Saving Fragment state in ViewPager

Question

I tried out the sample code from the API and it didn't really work so I implemented my own:

FragmentPagerSupport

public class FragmentPagerSupport extends FragmentActivity {

static final int NUM_ITEMS = 10;
MyAdapter mAdapter;
ViewPager mPager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    mAdapter = new MyAdapter(getSupportFragmentManager());
    Log.i("Pager", "mAdapter = " + mAdapter.toString());

    mPager = (ViewPager)findViewById(R.id.pager);
    if (mPager == null)
        Log.i("Pager", "mPager = null");
    else 
        Log.i("Pager", "mPager = " + mPager.toString());

    Log.i("Pager", "Setting Pager Adapter");
    mPager.setAdapter(mAdapter);
}

public static class MyAdapter extends FragmentPagerAdapter {
    public MyAdapter(FragmentManager fm) {
        super(fm);
        Log.i("Pager", "MyAdapter constructor");
    }

    @Override
    public int getCount() {
        Log.i("Pager", "MyAdapter.getCount()");
        return NUM_ITEMS;            
    }

    @Override
    public Fragment getItem(int position) {
        Log.i("Pager", "MyAdapter.getItem()");

        return TestFragment.newInstance(position);
    }
}

public static class TestFragment extends Fragment {

    public static TestFragment newInstance(int position) {
        Log.i("Pager", "TestFragment.newInstance()");

        TestFragment fragment = new TestFragment();

        Bundle args = new Bundle();
        args.putInt("position", position);
        fragment.setArguments(args);

        return fragment;
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        Log.i("Pager", "TestFragment.onCreateView()");

        LinearLayout layout = (LinearLayout)inflater.inflate(R.layout.fragment_item, null);
        int position = getArguments().getInt("position");

        TextView tv = (TextView)layout.findViewById(R.id.text);
        tv.setText("Fragment # " + position);
        tv.setTextColor(Color.WHITE);
        tv.setTextSize(30);

        return layout;
    }

}

}

main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <android.support.v4.view.ViewPager
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    />
</LinearLayout>

fragment_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <TextView  
    android:id="@+id/text"
    android:layout_width="match_parent" 
    android:layout_height="match_parent"
    android:gravity="center" 
    android:text="@string/hello"
    />
</LinearLayout>

My question is, instead of creating a new Fragment instance each time the user swipes left and right, how do I save a fragment's state (to some data structure) and then restore it?

The API demo doesn't seem to have any state information saving code at all.

Answer 1

Use ViewPager.setOffscreenPageLimit() in FragmentActivity

ViewPager pager = (ViewPager) findViewById(R.id.viewPager);
pager.setOffscreenPageLimit(2);

See more at How do I tell my custom FragmentPagerAdapter to stop destroying my fragments?

Answer 2

just override this method in FragmentpagerAdapter

@Override
        public void destroyItem(ViewGroup container, int position, Object object) {
            // TODO Auto-generated method stub
            super.destroyItem(ViewGroup container, int position, Object object);
        }

remove super.destroyItem(ViewGroup container, int position, Object object);

from your code

Answer 3

Update: In the getItem of MyAdapter you return a new instance of your Fragment. Android will not call getItem on every swipe. It will only call getItem to get a new instance, but will reuse the existing instances as long as they are availible.

About the state part of the Demo. I can't help you. I think the normal techniques for restoring state in Fragements/Activities apply here, so nothing special when loading it in a ViewPager (but I might be wrong).

Answer 4

If you override

public void destroyItem(View container, int position, Object object)

without calling super, the fragment will not be destroyed and will be reused.

Answer 5

The adapter should be extended from FragmentStatePagerAdapter instead of FragmentPageAdapter and the adapter will keep reference from position to fragment item. On the activity or fragment parent, set listener OnPageChangeListener for PageIndicator to detect the position of fragment item has been activated and update related data's state.

About the data state of fragment item, I think should save/restore state from Activity or Fragement parent.

The adapter can add some code as following:

public static class MyAdapter extends FragmentStatePagerAdapter {
    private SparseArray<TestFragment> mPageReferenceMap 
                              = new SparseArray<TestFragment>();
    ...

    @Override 
    public Object instantiateItem(ViewGroup viewGroup, int position) {
        Object obj = super.instantiateItem(viewGroup, position);

        //Add the reference when fragment has been create or restore
        if (obj instanceof TestFragment) {
                TestFragment f= (TestFragment)obj;
                mPageReferenceMap.put(position, f);
        }

        return obj;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        //Remove the reference when destroy it
        mPageReferenceMap.remove(position);

        super.destroyItem(container, position, object);
    }

    public TestFragment getFragment(int key) {

        return mPageReferenceMap.get(key);
    }
    ...
}

Hope this help.

Answer 6

I tripped on some similar ground... I wanted to keep my fragments saved within my adapter to avoid useless reloading since I only had 5 fragments in the FragmentPagerAdapter .

I basically had to create an array and override the getItem(position) method:

private StoryListFragment[] fragmentsArray
        = new StoryListFragment[getCount()];

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


@Override
public Fragment getItem(int position) {

    if(fragmentsArray[position]!=null)
        return fragmentsArray[position];
    StoryListFragment storyListFragment = null;
    switch(position){
        case(0):
            storyListFragment = StoryListFragment.newInstance(StoriesBank.NEWS);
            break;
        case(1):
            storyListFragment = StoryListFragment.newInstance(StoriesBank.FEATURES);
            break;
        case(2):
             storyListFragment=  StoryListFragment.newInstance(StoriesBank.ARTS);
            break;
    }

    fragmentsArray[position] =  storyListFragment;
    return storyListFragment;
}

@Override
public Object instantiateItem(ViewGroup container, int position) {
    StoryListFragment fragment = (StoryListFragment) super.instantiateItem(container, position);
    fragmentsArray[position] = fragment;
    return fragment;
}