Java – Access the ViewPager Fragment method from the activity

Here is my scenario :

I have ViewPager in my activity, which hosts 6 fragments. I disable pagination by swiping with my finger, so whenever I want to swipe, I use the relevant buttons and:

viewPager.setCurrentItem(viewPager.getCurrentItem() + 1, true);

In each fragment that slides (after the slide is done), I want to send a GET request to my server and get some data and display it in that fragment. To this end:

First method: I used this code in my fragment and it runs as soon as the fragment is visible:

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

However, there is a problem here: setUserVisibleHint executes exactly when the fragment is visible, so the slide animation has some lag (not smooth enough).

So I used the second method:
I added an OnPageChangeListener() :
to ViewPager in such a hosted activity

viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            int CurrentPossition = 0;
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { }
            @Override
            public void onPageSelected(int position) {
                CurrentPossition = position;
            }
            @Override
            public void onPageScrollStateChanged(int state) {
                if(state == ViewPager.SCROLL_STATE_IDLE && CurrentPossition != 0){
                    Toast.makeText(getBaseContext(),"finished" , Toast.LENGTH_SHORT).show();

try{
                        new fragment_two().sendGetRequest();;
                    }catch(Exception ex){
                        ex.printStackTrace();
                    }
                }
            }
        });

It works great, the toast is displayed as soon as the slide is done, but unlike the fully visible fragment, I get NullPointerException when sendGetRequest() runs.

Here is the stack trace:

04-08 20:15:37.840 12848-12848/com.example.mohamad.travelagency W/System.err: java.lang.NullPointerException
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:152)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:103)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.app.AlertDialog.resolveDialogTheme(AlertDialog.java:143)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.app.AlertDialog.<init>(AlertDialog.java:98)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.app.ProgressDialog.<init>(ProgressDialog.java:77)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err:     at com.example.mohamad.travelagency.fragment_two. GetServetData_L1(fragment_two.java:458)
04-08 20:15:37.850 12848-12848/com.example.mohamad.travelagency W/System.err:     at com.example.mohamad.travelagency.MainActivity$1.onPageScrollStateChanged(MainActivity.java :124)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.support.v4.view.ViewPager.dispatchOnScrollStateChanged(ViewPager.java:1811)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.support.v4.view.ViewPager.setScrollState(ViewPager.java:404)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.support.v4.view.ViewPager.access$000(ViewPager.java:91)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.support.v4.view.ViewPager$3.run(ViewPager.java:250)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.view.Choreographer.doCallbacks(Choreographer.java:574)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.view.Choreographer.doFrame(Choreographer.java:543)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.os.Handler.handleCallback(Handler.java:733)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:95)
04-08 20:15:37.860 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.os.Looper.loop(Looper.java:136)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:5271)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err:     at java.lang.reflect.Method.invokeNative(Native Method)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err:     at java.lang.reflect.Method.invoke(Method.java:515)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:851)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:667)
04-08 20:15:37.870 12848-12848/com.example.mohamad.travelagency W/System.err:     at dalvik.system.NativeStart.main(Native Method)

Any idea would be great.
Best regards

Answer:
Daniel Nugent’s code worked fine, except for displaying ProgressDialog during sending a GET request, I used:

final ProgressDialog dialog = new ProgressDialog(new MainActivity());

This code also returned NullPointerException, which I removed and now works fine.

Using ViewPager.OnPageChangeListener is the right approach, but you need to refactor your adapter slightly so that you retain a reference to each fragment contained in the FragmentPagerAdapter.

You can override it using the instantiateItem() method in the adapter, which is a simplified example:

 class PagerAdapter extends FragmentPagerAdapter {
        String tabTitles[] = new String[] { "One", "Two", "Three", "Four"};
        Context context;
        
This will contain your Fragment references:
        public Fragment[] fragments = new Fragment[tabTitles.length];
        
public PagerAdapter(FragmentManager fm, Context context) {
            super(fm);
            this.context = context;
        }
        @Override
        public int getCount() {
            return tabTitles.length;
        }
        @Override
        public Fragment getItem(int position) {
            switch (position) {
            case 0:
                return new FragmentOne();
            case 1:
                return new FragmentTwo();
            case 2:
                return new FragmentThree();   
            case 3:
                return new FragmentFour();
            }
            return null;
        }

This populates your Fragment reference array:
        @Override
        public Object instantiateItem(ViewGroup container, int position) {
            Fragment createdFragment = (Fragment) super.instantiateItem(container, position);
            fragments[position]  = createdFragment;
            return createdFragment;
        }
        
@Override
        public CharSequence getPageTitle(int position) {
             Generate title based on item position
            return tabTitles[position];
        }         
 }

Then, instead of creating a new fragment, use the fragment included in the adapter:

mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
  @Override
  public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) { }

@Override
  public void onPageSelected(int position) {

 do this instead, assuming your adapter reference
         is named mAdapter:
        Fragment frag = mAdapter.fragments[position];
        if (frag != null && frag instanceof FragmentTwo) {
          ((FragmentTwo)frag).sendGetRequest();
        }
  }

@Override
  public void onPageScrollStateChanged(int state) {  }
});

Note that if you use different Fragment classes in your adapter, you can implement the interface that defines sendGetRequest() and implement the sendGetRequest() method in each Fragment class.

If you do not take the interface approach, you will need to convert the fragment to your own fragment type, as shown in the example above, i.e.:

if (frag instanceof FragmentTwo) {
    ((FragmentTwo)frag).sendGetRequest();
}

Update

For using ViewPager2 and Kotlin, it looks like this:

    viewPager.registerOnPageChangeCallback(
            object: ViewPager2.OnPageChangeCallback() {
                override fun onPageSelected(position: Int) {
                    super.onPageSelected(position)
                    val frag: Fragment = mAdapter.fragments[position]
                    if (frag != null && frag is FragmentTwo) {
                        (frag as FragmentTwo).sendGetRequest()
                    }
                }
            }
    )