How to determine when Fragment becomes visible in ViewPager
How to determine when Fragment becomes visible in ViewPager
Question
Problem: Fragment onResume() in ViewPager is fired before the fragment becomes actually visible.
For example, I have 2 fragments with ViewPager and FragmentPagerAdapter. The second fragment is only available for authorized users and I need to ask the user to log in when the fragment becomes visible (using an alert dialog).
BUT the ViewPager creates the second fragment when the first is visible in order to cache the second fragment and makes it visible when the user starts swiping.
So the onResume() event is fired in the second fragment long before it becomes visible. That's why I'm trying to find an event which fires when the second fragment becomes visible to show a dialog at the appropriate moment.
How can this be done?
Accepted Answer
How to determine when Fragment becomes visible in ViewPager
You can do the following by overriding setUserVisibleHint in your Fragment:
public class MyFragment extends Fragment {
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if (isVisibleToUser) {
}
else {
}
}
}
Popular Answer
UPDATE: Android Support Library (rev 11) finally fixed the user visible hint issue, now if you use support library for fragments, then you can safely use getUserVisibleHint() or override setUserVisibleHint() to capture the changes as described by gorn's answer.
UPDATE 1 Here is one small problem with getUserVisibleHint(). This value is by default true.
// Hint provided by the app that this fragment is currently visible to the user.
boolean mUserVisibleHint = true;
So there might be a problem when you try to use it before setUserVisibleHint() was invoked. As a workaround you might set value in onCreate method like this.
public void onCreate(@Nullable Bundle savedInstanceState) {
setUserVisibleHint(false);
The outdated answer:
In most use cases, ViewPager only show one page at a time, but the pre-cached fragments are also put to "visible" state (actually invisible) if you are using FragmentStatePagerAdapter in Android Support Library pre-r11.
I override :
public class MyFragment extends Fragment {
@Override
public void setMenuVisibility(final boolean visible) {
super.setMenuVisibility(visible);
if (visible) {
// ...
}
}
// ...
}
To capture the focus state of fragment, which I think is the most suitable state of the "visibility" you mean, since only one fragment in ViewPager can actually place its menu items together with parent activity's items.
Read more... Read less...
This seems to restore the normal onResume() behavior that you would expect. It plays well with pressing the home key to leave the app and then re-entering the app. onResume() is not called twice in a row.
@Override
public void setUserVisibleHint(boolean visible)
{
super.setUserVisibleHint(visible);
if (visible && isResumed())
{
//Only manually call onResume if fragment is already visible
//Otherwise allow natural fragment lifecycle to call onResume
onResume();
}
}
@Override
public void onResume()
{
super.onResume();
if (!getUserVisibleHint())
{
return;
}
//INSERT CUSTOM CODE HERE
}
Here is another way using onPageChangeListener:
ViewPager pager = (ViewPager) findByViewId(R.id.viewpager);
FragmentPagerAdapter adapter = new FragmentPageAdapter(getFragmentManager);
pager.setAdapter(adapter);
pager.setOnPageChangeListener(new OnPageChangeListener() {
public void onPageSelected(int pageNumber) {
// Just define a callback method in your fragment and call it like this!
adapter.getItem(pageNumber).imVisible();
}
public void onPageScrolled(int arg0, float arg1, int arg2) {
// TODO Auto-generated method stub
}
public void onPageScrollStateChanged(int arg0) {
// TODO Auto-generated method stub
}
});
setUserVisibleHint() gets called sometimes before onCreateView() and sometimes after which causes trouble.
To overcome this you need to check isResumed() as well inside setUserVisibleHint() method. But in this case i realized setUserVisibleHint() gets called only if Fragment is resumed and visible, NOT when Created.
So if you want to update something when Fragment is visible, put your update function both in onCreate() and setUserVisibleHint():
@Override
public View onCreateView(...){
...
myUIUpdate();
...
}
....
@Override
public void setUserVisibleHint(boolean visible){
super.setUserVisibleHint(visible);
if (visible && isResumed()){
myUIUpdate();
}
}
UPDATE: Still i realized myUIUpdate() gets called twice sometimes, the reason is, if you have 3 tabs and this code is on 2nd tab, when you first open 1st tab, the 2nd tab is also created even it is not visible and myUIUpdate() is called. Then when you swipe to 2nd tab, myUIUpdate() from if (visible && isResumed()) is called and as a result,myUIUpdate() may get called twice in a second.
The other problem is !visible in setUserVisibleHint gets called both 1) when you go out of fragment screen and 2) before it is created, when you switch to fragment screen first time.
Solution:
private boolean fragmentResume=false;
private boolean fragmentVisible=false;
private boolean fragmentOnCreated=false;
...
@Override
public View onCreateView(...){
...
//Initialize variables
if (!fragmentResume && fragmentVisible){ //only when first time fragment is created
myUIUpdate();
}
...
}
@Override
public void setUserVisibleHint(boolean visible){
super.setUserVisibleHint(visible);
if (visible && isResumed()){ // only at fragment screen is resumed
fragmentResume=true;
fragmentVisible=false;
fragmentOnCreated=true;
myUIUpdate();
}else if (visible){ // only at fragment onCreated
fragmentResume=false;
fragmentVisible=true;
fragmentOnCreated=true;
}
else if(!visible && fragmentOnCreated){// only when you go out of fragment screen
fragmentVisible=false;
fragmentResume=false;
}
}
Explanation:
fragmentResume,fragmentVisible: Makes sure myUIUpdate() in onCreateView() is called only when fragment is created and visible, not on resume. It also solves problem when you are at 1st tab, 2nd tab is created even if it is not visible. This solves that and checks if fragment screen is visible when onCreate.
fragmentOnCreated: Makes sure fragment is not visible, and not called when you create fragment first time. So now this if clause only gets called when you swipe out of fragment.
Update
You can put all this code in BaseFragment code like this and override method.
To detect Fragment in ViewPager visible, I'm quite sure that only using setUserVisibleHint is not enough.
Here is my solution to check if a fragment is visible or invisible. First when launching viewpager, switch between page, go to another activity/fragment/ background/foreground`
public class BaseFragmentHelpLoadDataWhenVisible extends Fragment {
protected boolean mIsVisibleToUser; // you can see this variable may absolutely <=> getUserVisibleHint() but it not. Currently, after many test I find that
/**
* This method will be called when viewpager creates fragment and when we go to this fragment background or another activity or fragment
* NOT called when we switch between each page in ViewPager
*/
@Override
public void onStart() {
super.onStart();
if (mIsVisibleToUser) {
onVisible();
}
}
@Override
public void onStop() {
super.onStop();
if (mIsVisibleToUser) {
onInVisible();
}
}
/**
* This method will called at first time viewpager created and when we switch between each page
* NOT called when we go to background or another activity (fragment) when we go back
*/
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
mIsVisibleToUser = isVisibleToUser;
if (isResumed()) { // fragment have created
if (mIsVisibleToUser) {
onVisible();
} else {
onInVisible();
}
}
}
public void onVisible() {
Toast.makeText(getActivity(), TAG + "visible", Toast.LENGTH_SHORT).show();
}
public void onInVisible() {
Toast.makeText(getActivity(), TAG + "invisible", Toast.LENGTH_SHORT).show();
}
}
EXPLANATION You can check the logcat below carefully then I think you may know why this solution will work
First launch
Fragment1: setUserVisibleHint: isVisibleToUser=false isResumed=false
Fragment2: setUserVisibleHint: isVisibleToUser=false isResumed=false
Fragment3: setUserVisibleHint: isVisibleToUser=false isResumed=false
Fragment1: setUserVisibleHint: isVisibleToUser=true isResumed=false // AT THIS TIME isVisibleToUser=true but fragment still not created. If you do something with View here, you will receive exception
Fragment1: onCreateView
Fragment1: onStart mIsVisibleToUser=true
Fragment2: onCreateView
Fragment3: onCreateView
Fragment2: onStart mIsVisibleToUser=false
Fragment3: onStart mIsVisibleToUser=false
Go to page2
Fragment1: setUserVisibleHint: isVisibleToUser=false isResumed=true
Fragment2: setUserVisibleHint: isVisibleToUser=true isResumed=true
Go to page3
Fragment2: setUserVisibleHint: isVisibleToUser=false isResumed=true
Fragment3: setUserVisibleHint: isVisibleToUser=true isResumed=true
Go to background:
Fragment1: onStop mIsVisibleToUser=false
Fragment2: onStop mIsVisibleToUser=false
Fragment3: onStop mIsVisibleToUser=true
Go to foreground
Fragment1: onStart mIsVisibleToUser=false
Fragment2: onStart mIsVisibleToUser=false
Fragment3: onStart mIsVisibleToUser=true
Hope it help
In ViewPager2 and ViewPager from version androidx.fragment:fragment:1.1.0 you can just use onPause and onResume callbacks to determine which fragment is currently visible for the user. onResume callback is called when fragment became visible and onPause when it stops to be visible.
In case of ViewPager2 it is default behavior but the same behavior can be enabled for old good ViewPager easily.
To enable this behavior in the first ViewPager you have to pass FragmentPagerAdapter.BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT parameter as second argument of FragmentPagerAdapter constructor.
FragmentPagerAdapter(fragmentManager, BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT)
Note: setUserVisibleHint() method and FragmentPagerAdapter constructor with one parameter are now deprecated in the new version of Fragment from android jetpack.