How to get existing fragments when using FragmentPagerAdapter

Summary of the problem

Note: In this answer I'm going to reference FragmentPagerAdapter and its source code. But the general solution should also apply to FragmentStatePagerAdapter.

If you're reading this you probably already know that FragmentPagerAdapter/FragmentStatePagerAdapter is meant to create Fragments for your ViewPager, but upon Activity recreation (whether from a device rotation or the system killing your App to regain memory) these Fragments won't be created again, but instead their instances retrieved from the FragmentManager. Now say your Activity needs to get a reference to these Fragments to do work on them. You don't have an id or tag for these created Fragments because FragmentPagerAdapter set them internally. So the problem is how to get a reference to them without that information...

Problem with current solutions: relying on internal code

A lot of the solutions I've seen on this and similar questions rely on getting a reference to the existing Fragment by calling FragmentManager.findFragmentByTag() and mimicking the internally created tag: "android:switcher:" + viewId + ":" + id. The problem with this is that you're relying on internal source code, which as we all know is not guaranteed to remain the same forever. The Android engineers at Google could easily decide to change the tag structure which would break your code leaving you unable to find a reference to the existing Fragments.

Alternate solution without relying on internal tag

Here's a simple example of how to get a reference to the Fragments returned by FragmentPagerAdapter that doesn't rely on the internal tags set on the Fragments. The key is to override instantiateItem() and save references in there instead of in getItem().

public class SomeActivity extends Activity {
    private FragmentA m1stFragment;
    private FragmentB m2ndFragment;

    // other code in your Activity...

    private class CustomPagerAdapter extends FragmentPagerAdapter {
        // other code in your custom FragmentPagerAdapter...

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

        @Override
        public Fragment getItem(int position) {
            // Do NOT try to save references to the Fragments in getItem(),
            // because getItem() is not always called. If the Fragment
            // was already created then it will be retrieved from the FragmentManger
            // and not here (i.e. getItem() won't be called again).
            switch (position) {
                case 0:
                    return new FragmentA();
                case 1:
                    return new FragmentB();
                default:
                    // This should never happen. Always account for each position above
                    return null;
            }
        }

        // Here we can finally safely save a reference to the created
        // Fragment, no matter where it came from (either getItem() or
        // FragmentManger). Simply save the returned Fragment from
        // super.instantiateItem() into an appropriate reference depending
        // on the ViewPager position.
        @Override
        public Object instantiateItem(ViewGroup container, int position) {
            Fragment createdFragment = (Fragment) super.instantiateItem(container, position);
            // save the appropriate reference depending on position
            switch (position) {
                case 0:
                    m1stFragment = (FragmentA) createdFragment;
                    break;
                case 1:
                    m2ndFragment = (FragmentB) createdFragment;
                    break;
            }
            return createdFragment;
        }
    }

    public void someMethod() {
        // do work on the referenced Fragments, but first check if they
        // even exist yet, otherwise you'll get an NPE.

        if (m1stFragment != null) {
            // m1stFragment.doWork();
        }

        if (m2ndFragment != null) {
            // m2ndFragment.doSomeWorkToo();
        }
    }
}

or if you prefer to work with tags instead of class member variables/references to the Fragments you can also grab the tags set by FragmentPagerAdapter in the same manner: NOTE: this doesn't apply to FragmentStatePagerAdapter since it doesn't set tags when creating its Fragments.

@Override
public Object instantiateItem(ViewGroup container, int position) {
    Fragment createdFragment = (Fragment) super.instantiateItem(container, position);
    // get the tags set by FragmentPagerAdapter
    switch (position) {
        case 0:
            String firstTag = createdFragment.getTag();
            break;
        case 1:
            String secondTag = createdFragment.getTag();
            break;
    }
    // ... save the tags somewhere so you can reference them later
    return createdFragment;
}

Note that this method does NOT rely on mimicking the internal tag set by FragmentPagerAdapter and instead uses proper APIs for retrieving them. This way even if the tag changes in future versions of the SupportLibrary you'll still be safe.

Don't forget that depending on the design of your Activity, the Fragments you're trying to work on may or may not exist yet, so you have to account for that by doing null checks before using your references.

Also, if instead you're working with FragmentStatePagerAdapter, then you don't want to keep hard references to your Fragments because you might have many of them and hard references would unnecessarily keep them in memory. Instead save the Fragment references in WeakReference variables instead of standard ones. Like this:

WeakReference<Fragment> m1stFragment = new WeakReference<Fragment>(createdFragment);
// ...and access them like so
Fragment firstFragment = m1stFragment.get();
if (firstFragment != null) {
    // reference hasn't been cleared yet; do work...
}

I have found answer on my question based on following post: reusing fragments in a fragmentpageradapter

Few things I have learned:

1. getItem(int position) in the FragmentPagerAdapter is rather misleading name of what this method actually does. It creates new fragments, not returning existing ones. In so meaning, the method should be renamed to something like createItem(int position) in the Android SDK. So this method does not help us getting fragments.
2. Based on explanation in the post support FragmentPagerAdapterholds reference to old fragments you should leave the creation of the fragments to the FragmentPagerAdapter and in so meaning you have no reference to the Fragments or their tags. If you have fragment tag though, you can easily retrieve reference to it from the FragmentManager by calling findFragmentByTag(). We need a way to find out tag of a fragment at given page position.

Solution

Add following helper method in your class to retrieve fragment tag and send it to the findFragmentByTag() method.

private String getFragmentTag(int viewPagerId, int fragmentPosition)
{
     return "android:switcher:" + viewPagerId + ":" + fragmentPosition;
}

NOTE! This is identical method that FragmentPagerAdapter use when creating new fragments. See this link http://code.google.com/p/openintents/source/browse/trunk/compatibility/AndroidSupportV2/src/android/support/v2/app/FragmentPagerAdapter.java#104

you don't need to override instantiateItem nor rely on compatibility with internal makeFragmentName method by manually creating fragment tags .
instantiateItem is a public method so you can and actually should call it in onCreate method of your activity surrounded with calls to startUpdate and finishUpdate methods as described in PagerAdapter javadoc:

A call to the PagerAdapter method startUpdate(ViewGroup) indicates that the contents of the ViewPager are about to change. One or more calls to instantiateItem(ViewGroup, int) and/or destroyItem(ViewGroup, int, Object) will follow, and the end of an update will be signaled by a call to finishUpdate(ViewGroup).

You can then by the way of the above, store references to instances of your fragments on local vars if you need. See example:

public class MyActivity extends AppCompatActivity {

    Fragment0 tab0; Fragment1 tab1;

    @Override protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.myLayout);
        ViewPager viewPager = (ViewPager) findViewById(R.id.myViewPager);
        MyPagerAdapter adapter = new MyPagerAdapter(getSupportFragmentManager());
        viewPager.setAdapter(adapter);
        ((TabLayout) findViewById(R.id.tabs)).setupWithViewPager(viewPager);

        adapter.startUpdate(viewPager);
        tab0 = (Fragment0) adapter.instantiateItem(viewPager, 0);
        tab1 = (Fragment1) adapter.instantiateItem(viewPager, 1);
        adapter.finishUpdate(viewPager);
    }

    class MyPagerAdapter extends FragmentPagerAdapter {

        public MyPagerAdapter(FragmentManager manager) {super(manager);}

        @Override public int getCount() {return 2;}

        @Override public Fragment getItem(int position) {
            if (position == 0) return new Fragment0();
            if (position == 1) return new Fragment1();
            return null;  // or throw some exception
        }

        @Override public CharSequence getPageTitle(int position) {
            if (position == 0) return getString(R.string.tab0);
            if (position == 1) return getString(R.string.tab1);
            return null;  // or throw some exception
        }
    }
}

instantiateItem will first try to get references to existing fragment instances from FragmentManager. Only if they don't exist yet, it will create new ones using getItem method from your adapter and "store" them in the FragmentManager for any future use.

It's important to note, that even if you don't need to obtain references to your fragments, you still should call instantiateItem for all your tabs surrounded by startUpdate/finishUpdate in your onCreate method like this:

    adapter.startUpdate(viewPager);
    // ignoring return values of the below 2 calls, just side effects matter:
    adapter.instantiateItem(viewPager, 0);
    adapter.instantiateItem(viewPager, 1);
    adapter.finishUpdate(viewPager);

If you don't do so, then you are risking that your fragment instances will never be committed to FragmentManager : when your activity becomes foreground instantiateItem will be called automatically to obtain your fragments, but startUpdate/finishUpdate may not (depending on implementation details) and what they basically do is begin/commit a FragmentTransaction.
This may result in references to the created fragment instances being lost very quickly (for example when you rotate your screen) and recreated much more often than necessary. Depending on how "heavy" your fragments are, it may have a non-negligible performance consequences.
Moreover, in such case instances of fragments stored on local vars may become stale: if android platform tries to obtain them from FragmentManager for whatever reason, it will fail and thus will create and use new ones, while your vars will still be referencing the old ones.