How to determine when Fragment becomes visible in

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?

Answer 1

Note that setUserVisibleHint(false) is not called on activity / fragment stop. You'll still need to check start/stop to properly register/unregister any listeners/etc.

Also, you'll get setUserVisibleHint(false) if your fragment starts in a non-visible state; you don't want to unregister there since you've never registered before in that case.

@Override
public void onStart() {
    super.onStart();

    if (getUserVisibleHint()) {
        // register
    }
}

@Override
public void onStop() {
    if (getUserVisibleHint()) {
        // unregister
    }

    super.onStop();
}

@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
    super.setUserVisibleHint(isVisibleToUser);

    if (isVisibleToUser && isResumed()) {
        // register

        if (!mHasBeenVisible) {
            mHasBeenVisible = true;
        }
    } else if (mHasBeenVisible){
        // unregister
    }
}

Answer 2

Override Fragment.onHiddenChanged() for that.

public void onHiddenChanged(boolean hidden)

Called when the hidden state (as returned by isHidden()) of the fragment has changed. Fragments start out not hidden; this will be called whenever the fragment changes state from that.

Parameters
hidden - boolean: True if the fragment is now hidden, false if it is not visible.

Answer 3

I figured out that onCreateOptionsMenu and onPrepareOptionsMenu methods called only in the case of the fragment really visible. I could not found any method which behaves like these, also I tried OnPageChangeListener but it did not work for the situations, for example, I need a variable initialized in onCreate method.

So these two methods can be used for this problem as a workaround, specifically for little and short jobs.

I think, this is the better solution but not the best. I will use this but wait for better solution at the same time.

Regards.

Answer 4

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.

Answer 5

Another solution posted here overriding setPrimaryItem in the pageradapter by kris larson almost worked for me. But this method is called multiple times for each setup. Also I got NPE from views, etc. in the fragment as this is not ready the first few times this method is called. With the following changes this worked for me:

private int mCurrentPosition = -1;

@Override
public void setPrimaryItem(ViewGroup container, int position, Object object) {
    super.setPrimaryItem(container, position, object);

    if (position == mCurrentPosition) {
        return;
    }

    if (object instanceof MyWhizBangFragment) {
        MyWhizBangFragment fragment = (MyWhizBangFragment) object;

        if (fragment.isResumed()) {
            mCurrentPosition = position;
            fragment.doTheThingYouNeedToDoOnBecomingVisible();
        }
    }
}

Answer 6

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
}