android - 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?