Develop Reference

Loading Fragment UI on-demand

Problem:
I am currently running into a problem where my app is trying to load too many fragments when it opens for the first time.
I have BottomNavigationView with ViewPager that loads 4 fragments - each one of the Fragment contains TabLayout with ViewPager to load at least 2 more fragments.
As you can imagine, that is a lot of UI rendering (10+ fragments) - especially when some of these fragments contain heavy components such as calendar, bar graphs, etc.
Currently proposed solution:
Control the UI loading when the fragment is required - so until the user goes to that fragment for the first time, there is no reason to load it.
It seems like it's definitely possible as many apps, including the Play Store, are doing it. Please see the example here
In the video example above - the UI component(s) are being loaded AFTER the navigation to the tab is completed. It even has an embedded loading symbol.
1) I am trying to figure out how to do exactly that - at what point would I know that this fragment UI need to be created vs it already is created?
2) Also, what is the fragment lifecycle callback where I would start the UI create process? onResume() means UI is visible to the user so loading the UI there will be laggy and delayed.
Hope this is clear enough.
EDIT:
I'm already using the FragmentStatePagerAdapter as ViewPager adapter. I noticed that the super(fm) method in the constructor is deprecated now:
ViewPagerAdapter(FragmentManager fm) {
super(fm); // this is deprecated
}
So I changed that to:
ViewPagerAdapter(FragmentManager fm) {
super(fm, BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT);
}
BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT: Indicates that only the current fragment will be in the Lifecycle.State.RESUMED state. All other Fragments are capped at Lifecycle.State.STARTED.
This seems useful as the onResume() of the Fragment will only be called when the Fragment is visible to the user. Can I use this indication somehow to load the UI then?

The reason your app loads multiple Fragments at the startup is most probably, you're initializing them all at once. Instead, you can initialize them when you need them. Then use show\ hide to attach\ detach from window without re-inflating whole layout.
Simple explanation: You'll create your Fragment once user clicks on BottomNavigationView's item. On clicked item, you'll check if Fragment is not created and not added, then create it and add. If it's already created then use show() method to show already available Fragment and use hide() to hide all other fragments of BottomNavigationView.
As per your case show()/hide is better than add()/replace because as you said you don't want to re-inflate the Fragment when you want show them
public class MainActivity extends AppCompatActivity{
FragmentOne frg1;
FragmentTwo frg2;
#Override
public boolean onNavigationItemSelected(MenuItem item){
switch(item.getId()){
case R.id.fragment_one:
if (frg2 != null && frg2.isAdded(){
fragmentManager.beginTransaction().hide(frg2).commit();
}
if(frg1 != null && !frg1.isAdded){
frg1 = new FragmenOne();
fragmentManager.beginTransaction().add(R.id.container, frg1).commit();
}else if (frg1 != null && frg1.isAdded) {
fragmentManager.beginTransaction().show(frg1).commit();
}
return true;
case R.id.fragment_two:
// Reverse of what you did for FragmentOne
return true;
}
}
}
And for your ViewPager as you can see from the example you're referring to; PlayStore is using setOffscreenPageLimit. This will let you choose how many Views should be kept alive, otherwise will be destroyed and created from start passing through all lifecycle events of the Fragment (in case view is Fragment). In PlayStore app's case that's probably 4-5 that why it started loading again when you re-selected "editor's choice" tab. If you do the following only selected and neighboring (one in the right) Fragments will be alive other Fragments outside screen will be destroyed.
public class FragmentOne extends Fragment{
ViewPager viewPager;
#Override
public void onCreateView(){
viewPager = .... // Initialize
viewpAger.setOffscreenPageLimit(1); // This will keep only 2 Fragments "alive"
}
}
Answer to both questions
If you use show/hide you won't need to know when to inflate your view. It will be handled automatically and won't be laggy since it's just attaching/detaching views not inflating.

It depends upon how you initialize your fragment in your activity. May be you are initializing all your fragment in onCreate method of your activity instead of that you can initialize it when BottomNavigation item is selected like below :
Fragment one,two,three,four;
#Override
public boolean onNavigationItemSelected(MenuItem item){
Fragment fragment;
switch(item.getId()){
case R.id.menu_one:{
if(one==null)
one = Fragment()
fragment = one;
break;
}
case R.id.menu_two:{
if(two==null)
two = Fragment()
fragment = two;
break;
}
}
getFragmentManager().beginTransaction().replace(fragment).commit();
}
To decide how many page is load in you view pager at one time you can use :
setOffscreenPageLimit.
viewPager.setOffscreenPageLimit(number)
To get the resume and pause functionality on fragments you can take an example from this link.
Please try this.

i was worked with the same kind of the Application, There were multiple tabs and also Tabs have multiple inner tabs.
i was used the concept of ViewPager method, In which there is one method of onPageSelected() for that method we were getting the page position.
By the Use of this position we are checking the current Fragment and called their custom method that we created inside that fragment like onPageSelected() defined inside that fragment.
With this custom method onPageSelected() inside the Fragment we checked that weather the list are available or not if list have data then we are not making the call of Api otherwise we are calling the Api and loading that list.
I think you have same kind of requirement to follow if your Tabs have inner Tab or viewpager you can follow same concept inside of that so if your current fragment of viewpager method onpageSelected called at that time your viewpager fragment initialized.
you have to call just initialization like data binding or view initialization need to be called in onCreate() method and other list attachment and api call to be managed by the custom method onPageSelected that will be called based on ViewPager onPageSelected.
let me Know if you need any help for same.

You can try to have Fragments with FrameLayouts only in ViewPager. The actual Fragments could be added to FrameLayout in onResume() (after checking if this Fragment isn't already attached). It should work if BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT works as expected.

I would recommend you use BottomNavigationView.setOnNavigationItemSelectedListener to toggle between the fragment UI whenever it is needed.
navigationView.setOnNavigationItemSelectedListener(item -> {
switch(item.getItemId()) {
case R.id.item1:
// you can replace the code findFragmentById() with findFragmentByTag("dashboard");
// if you only have one framelayout to hold the fragment
fragment = getSupportFragmentManager().findFragmentById(R.id.fragment_container);
if (fragment == null) {
fragment = new ExampleFragment();
// if the fragment is identified by tag, add another
// argument to this method:
// replace(R.id.fragment_container, fragment, "dashboard")
getSupportFragmentManager().begintransaction()
.replace(R.id.fragment_container, fragment)
.commit();
}
break;
}
}
The idea is simple, when the user swipes or selects a different tab, the fragment that was visible is replaced by the new fragment.

Just load fragments one by one. Create the main fragment layout with many placeholders and stubs and then just load them in the order you like.
Use FragmentTransaction.replace() from the main fragment after it loads.

Have you tried the setUserVisibleHint() method of a fragment
override fun setUserVisibleHint(isVisibleToUser: Boolean) {
super.setUserVisibleHint(isVisibleToUser)
if(isVisibleToUser){
// Do you stuff here
}
}
This will only get called when a fragment is visible to the user

How about you maintain just one ViewPager? Sounds crazy? In that case, you just change the dataset of PagerAdapter when you switch between the bottom tabs. Let's see how you can accomplish this,
As you mentioned, you have 4 fragments, which are assigned to each individual tabs of the bottom navigation view. Each performs some redundant work i.e. holding a viewPager with tab layout and setting the same kind of adapters. So, if we can combine these 4 redundant tasks into one then we will be able to get rid of 4 fragments. And as there will be just one viewPager with one single adapter then we will be able to reduce the fragment loading count from ~10 to 2 if we set offScreenPageLimit to 1. Let's see some example,
activity.xml should look like
<LinearLayout>
<TabLayout />
<ViewPager />
<BottomNavigationView />
</LinearLayout>
It's optional but I would recommend to create a base PagerFragment abstract class with abstract method getTabTitle()
public abstract class PagerFragment extends Fragment {
public abstract String getTabTitle();
}
Now it's time to make our PagerAdapter class
public class SectionsPagerAdapter extends FragmentStatePagerAdapter {
public Map<Integer, List<PagerFragment>> map = ...; // If you are concerned about memory then I could recommend to store DataObject instead of PagerFragment and instantiate fragment on demand using that data.
public int currentTabId = R.id.first_bottom_tab_id;
private List<PagerFragment> getCurrentFragments() {
return map.get(currentTabId);
}
public void setCurrentTabId(int tabId) {
this.currentTabId = tabId;
}
public SectionsPagerAdapter(FragmentManager manager) {
super(manager);
}
#Override
public Fragment getItem(int position) {
return getCurrentFragments().get(position);
}
#Override
public int getCount() {
return getCurrentFragments().size();
}
#Override
public int getItemPosition(#NonNull Object object) {
return POSITION_NONE;
}
#Override
public CharSequence getPageTitle(int position) {
return getCurrentFragments().get(position).getTabTitle();
}
}
And finally, in Activity
SectionsPagerAdapter pagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(pagerAdapter);
viewPager.setOffscreenPageLimit(1);
viewPagerTab.setViewPager(viewPager);
bottomNavigationView.setOnNavigationItemSelectedListener(menuItem -> {
pagerAdapter.setCurrentTabId(menuItem.getItemId())
pagerAdapter.notifyDataSetChanged();
viewPagerTab.setViewPager(viewPager);
}
This is the basic idea. You can mix some of your own ideas with it to make a wonderful result. Let me know if it is useful?
UPDATE
Answer to your questions,
I think with my solution you can achieve exactly the same behavior of the video as I already did it in a project. In my solution, if you set offset page limit to 1 then only adjacent fragment's is created in advance. So, fragment creation will be handled by adapter and viewpager you don't need to worry about it.
In my above solution, you should create UI in onCreateView().

ViewPage listener fired after fragment creation

Short Story:
How to detect a ViewPager page scrolling/changing before fragment lifecycle execution for the new page?
Long Story:
I have a ViewPager with assigned fragments to be displayed using ViewPagerAdapter, one of these fragments is meant to be displayed with different data according to current page selected in the pager.
for example, if current page selected is 2 it would display A data, and if the current page selected is 4 it would display B data.
the straight forward solution is to set the data according to the current page using OnPageChangeListener or SimpleOnPageChangeListener, but both are not applicable as the fragment WHOLE life cycle is being called before any of these listeners methods being called, so the data would be set after fragment creation here.
the second straight forward solution is to make the changes after receiving the call from the listeners which is so bad regarding user experience and design wise.
So would be the best way to set fragment credentials when changing the current page of the ViewPager before onResume() method of the fragment to be executed?
What I'm doing:
in MyFragment.java:
// it goes here first
#Override
public void onResume() {
super.onResume();
// check the Data Applied
if(dataA)
doSomething();
else
doSomethingElse();
}
in MainActivity.java:
pager.addOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
#Override
public void onPageSelected(int position) {
// then it goes here
// setting the Data of the fragment
if (position == 2)
setDataA();
else (position == 4)
setDataB();
}
});

Why don't you use callback Interface? If you set interface, you can even get the call back on the fragment onAttach() or where you want.
Example Implementation:
Activity:
public class MyActivity extends AppCompatActivity implements FragmentListener {
#Override
public void onFragmentSelected(String value) {
// set data here.
}
public interface FragmentListener {
void onFragmentSelected(String value);
}
}
In your viewPager Fragments:
public class MyFragment extends Fragment{
#Override
public void onAttach(Context context) {
super.onAttach(context);
if(context instanceof MyActivity){
((MyActivity)context).onFragmentSelected("Your_Identification");
}
}
}
Do this in all your viewPager fragments so you will get which fragment attached from the frgment onAttach() itself. Or choose when it should be called.
Hope it Helps:)

Updating listview fragment on viewpager tab changes

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()

Notifying activity of fragment swiped into view via a pager

I am having trouble implementing a feature in my Android app.
Here's the setup:
ItemPagerActivity: An activity that contains a fragment that displays a pager.
ItemPagerFragment: The fragment containing a pager that loads other fragments. A cursor is used to load the fragments.
ItemFragment: The fragment in the pager, which performs an asynchronous task to load its data.
What I want is the following:
as a I swipe pages, the data in the currently displayed ItemFragment is communicated to the ItemPagerActivity (specifically, the name of the item will be used as the activity's title).
I've defined a listener in ItemFragment that notifies when the data is loaded:
public class ItemFragment ... {
public interface OnItemLoadedListener {
public void onItemLoaded(Item item);
}
private Collection<OnItemLoadListener> listeners;
private class LoadItemTask extends AsyncTask<...> {
...
public void onPostExecute(Item item) {
notifyItemLoaded(item);
...
}
}
}
If this fragment was wrapped by an Activity, then I could set the activity's title simply by doing the following:
public class ItemActivity {
public void onCreate(...) {
...
ItemFragment fragment = new ItemFragment();
fragment.registerItemLoadedListener(new ItemLoadedListener() {
public void onItemLoaded(Item item) {
setTitle("Item: " + item.getName());
}
});
...
}
}
So that's easy enough, and works as expected: when the activity starts, it creates the fragment, which loads the item, which notifies the activity, and the title is updated correctly.
But with ItemPagerFragment, the fragments are loaded pre-emptively: swiping to Fragment 3 may mean that Fragment 4 and Fragment 5 are created. Receiving notifications from the ItemFragment class when items are loaded is not correct here because the fragment displayed may not match the fragment that performed the last load.
Now the ViewPager class has a OnPageChangeListener which could be a solution: when I swipe, this listener is invoked with the current page number. From that page number, I need to (somehow) get the fragment representing that page from the adapter, get the Item data out of the fragment, and notify listeners that the Item is now loaded:
public class ItemPagerFragment ... {
private Collection<OnItemLoadedListener> listeners;
public View onCreateView(...) {
...
ViewPager pager = (ViewPager) view.findViewById(R.id.pager):
pager.setOnPageChangeListener(new OnPageChangeListener() {
public void onPageChange(int pageNumber) {
ItemFragment fragment = getItemFragment(pageNumber);
Item item = fragment.getLoadedItem();
notifyItemLoaded(item);
}
});
...
}
}
The ItemPagerActivity class would then register as a listener on the ItemPagerFragment class as follows:
class ItemPagerActivity ... {
public void onCreate(...) {
...
ItemPagerFragment fragment = new ItemPagerFragment();
fragment.registerOnItemLoadedListener(new OnItemLoadedListener() {
public void onItemLoaded(Item item) {
setTitle("Item: " + item.getName());
}
});
...
}
}
This looks good, but there are a number of problems:
The OnPageChangeListener may be invoked before a fragment has loaded its data (i.e., the fragment is swiped into view before the item has asynchronously loaded). So the call to fragment.getLoadedItem() may return null.
The OnPageChangeListener is not invoked for the initial page (only when a page changes, e.g. after a swipe action) so the activity title will be incorrect for the initial page.
The ViewPager class allows for only one OnPageChangeListener. This is a problem because I am also using the ViewPageIndicator library, which wants to assign a listener to the ViewPager.
I'm assuming that this pattern (notifying the activity of the data in a fragment that has been swiped into view) might be common, so I am wondering if there are any good solutions for this pattern, and to the three specific problems that I have identified above.

...so I am wondering if there are any good solutions for this pattern,
and to the three specific problems that I have identified above.
I don't know if I would call it a pattern but the OnPageChangeListener is the way to go.
The OnPageChangeListener may be invoked before a fragment has loaded
its data (i.e., the fragment is swiped into view before the item has
asynchronously loaded). So the call to fragment.getLoadedItem() may
return null.
First, your code should handle the "no data available situation" from the start. Your AsyncTasks will have the job of loading the data and also update the title only if the fragment for which they are working is the visible one(a position field in the ItemFragment tested against the ViewPager's getCurrentItem() method). The OnPageChangeListener will handle the update of the title after the data was loaded, as the user switches between pages and the data is available(it will return null if no data is available). To get the ItemFragment for a certain position you could use the code below:
ItemFragment itf = getSupportFragmentManager()
.findFragmentByTag(
"android:switcher:" + R.id.theIdOfTheViewPager + ":"
+ position);
if (itf != null) {
Item item = fragment.getLoadedItem();
notifyItemLoaded(item);
}
The OnPageChangeListener is not invoked for the initial page (only
when a page changes, e.g. after a swipe action) so the activity title
will be incorrect for the initial page.
See above.
The ViewPager class allows for only one OnPageChangeListener. This is
a problem because I am also using the ViewPageIndicator library, which
wants to assign a listener to the ViewPager
I admit I don't have much knowledge on the ViewPagerIndicator library but at a quick look on its site I saw:
(Optional) If you use an OnPageChangeListener with your view pager you
should set it in the indicator rather than on the pager directly.
titleIndicator.setOnPageChangeListener(mPageChangeListener);
I don't see where is the limitation.

For my purposes, it worked to use ViewPager.OnPageChangeListener.onPageSelected() in conjunction with Fragment.onActivityCreated() to perform an action when the Fragment is visible. Fragment.getUserVisibleHint() helps too.

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");
}
}