Android FragmentStatePagerAdapter

Question

I'm working with a FragmentStatePagerAdapter using this example.

The MyAdapter class is implemented as follows:

public static class MyAdapter extends FragmentStatePagerAdapter {         public MyAdapter(FragmentManager fm) {             super(fm);         }          @Override         public int getCount() {             return NUM_ITEMS;         }          @Override         public Fragment getItem(int position) {             return ArrayListFragment.newInstance(position);         }     } 

The ListFragment class includes the following method to create a new instance:

    /**      * Create a new instance of CountingFragment, providing "num"      * as an argument.      */     static ArrayListFragment newInstance(int num) {         ArrayListFragment f = new ArrayListFragment();          // Supply num input as an argument.         Bundle args = new Bundle();         args.putInt("num", num);         f.setArguments(args);          return f;     } 

When I create a new fragment state pager adapter in my activity, getItem is called, which in turn calls the newInstance method in the ListFragment class. This is great when I want to create a new fragment.

But it's not clear to me how to modify getItem (if even needed) to get the fragment object when it already exists and the user pages from, for example, page 2 to page 1. I'd like my Activity to retrieve that existing, previously created fragment so that it can run an inner class AsyncMethod, which resides in the fragment class.

Answer 1

I think the solution is much simpler than the answers provided.

If you take a look at the source of FragmentStatePagerAdapter.instantiateItem(), you'll notice that instantiateItem() handles this logic for you, and thus your implementation of getItem() should always return a new instance.

So in order to return an existing Fragment, simply do (assuming you're calling from ViewPager):

Fragment f = (Fragment) getAdapter().instantiateItem(this, getCurrentItem()); 

Answer 2

While @imiric's solution is very concise, it still bothers me that it requires knowledge of the implementation of the class. My solution involves adding the following to your adapter, which should behave nicely with a FragmentStatePagerAdapter possibly destroying fragments:

    private SparseArray<WeakReference<Fragment>> mFragments = new SparseArray<>();      @Override     public Object instantiateItem(ViewGroup container, int position) {         Fragment f = (Fragment) super.instantiateItem(container, position);         mFragments.put(position, new WeakReference<>(f));  // Remember what fragment was in position         return f;     }      @Override     public void destroyItem(ViewGroup container, int position, Object object) {         super.destroyItem(container, position, object);         mFragments.remove(position);     }      public Fragment getFragment(int position) {         WeakReference<Fragment> ref = mFragments.get(position);         Fragment f = ref != null ? ref.get() : null;         if (f == null) {             Log.d(TAG, "fragment for " + position + " is null!");         }         return f;     }