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));
Related
I have a ViewPager using a FragmentPagerAdapter for displaying three tabs, each represented by its ow fragment. One of these fragments contains a list, that should be updated on switching / swiping to that tab. But I don't find any way to make it happen. I tried using the onResume method, but the fragments seem not to be paused and resumed on tab change. I also tried using ViewPager.OnPageChangeListener in my MainActivity:
#Override
public void onPageSelected(int position)
{
FragmentRefreshInterface currentFragment = (FragmentRefreshInterface) mSectionsPagerAdapter.getItem(position);
currentFragment.onRefreshed();
}
And in the fragment I use the following:
#Override
public void onRefreshed()
{
List<Record> records = mRecordingService.getRecords();
mRecordAdapter.clear();
mRecordAdapter.add(record);
}
But using this code I can't access my RecordingService class that is used to provide the database functions (because mRecordingService seems to be null). I initialize it in the fragment like this:
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
mRecordingService = new RecordingService(getContext());
}
Using the onPageChangeListener is the correct way to do it. I believe the reason why your code is not working, is because you are calling getItem on your pager adapter: getItem() actually returns a new instance of the fragment. In order to get the current instance, you use instantiateItem() (which returns a reference to the fragment actually being used).
Change your code to look something like this:
#Override
public void onPageSelected(int position)
{
FragmentRefreshInterface currentFragment = (FragmentRefreshInterface) mSectionsPagerAdapter.instantiateItem(viewPager,position);
currentFragment.onRefreshed();
}
And it should work.
I suggest that the code you have in onRefreshed() go in onResume() instead. Fragment doesn't have an onRefreshed() method. You must be implementing another interface that declares this method.
Since you are storing data in a database, you should be use a CursorAdapter or subclass such as SimpleCursorAdapter. If you do this correctly, the ListView will automatically update when you add a record to the database. Then the service can add records without needing to access the service from the fragment.
In your MainActivity:
private FirstFragment firstFragment;
private WantedFragment wantedFragment;
private ThirdFragment thirdfragment;
In getItem
switch(postition){
//return first, wanted, third fragments depending on position
}
onPageSelected:
if(position == 1) // position of the wanted fragment
wantedfragment.onRefreshed()
Trying to programmatically add a fragment page to my ViewPager, I get:
java.lang.IllegalStateException: The application's PagerAdapter changed the adapter's contents without calling PagerAdapter#notifyDataSetChanged!
Expected adapter item count: 3, found: 2
Pager id: com.my.app:id/view_pager
Pager class: class android.support.v4.view.ViewPager
Problematic adapter: class com.my.app.ui.BaseFragmentPagerAdapter
at android.support.v4.view.ViewPager.populate(ViewPager.java:1000)
at android.support.v4.view.ViewPager.populate(ViewPager.java:952)
at ...
I'm simply calling these few lines on my FragmentPagerAdapter implementation:
adapter.addFragment(new Fragment(), "FIRST");
adapter.addFragment(new Fragment(), "SECOND");
pager.setAdapter(adapter);
//later... (on click of a button)
adapter.addFragment(new Fragment(), "THIRD");
adapter.notifyDataSetChanged();
It actually adds the third page, but when I try to swipe there, it fails with the above mentioned exception. Until today I thought I had a pretty complete understanding of how adapters work. Now I can't figure out what's wrong.
From debugging, it seems that all the time adapter.getCount() correctly returns 3 (after adding the third page), but when I'm there to the third page it eventually returns 2 and breaks, as if someone called destroyItem() on it, but that's not me.
Here's my simple class:
public class BaseFragmentPagerAdapter extends FragmentPagerAdapter {
private SparseArray<Fragment> mFragments;
private ArrayList<String> mFragmentTitles;
public BaseFragmentPagerAdapter(FragmentManager manager) {
super(manager);
this.mFragments = new SparseArray<>();
this.mFragmentTitles = new ArrayList<>();
}
public void addFragment(Fragment f, String title) {
this.mFragments.append(mFragments.size() , f);
this.mFragmentTitles.add(title);
}
#Override
public Fragment getItem(int position) {
return this.mFragments == null ? null : this.mFragments.get(position) ;
}
#Override
public int getItemPosition(Object object) {
return this.mFragments.indexOfValue((Fragment) object);
}
#Override
public void destroyItem(ViewGroup container, int position, Object object) {
super.destroyItem(container, position, object);
this.mFragments.remove(position);
this.mFragmentTitles.remove(position);
}
#Override
public CharSequence getPageTitle(int position) {
return mFragmentTitles.get(position);
}
#Override
public int getCount() {
return mFragmentTitles.size();
}
}
Note that nothing changes if I use a FragmentStatePagerAdapter rather than FragmentPagerAdapter.
I will answer this myself since I found the answer while writing the question (as often). I'm not sure this is the best solution, but it worked.
Basically, when going to page 3, since it's not directly swipable-to, the adapter will call destroyItem() on page 1. Shouldn't FragmentPagerAdapter hold all items in memory without destroying them?
Well, I do hold fragments in memory through the mFragments fields. The call to destroyItem() destroys the associated view of the fragment, but should not destroy the fragment itself (I might be slightly wrong here, but you get the point).
So it's up to you (me) to keep the fragments in memory, and not invalidating them on destroyItem(). Specifically, I had to remove these two lines:
#Override
public void destroyItem(ViewGroup container, int position, Object object) {
super.destroyItem(container, position, object);
//removed: this.mFragments.remove(position);
//removed: this.mFragmentTitles.remove(position);
}
This way getCount() keeps returning correctly 3, and when you are back to page 1, the adapter can get its fragment through getItem().
Edit
After dealing with it for a day, I can say that, at a first glance, having a FragmentPagerAdapter that does not hold fragments in memory makes no sense to me.
It is documented that it should be used just for a few static fragments. ViewPager, by default, holds the current item, the one before and the one after, but you can tune this setting through viewPager.setOffscreenPageLimit().
If you destroy them during destroyItem(), things get bad. It will be easy to reinstantiate them through some logic in getItem(), but it is quite hard to save their instance state. For instance, onSaveInstanceState(Bundle outstate) is not called after destroyItem().
If your fragments are many and you want to accept the possibility that one of more get destroyed, you just switch to FragmentStatePagerAdapter, but that's another story: it automatically calls onSaveInstanceState and lets you retain what you need to retain.
Using FragmentPagerAdapter, thus renouncing on the state-saving features of FragmentStatePagerAdapter, makes no sense if you don't retain. I mean, either you retain the instances, or you save their state (as suggested in the comments). For the latter, though, I would go for FragmentStatePagerAdapter that makes it easy.
(Note: I'm not talking about retaining instances when the activity gets destroyed, but rather when a page of the ViewPager goes through destroyItem and the associated fragment goes through onDestroyView()).
Thats a big problem for me right now because i need to call a method from an interface
all my fragments in my viewpager are implementing. I need to do something like this:
#Override
public void onPageSelected(int position) {
this.getActivity().getActionBar().setSelectedNavigationItem(position);
FragmentVisible fragment = (FragmentVisible) this.fragmentPager.instantiateItem(this.viewPager, position);
if (fragment != null) {
fragment.fragmentBecameVisible();
}
}
This works for the "normal startup" but when i rotate the screen i get nullpointer exceptions
because onPageSelected gets called before onViewCreated. I need my views to get updated everytime
a fragment gets visible. First i hoped onResume would get called everytime but it doesnt. For that
i implemented the interface:
public interface FragmentVisible {
public void fragmentBecameVisible();
}
Does someone has an idea how to solve this?
Per the FragmentPagerAdapter's setPrimaryItem() method (called when the ViewPager sets the current page), it calls setUserVisibleHint(true) for the current page's fragment. You can override that method in your Fragment and do your fragmentBecameVisible() method in there.
I have a ViewPager with 3 Fragments and my FragmentPagerAdapter:
private class test_pager extends FragmentPagerAdapter {
public test_pager(FragmentManager fm) {
super(fm);
}
#Override
public Fragment getItem(int i) {
return fragments[i];
}
#Override
public long getItemId(int position) {
if (position == 1) {
long res = fragments[position].hashCode()+fragment1_state.hashCode();
Log.d(TAG, "getItemId for position 1: "+res);
return res;
} else
return fragments[position].hashCode();
}
#Override
public int getCount() {
return fragments[2] == null ? 2 : 3;
}
#Override
public int getItemPosition(Object object) {
Fragment fragment = (Fragment) object;
for (int i=0; i<3; i++)
if (fragment.equals(fragments[i])){
if (i==1) {
return 1; // not sure if that makes a difference
}
return POSITION_UNCHANGED;
}
return POSITION_NONE;
}
}
In one of the page (#1), I keep changing the fragment to be displayed. The way I remove the old fragment is like this:
FragmentManager fm = getSupportFragmentManager();
fm.beginTransaction().remove(old_fragment1).commit();
And then just changing the value of fragments[1]
I found that I cannot really add or replace the new one or it will complain the ViewPager is trying to add it too with another tag... (am I doing something wrong here?)
All the fragments I display have setRetainInstance(true); in their onCreate function.
My problem is that this usually works well for the first few replacement, but then when I try to reuse a fragment, sometimes (I have not really figured out the pattern, the same fragment may be displayed several times before this happens) it will only show a blank page.
Here is what I have found happened in the callback functions of my Fragment I am trying to display when the problem happens:
onAttach is called (but at that time, getView is still null)
onCreateView is not called (that's expected)
onViewStateRestored is not called (why not?)
onResume is not called (I really thought it would...)
If it changes anything, I am using the support package, my activity is a SherlockFragmentActivity
EDIT (to answer Marco's comment):
The fragments are instantiated in the onCreate function of the Activity, I fill an ArrayList with those fragments:
char_tests = new ArrayList<Fragment>(Arrays.asList(
new FragmentOptionA(), new FragmentOptionB(), new FragmentOptionC()));
The I pick from that list to set fragments[1] (that's all done in the UI thread)
I fixed this by changing test_pager to extends FragmentStatePagerAdapter instead.
I am still confused as to what PagerAdapter should be used depending on the usage. The only thing I can find in the documentation says that FragmentPagerAdapter is better for smaller number of pages that would be kept in memory and FragmentPagerStateAdapter better for a larger number of pages where they would be destroyed and save memory...
When trying to do (fancy?) things with Fragments, I found FragmentStatePagerAdapter is better when pages are removed and re-inserted like in this case. And FragmentPagerAdapter is better when pages move position (see bug 37990)
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");
}
}