How to communicate with fragments when using viewpager?

Question

I'm currently working with an application that has "swipey-tabs" and uses the following PagerAdapter:

public class TabsPagerAdapter extends FragmentPagerAdapter {

    public TabsPagerAdapter(FragmentManager fm) {
        super(fm);
    }


    @Override
    public Fragment getItem(int index) {
        switch (index) {
            case 0:
                return new HomeFragment();
            case 1:
                return new ListFragment();
            case 2:
                return new ChartFragment();
        }

        return null;
    }

    @Override
    public int getCount() {

        return 3;
    }

}

I'm using the following code to communicate with one of my fragments from the main activity:

   @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data)
    {
        super.onActivityResult(requestCode, resultCode, data);
        if(requestCode == 1)
        {
            if (resultCode == Activity.RESULT_OK)
            {
                String className = data.getExtras().getString("class_name");
                if (className.trim().length() > 0) {
                    HomeFragment homeFrag = (HomeFragment) getSupportFragmentManager().findFragmentById(R.id.home_fragment);
                    if(homeFrag != null) {
                        Log.d("MainActivity", "homeFrag is not null");
                        homeFrag.newClass(className);
                    }else{
                        Log.d("MainActivity", "homeFrag is null");
                        HomeFragment newFragment = new HomeFragment();

                        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
                        transaction.replace(R.id.home_fragment, newFragment);
                        transaction.commit();
                        newFragment.newClass(className);
                    }
                    addSubjectToDataBase(className);
                }

            }
        }
    }

My problem is that when I try to communicate with the fragment from the main activity, I seem to get multiple fragments existing at the same time. When I change the orientation of the screen or restart the activity, I get the modified version of the fragment, but when I swipe back and forth to view the fragments, I get the old, non-modified version of the fragment. The fragment manager doesn't seem to be registering the fragments created by the adapter because though I know the fragments have been created, the fragment manager always returns a null home fragment.

How should I go about avoiding this problem, and what is the best way to communicate with the fragment from the main activity?

The information that I'm updating the fragment with is coming from user input from another activity that is launched when they press an add button. — Matthias, Jul 19 '15 at 08:29

score 1 · Answer 1 · edited May 23 '17 at 11:45

After reading through this post I worked out a solution to my own question. The problem was that since FragmentPagerAdapter manages its own fragments, the fragment that I was trying to access wasn't the same as the one being used by the view pager.

I worked out a way of accessing the tags of the fragments that are used by the view pager by modifying my modifying my PagerAdapter as such:

public class TabsPagerAdapter extends FragmentPagerAdapter {

    protected String homeTag;
    protected String listTag;
    protected String chartTag;

    public TabsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int index) {
        switch (index) {
            case 0:
                Log.d("Adapter", "returning new homeFragment");
                return new HomeFragment();
            case 1:
                return new ListFragment();
            case 2:
                return new ChartFragment();
        }

        return null;
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Fragment createdFragment = (Fragment) super.instantiateItem(container, position);

        switch (position){
            case 0:
                homeTag = createdFragment.getTag();
                break;
            case 1:
                listTag = createdFragment.getTag();
                break;
            case 2:
                chartTag = createdFragment.getTag();
                break;
        }

        return createdFragment;
    }

    public String getHomeTag(){
        return  homeTag;
    }

    public String getListTag(){
        return listTag;
    }

    public String getChartTag(){
        return chartTag;
    }
    @Override
    public int getCount() {

        return 3;
    }

}

Then, it was a simple matter to modify my code to access the correct fragments after I had access the proper fragment tags:

HomeFragment homeFrag = (HomeFragment) getSupportFragmentManager().findFragmentByTag(mAdapter.getHomeTag());
                    if(homeFrag != null) {
                        Log.d("MainActivity", "homeFrag is not null");
                        homeFrag.newClass(className);
                    }else{
                        Log.d("MainActivity", "homeFrag is null");
                        HomeFragment newFragment = new HomeFragment();

                        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
                        transaction.replace(R.id.pager, newFragment, madapter.get);
                        transaction.commit();
                        newFragment.newClass(className);

Jibbo · Answer 2 · 2015-07-19T10:46:20.260

It's not elegant but I would do something like this (Maybe someone will post a better solution):

In a singleton, I'll put an array of fragment:

public static Fragment[] sShownFragments = new Fragment[3];

Then in the getItem() method:

if(SingletonClass.sShownFragments[index]!=null){
    return mFragments[index];
}

switch (index) {
        case 0:
            SingletonClass.sShownFragments[index] = new HomeFragment();
            break;
        case 1:
            SingletonClass.sShownFragments[index] = new ListFragment();
        case 2:
            SingletonClass.sShownFragments[index] = new ChartFragment();
        default:
            SingletonClass.sShownFragments[index] = new Fragment();
}
return SingletonClass.sShownFragments[index];

Then in the getCount(): return SingletonClass.sShownFragments.length();

Now in your onActivityResult():

 \\some code ...
 ((HomeFragment)SingletonClass.sShownFragments[0]).newClass(className);
 \\ some other code...

How to communicate with fragments when using viewpager?

2 Answers2