Retrieve a Fragment from a ViewPager

Question

I'm using a ViewPager together with a FragmentStatePagerAdapter to host three different fragments:

• [Fragment1]
• [Fragment2]
• [Fragment3]

When I want to get Fragment1 from the ViewPager in the FragmentActivity.

What is the problem, and how do I fix it?

Answer 1

The easiest and the most concise way. If all your fragments in ViewPager are of different classes you may retrieve and distinguish them as following:

public class MyActivity extends Activity
{

    @Override
    public void onAttachFragment(Fragment fragment) {
        super.onAttachFragment(fragment);
        if (fragment.getClass() == MyFragment.class) {
            mMyFragment = (MyFragment) fragment;
        }
    }

}

Answer 2

I implemented this easy with a bit different approach.

My custom FragmentAdapter.getItem method returned not new MyFragment(), but the instance of MyFragment that was created in FragmentAdapter constructor.

In my activity I then got the fragment from the adapter, check if it is instanceOf needed Fragment, then cast and use needed methods.

Answer 3

For grabbing fragments out of a ViewPager there are a lot of answers on here and on other related SO threads / blogs. Everyone I have seen is broken, and they generally seem to fall into one of the two types listed below. There are some other valid solutions if you only want to grab the current fragment, like this other answer on this thread.

If using FragmentPagerAdapter see below. If using FragmentStatePagerAdapter its worth looking at this. Grabbing indexes that are not the current one in a FragmentStateAdapter is not as useful as by the nature of it these will be completely torn down went out of view / out of offScreenLimit bounds.

THE UNHAPPY PATHS

Wrong: Maintain your own internal list of fragments, added to when FragmentPagerAdapter.getItem() is called

• Usually using a SparseArray or Map
• Not one of the many examples I have seen accounts for lifecycle events so this solution is fragile. As getItem is only called the first time a page is scrolled to (or obtained if your ViewPager.setOffscreenPageLimit(x) > 0) in the ViewPager, if the hosting Activity / Fragment is killed or restarted then the internal SpaseArray will be wiped out when the custom FragmentPagerActivity is recreated, but behind the scenes the ViewPagers internal fragments will be recreated, and getItem will NOT be called for any of the indexes, so the ability to get a fragment from index will be lost forever. You can account for this by saving out and restoring these fragment references via FragmentManager.getFragment() and putFragment but this starts to get messy IMHO.

Wrong: Construct your own tag id matching what is used under the hood in FragmentPagerAdapter and use this to retrieve the page Fragments from the FragmentManager

• This is better insomuch as it copes with the losing-fragment-references problem in the first internal-array solution, but as rightly pointed out in the answers above and elsewhere on the net - it feels hacky as its a private method internal to ViewPager that could change at any time or for any OS version.

The method thats recreated for this solution is

private static String makeFragmentName(int viewId, long id) {
    return "android:switcher:" + viewId + ":" + id;
}

A HAPPY PATH: ViewPager.instantiateItem()

A similar approach to getItem() above but non-lifecycle-breaking is to this is to hook into instantiateItem() instead of getItem() as the former will be called everytime that index is created / accessed. See this answer

A HAPPY PATH: Construct your own FragmentViewPager

Construct your own FragmentViewPager class from the source of the latest support lib and change the method used internally to generate the fragment tags. You can replace it with the below. This has the advantage that you know the tag creation will never change and your not relying on a private api / method, which is always dangerous.

/**
 * @param containerViewId the ViewPager this adapter is being supplied to
 * @param id pass in getItemId(position) as this is whats used internally in this class
 * @return the tag used for this pages fragment
 */
public static String makeFragmentName(int containerViewId, long id) {
    return "android:switcher:" + containerViewId + ":" + id;
}

Then as the doc says, when you want to grab a fragment used for an index just call something like this method (which you can put in the custom FragmentPagerAdapter or a subclass) being aware the result may be null if getItem has not yet been called for that page i.e. its not been created yet.

/**
 * @return may return null if the fragment has not been instantiated yet for that position - this depends on if the fragment has been viewed
 * yet OR is a sibling covered by {@link android.support.v4.view.ViewPager#setOffscreenPageLimit(int)}. Can use this to call methods on
 * the current positions fragment.
 */
public @Nullable Fragment getFragmentForPosition(int position)
{
    String tag = makeFragmentName(mViewPager.getId(), getItemId(position));
    Fragment fragment = getSupportFragmentManager().findFragmentByTag(tag);
    return fragment;
}

This is a simple solution and solves the issues in the other two solutions found everywhere on the web

Answer 4

Add next methods to your FragmentPagerAdapter:

public Fragment getActiveFragment(ViewPager container, int position) {
String name = makeFragmentName(container.getId(), position);
return  mFragmentManager.findFragmentByTag(name);
}

private static String makeFragmentName(int viewId, int index) {
    return "android:switcher:" + viewId + ":" + index;
}

getActiveFragment(0) has to work.

Here is the solution implemented into ViewPager https://gist.github.com/jacek-marchwicki/d6320ba9a910c514424d. If something fail you will see good crash log.

Answer 5

Easy way to iterate over fragments in fragment manager. Find viewpager, that has section position argument, placed in public static PlaceholderFragment newInstance(int sectionNumber).

public PlaceholderFragment getFragmentByPosition(Integer pos){
    for(Fragment f:getChildFragmentManager().getFragments()){
        if(f.getId()==R.id.viewpager && f.getArguments().getInt("SECTNUM") - 1 == pos) {
            return (PlaceholderFragment) f;
        }
    }
    return null;
}

Answer 6

I handled it by first making a list of all the fragments (List<Fragment> fragments;) that I was going to use then added them to the pager making it easier to handle the currently viewed fragment.

So:

@Override
onCreate(){
    //initialise the list of fragments
    fragments = new Vector<Fragment>();

    //fill up the list with out fragments
    fragments.add(Fragment.instantiate(this, MainFragment.class.getName()));
    fragments.add(Fragment.instantiate(this, MenuFragment.class.getName()));
    fragments.add(Fragment.instantiate(this, StoresFragment.class.getName()));
    fragments.add(Fragment.instantiate(this, AboutFragment.class.getName()));
    fragments.add(Fragment.instantiate(this, ContactFragment.class.getName()));


    //Set up the pager
    pager = (ViewPager)findViewById(R.id.pager);
    pager.setAdapter(new MyFragmentPagerAdapter(getSupportFragmentManager(), fragments));
    pager.setOffscreenPageLimit(4);
}

so then this can be called:

public Fragment getFragment(ViewPager pager){   
    Fragment theFragment = fragments.get(pager.getCurrentItem());
    return theFragment;
}

so then i could chuck it in an if statement that would only run if it was on the correct fragment

Fragment tempFragment = getFragment();
if(tempFragment == MyFragmentNo2.class){
    MyFragmentNo2 theFrag = (MyFragmentNo2) tempFragment;
    //then you can do whatever with the fragment
    theFrag.costomFunction();
}

but thats just my hack and slash approach but it worked for me, I use it do do relevent changes to my currently displayed fragment when the back button is pushed.