAW_TITLE_LOTTO);
case 1:
return CheckerFragment.newInstance(MainFragment.DRAW_TITLE_DAILY);
case 2:
return CheckerFragment.newInstance(MainFragment.DRAW_TITLE_EURO);
default:
return null;
}
}
#Override
public CharSequence getPageTitle(int position) {
return mTabTitles[position];
}
#Override
public int getCount() {
return 3;
}
}
I know that the ViewPager will always create the Fragment either side of the current Fragment. So say my 3 CheckerFragments are called A, B and C and the current Fragment is A. B has already been created. But my problem is that even though I am still looking at Fragment A, Fragment B is the 'active' Fragment. Every input I make is actually corresponding to Fragment B and not A. The active Fragment is always the one which has been created last by the ViewPager.
I've looked at quite a few things to see if anyone has had the same problem but i'm finding it difficult to even describe what's wrong. I think it's something to with the fact that all of the ViewPagers fragments are of the same type ie - CheckerFragment. I have a working implementation of a ViewPager inside a fragment elsewhere in the application and the only difference I can tell is that each page is a different type of Fragment.
Any help would be appreciated!
*EDIT
PagerAdapter adapter = new PagerAdapter(getChildFragmentManager(), tabTitles);
ViewPager viewPager = (ViewPager)view.findViewById(R.id.viewPagerChecker);
viewPager.setAdapter(adapter);

I feel pretty stupid but I found out what the issue was. In my CheckerFragment I would call getArguments() to retrieve a String extra and I would use this to determine how to layout the fragment. Problem was I made this extra a static member of CheckerFragment. So every time a new Fragment was created it was using the most recent extra.
Moral of the story - Don't make your fragments extra a static member if you plan on making multiple instances of that fragment.

Replace a fragment within a Viewpager(notifyDataSetChanged does not work)

My MainActivity contains a viewPager.
In the MainActivity.java, I set the adapter for viewpager. The adapter extends FragmentStatePagerAdapter. The fragment I want to replace is a
cameraFragment. So when the user clicks on the switch camera button, I want to now show the camera fragment, this time with a front camera on.
On clicking the switch Camera button, I remove the fragment from the arraylist of fragments I had passed to the custom adapter. I add the new fragment and call notifydatasetchanged. However, this does not result in the new fragment being added. How do I achieve dynamic replacement of fragments within a viewpager which is backed my a custom fragment state pager adapter?
Code :
mainPageFragments = new ArrayList<>();
mainPageFragments.add(new ResultsFragment_());
mainPageFragments.add(DemoCameraFragment_.newInstance(false));
pagerAdapter = new MainViewPagerAdapter(getSupportFragmentManager(),mainPageFragments);
To replace the fragment : On receiving the related event I do,
mainPageFragments.remove(1);
if (event.getCameraState().equals(CameraSwitchButton.CameraTypeEnum.BACK)) {
mainPageFragments.add(DemoCameraFragment.newInstance(false));
} else {
mainPageFragments.add(DemoCameraFragment.newInstance(true));
}
// Not Working...
pagerAdapter.notifyDataSetChanged();
Adapter Code :
public class MainViewPagerAdapter extends FragmentStatePagerAdapter {
ArrayList<Fragment> fragmentsArray;
public MainViewPagerAdapter(FragmentManager fm, ArrayList<Fragment> fragmentsArray) {
super(fm);
this.fragmentsArray = fragmentsArray;
}
#Override
public Fragment getItem(int position) {
return fragmentsArray.get(position);
}
#Override
public int getCount() {
return fragmentsArray.size();
}
#Override
public int getItemPosition(Object object) {
return super.getItemPosition(object);
}
}

Your MainViewPagerAdapter.getItemPosition is the cause of your issue.
Default implementation always returns POSITION_UNCHANGED. For pager to remove your fragment you have to return PagerAdapter.POSITION_NONE for the fragments that are removed.
Additionally your current design contradicts with the idea of FragmentStatePagerAdapter. From the FragmentStatePagerAdapter documentation: "This version of the pager is more useful when there are a large number of pages, working more like a list view. When pages are not visible to the user, their entire fragment may be destroyed, only keeping the saved state of that fragment. This allows the pager to hold on to much less memory associated with each visited page as compared to FragmentPagerAdapter at the cost of potentially more overhead when switching between pages."
Your current implementation holds all fragments in an array, and so defeats this mechanism. Correct implementation would be to create fragments in MainViewPagerAdapter.getItem method and let the adapter to handle fragments lifecycles as needed.

Thanks to #Okas. I made the following change to getItemPosition within my FragmentStatePagerAdapter subclass.
#Override
public int getItemPosition(Object object) {
if (object instanceof DemoCameraFragment_)
return POSITION_NONE;
return super.getItemPosition(object);
}
I added logs to the OnCreate of both my fragments to confirm if they were getting recreated or not. As per my requirement, only the second fragment is recreated.

How can make my ViewPager load only one page at a time ie setOffscreenPageLimit(0);

I understand the lowest number I can give setOffscreenPageLimit(int) is 1. but I need to load one page at a time because memory problems.
Am i going to have to use the old style tabhost etc? or is there a way/hack I can make my viewPager load one page at a time?
My Adapter extends BaseAdapter with the ViewHolder patern.

I was having the same problem and I found the solution for it:
Steps:
1) First Download the CustomViewPager Class from this link.
2) Use that class as mentioned below:
In Java:
CustomViewPager mViewPager;
mViewPager = (CustomViewPager) findViewById(R.id.swipePager);
mViewPager.setOffscreenPageLimit(0);
In XML:
<com.yourpackagename.CustomViewPager
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/swipePager"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
Now only one page will be loaded at once.
P.S: As per the question's requirement, I have posted the solution for Viewpager. I haven't tried the same with TabLayout yet. If I will find any solution for that I will update the answer.
In this file, KeyEventCompat is used it may not found by the android studio because KeyEnentCompat class was deprecated in API level 26.0.0 so you need to replace KeyEventCompat to event for more details you can view
https://developer.android.com/sdk/support_api_diff/26.0.0-alpha1/changes/android.support.v4.view.KeyEventCompat

As far as I know, that is not possible when using the ViewPager. At least not, when you want your pages to be swipe-able.
The explaination therefore is very simple:
When you swipe between two pages, there is a Point when both pages need to be visible, since you cannot swipe between two things when one of those does not even exist at that point.
See this question for more: ViewPager.setOffscreenPageLimit(0) doesn't work as expected
CommonsWare provided a good explaination in the comments of his answer.

but I need to load one page at a time because memory problems.
That presumes that you are getting OutOfMemoryErrors.
Am i going to have to use the old style tabhost etc?
Yes, or FragmentTabHost, or action bar tabs.
or is there a way/hack I can make my viewPager load one page at a time?
No, for the simple reason that ViewPager needs more than one page at a time for the sliding animation. You can see this by using a ViewPager and swiping.
Or, you can work on fixing your perceived memory problems. Assuming this app is the same one that you reported on earlier today, you are only using 7MB of heap space. That will only result in OutOfMemoryErrors if your remaining heap is highly fragmented. There are strategies for memory management (e.g., inBitmap on BitmapOptions for creating bitmaps from external sources) that help address such fragmentation concerns.
My Adapter extends BaseAdapter with the ViewHolder patern.
BaseAdapter is for use with AdapterView, not ViewPager.

I have an Answer for this. The above said method setUserVisibleHint() is deprecated and you can use setMaxLifecycle() method. For loading only the visible fragment you have to set the behaviour to BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT in the viewpager adapter. ie; in the Constructor. And for handling the fragment use onResume() method in the fragment.
In this way you can load only one fragment at a time in the viewpager.
public static class MyAdapter extends FragmentStatePagerAdapter {
public MyAdapter(FragmentManager fm) {
super(fm, BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT);
}
#Override
public int getCount() {
return NUM_ITEMS;
}
#Override
public Fragment getItem(int position) {
return ArrayListFragment.newInstance(position);
}
}
In Kotlin:
class MyAdapter(fm: FragmentManager) : FragmentStatePagerAdapter(fm,BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT )
Also use with FragmentPagerAdapter (now deprecated) in same way

By using this method you can load one page at time in tab layout with view pager`
#Override
public void onResume() {
super.onResume();
if (getUserVisibleHint() && !isVisible) {
Log.e("~~onResume: ", "::onLatestResume");
//your code
}
isVisible = true;
}
#Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if (isVisibleToUser && isVisible) {
Handler handler = new Handler();
handler.postDelayed(new Runnable() {
#Override
public void run() {
//your code
}
}, 500);
}
}
`

Override the setUserVisibleHint and add postDelayed like below in your every fragments.
override fun setUserVisibleHint(isVisibleToUser: Boolean) {
if (isVisibleToUser)
Handler().postDelayed({
if (activity != null) {
// Do you stuff here
}
}, 200)
super.setUserVisibleHint(isVisibleToUser)
}
I can manage by this way and its working fine now for me.

First, copy in the SmartFragmentStatePagerAdapter.java which provides the intelligent caching of registered fragments within our ViewPager. It does so by overriding the instantiateItem() method and caching any created fragments internally. This solves the common problem of needing to access the current item within the ViewPager.
Now, we want to extend from SmartFragmentStatePagerAdapter copied above when declaring our adapter so we can take advantage of the better memory management of the state pager:
public abstract class SmartFragmentStatePagerAdapter extends FragmentStatePagerAdapter {
// Sparse array to keep track of registered fragments in memory
private SparseArray<Fragment> registeredFragments = new SparseArray<Fragment>();
public SmartFragmentStatePagerAdapter(FragmentManager fragmentManager) {
super(fragmentManager);
}
// Register the fragment when the item is instantiated
#Override
public Object instantiateItem(ViewGroup container, int position) {
Fragment fragment = (Fragment) super.instantiateItem(container, position);
registeredFragments.put(position, fragment);
return fragment;
}
// Unregister when the item is inactive
#Override
public void destroyItem(ViewGroup container, int position, Object object) {
registeredFragments.remove(position);
super.destroyItem(container, position, object);
}
// Returns the fragment for the position (if instantiated)
public Fragment getRegisteredFragment(int position) {
return registeredFragments.get(position);
}
}
// Extend from SmartFragmentStatePagerAdapter now instead for more dynamic ViewPager items
public static class MyPagerAdapter extends SmartFragmentStatePagerAdapter {
private static int NUM_ITEMS = 3;
public MyPagerAdapter(FragmentManager fragmentManager) {
super(fragmentManager);
}
// Returns total number of pages
#Override
public int getCount() {
return NUM_ITEMS;
}
// Returns the fragment to display for that page
#Override
public Fragment getItem(int position) {
switch (position) {
case 0: // Fragment # 0 - This will show FirstFragment
return FirstFragment.newInstance(0, "Page # 1");
case 1: // Fragment # 0 - This will show FirstFragment different title
return FirstFragment.newInstance(1, "Page # 2");
case 2: // Fragment # 1 - This will show SecondFragment
return SecondFragment.newInstance(2, "Page # 3");
default:
return null;
}
}
// Returns the page title for the top indicator
#Override
public CharSequence getPageTitle(int position) {
return "Page " + position;
}
}

You actually don't need a custom ViewPager.
I had the same issue and I did like this.
Keep the setOffscreenPageLimit() as 1.
Use fragment's onResume and onPause lifecycle methods.
Initialize and free-up memories on these lifecycle methods.

I know this is an old post, but I stumbled upon this issue and found a good fix if your loading fragments. Simply, check if the user is seeing the fragment or not by overriding the setUserVisibleHint(). After that load the data.
#Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if (isVisibleToUser) {
getData(1, getBaseUrl(), getLink());
}
}

FragmentPagerAdapter doesn't recreate Fragments on orientation change?

I have a ViewPager (extends FragmentPagerAdapter) which holds two Fragments. What I need is just refresh a ListView for each Fragment when I swipe among them. For this I have implemented ViewPager.OnPageChangeListener interface (namely onPageScrollStateChanged). In order to hold references to Fragments I use a HashTable. I store references to Fragments in HashTable in getItem() method:
#Override
public Fragment getItem(int num) {
if (num == 0) {
Fragment itemsListFragment = new ItemsListFragment();
mPageReferenceMap.put(num, itemsListFragment);
return itemsListFragment;
} else {
Fragment favsListFragment = new ItemsFavsListFragment();
mPageReferenceMap.put(num, favsListFragment);
return favsListFragment;
}
}
So when I swipe from one Fragment to another the onPageScrollStateChanged triggers where I use the HashTable to call required method in both Fragments (refresh):
public void refreshList() {
((ItemsListFragment) mPageReferenceMap.get(0)).refresh();
((ItemsFavsListFragment) mPageReferenceMap.get(1)).refresh();
}
Everything goes fine until orientation change event happens. After it the code in refresh() method, which is:
public void refresh() {
mAdapter.changeCursor(mDbHelper.getAll());
getListView().setItemChecked(-1, true); // The last row from a exception trace finishes here (my class).
}
results in IllegalStateException:
java.lang.IllegalStateException: Content view not yet created
at android.support.v4.app.ListFragment.ensureList(ListFragment.java:328)
at android.support.v4.app.ListFragment.getListView(ListFragment.java:222)
at ebeletskiy.gmail.com.passwords.ui.ItemsFavsListFragment.refresh(ItemsFavsListFragment.java:17)
Assuming the Content view is not created indeed I set the boolean variable in onActivityCreated() method to true and used if/else condition to call getListView() or not, which shown the activity and content view successfully created.
Then I was debugging to see when FragmentPagerAdapter invokes getItem() and it happens the method is not called after orientation change event. So looks like it ViewPager holds references to old Fragments. This is just my assumption.
So, is there any way to enforce the ViewPager to call getItem() again, so I can use proper references to current Fragments? May be some other solution? Thank you very much.

Then I was debugging to see when FragmentPagerAdapter invokes getItem() and it happens the method is not called after orientation change event. So looks like it ViewPager holds references to old Fragments.
The fragments should be automatically recreated, just like any fragment is on an configuration change. The exception would be if you used setRetainInstance(true), in which case they should be the same fragment objects as before.
So, is there any way to enforce the ViewPager to call getItem() again, so I can use proper references to current Fragments?
What is wrong with the fragments that are there?

I've spent some days searching for a solution for this problem, and many points was figured out:
use FragmentPagerAdapter instead of FragmentStatePagerAdapter
use FragmentStatePagerAdapter instead of FragmentPagerAdapter
return POSITION_NONE on getItemPosition override of FragmentPagerAdapter
don't use FragmentPagerAdapter if you need dynamic changes of Fragments
and many many many others...
In my app, like Eugene, I managed myself the instances of created fragments. I keep that in one HashMap<String,Fragment> inside some specialized class, so the fragments are never released, speeding up my app (but consuming more resources).
The problem was when I rotate my tablet (and phone). The getItem(int) wasn't called anymore for that fragment, and I couldn't change it.
I really spent many time until really found a solution, so I need share it with StackOverflow community, who helps me so many many times...
The solution for this problem, although the hard work to find it, is quite simple:
Just keep the reference to FragmentManager in the constructor of FragmentPagerAdapter extends:
public class Manager_Pager extends FragmentPagerAdapter {
private final FragmentManager mFragmentManager;
private final FragmentActivity mContext;
public Manager_Pager(FragmentActivity context) {
super( context.getSupportFragmentManager() );
this.mContext = context;
this.mFragmentManager = context.getSupportFragmentManager();
}
#Override
public int getItemPosition( Object object ) {
// here, check if this fragment is an instance of the
// ***FragmentClass_of_you_want_be_dynamic***
if (object instanceof FragmentClass_of_you_want_be_dynamic) {
// if true, remove from ***FragmentManager*** and return ***POSITION_NONE***
// to force a call to ***getItem***
mFragmentManager.beginTransaction().remove((Fragment) object).commit();
return POSITION_NONE;
}
//don't return POSITION_NONE, avoid fragment recreation.
return super.getItemPosition(object);
}
#Override
public Fragment getItem( int position ) {
if ( position == MY_DYNAMIC_FRAGMENT_INDEX){
Bundle args = new Bundle();
args.putString( "anything", position );
args.putString( "created_at", ALITEC.Utils.timeToStr() );
return Fragment.instantiate( mContext, FragmentClass_of_you_want_be_dynamic.class.getName(), args );
}else
if ( position == OTHER ){
//...
}else
return Fragment.instantiate( mContext, FragmentDefault.class.getName(), null );
}
}
Thats all. And it will work like a charm...

You can clear the saved instance state
protected void onCreate(Bundle savedInstanceState) {
clearBundle(savedInstanceState);
super.onCreate(savedInstanceState, R.layout.activity_car);
}
private void clearBundle(Bundle savedInstanceState) {
if (savedInstanceState != null) {
savedInstanceState.remove("android:fragments");
savedInstanceState.remove("android:support:fragments");
savedInstanceState.remove("androidx.lifecycle.BundlableSavedStateRegistry.key");
savedInstanceState.remove("android:lastAutofillId");
}
}