[android] ViewPager and fragments — what's the right way to store fragment's state?

To get the fragments after orientation change you have to use the .getTag().

    getSupportFragmentManager().findFragmentByTag("android:switcher:" + viewPagerId + ":" + positionOfItemInViewPager)

For a bit more handling i wrote my own ArrayList for my PageAdapter to get the fragment by viewPagerId and the FragmentClass at any Position:

public class MyPageAdapter extends FragmentPagerAdapter implements Serializable {
private final String logTAG = MyPageAdapter.class.getName() + ".";

private ArrayList<MyPageBuilder> fragmentPages;

public MyPageAdapter(FragmentManager fm, ArrayList<MyPageBuilder> fragments) {
    super(fm);
    fragmentPages = fragments;
}

@Override
public Fragment getItem(int position) {
    return this.fragmentPages.get(position).getFragment();
}

@Override
public CharSequence getPageTitle(int position) {
    return this.fragmentPages.get(position).getPageTitle();
}

@Override
public int getCount() {
    return this.fragmentPages.size();
}


public int getItemPosition(Object object) {
    //benötigt, damit bei notifyDataSetChanged alle Fragemnts refrehsed werden

    Log.d(logTAG, object.getClass().getName());
    return POSITION_NONE;
}

public Fragment getFragment(int position) {
    return getItem(position);
}

public String getTag(int position, int viewPagerId) {
    //getSupportFragmentManager().findFragmentByTag("android:switcher:" + R.id.shares_detail_activity_viewpager + ":" + myViewPager.getCurrentItem())

    return "android:switcher:" + viewPagerId + ":" + position;
}

public MyPageBuilder getPageBuilder(String pageTitle, int icon, int selectedIcon, Fragment frag) {
    return new MyPageBuilder(pageTitle, icon, selectedIcon, frag);
}


public static class MyPageBuilder {

    private Fragment fragment;

    public Fragment getFragment() {
        return fragment;
    }

    public void setFragment(Fragment fragment) {
        this.fragment = fragment;
    }

    private String pageTitle;

    public String getPageTitle() {
        return pageTitle;
    }

    public void setPageTitle(String pageTitle) {
        this.pageTitle = pageTitle;
    }

    private int icon;

    public int getIconUnselected() {
        return icon;
    }

    public void setIconUnselected(int iconUnselected) {
        this.icon = iconUnselected;
    }

    private int iconSelected;

    public int getIconSelected() {
        return iconSelected;
    }

    public void setIconSelected(int iconSelected) {
        this.iconSelected = iconSelected;
    }

    public MyPageBuilder(String pageTitle, int icon, int selectedIcon, Fragment frag) {
        this.pageTitle = pageTitle;
        this.icon = icon;
        this.iconSelected = selectedIcon;
        this.fragment = frag;
    }
}

public static class MyPageArrayList extends ArrayList<MyPageBuilder> {
    private final String logTAG = MyPageArrayList.class.getName() + ".";

    public MyPageBuilder get(Class cls) {
        // Fragment über FragmentClass holen
        for (MyPageBuilder item : this) {
            if (item.fragment.getClass().getName().equalsIgnoreCase(cls.getName())) {
                return super.get(indexOf(item));
            }
        }
        return null;
    }

    public String getTag(int viewPagerId, Class cls) {
        // Tag des Fragment unabhängig vom State z.B. nach bei Orientation change
        for (MyPageBuilder item : this) {
            if (item.fragment.getClass().getName().equalsIgnoreCase(cls.getName())) {
                return "android:switcher:" + viewPagerId + ":" + indexOf(item);
            }
        }
        return null;
    }
}

So just create a MyPageArrayList with the fragments:

    myFragPages = new MyPageAdapter.MyPageArrayList();

    myFragPages.add(new MyPageAdapter.MyPageBuilder(
            getString(R.string.widget_config_data_frag),
            R.drawable.ic_sd_storage_24dp,
            R.drawable.ic_sd_storage_selected_24dp,
            new WidgetDataFrag()));

    myFragPages.add(new MyPageAdapter.MyPageBuilder(
            getString(R.string.widget_config_color_frag),
            R.drawable.ic_color_24dp,
            R.drawable.ic_color_selected_24dp,
            new WidgetColorFrag()));

    myFragPages.add(new MyPageAdapter.MyPageBuilder(
            getString(R.string.widget_config_textsize_frag),
            R.drawable.ic_settings_widget_24dp,
            R.drawable.ic_settings_selected_24dp,
            new WidgetTextSizeFrag()));

and add them to the viewPager:

    mAdapter = new MyPageAdapter(getSupportFragmentManager(), myFragPages);
    myViewPager.setAdapter(mAdapter);

after this you can get after orientation change the correct fragment by using its class:

        WidgetDataFrag dataFragment = (WidgetDataFrag) getSupportFragmentManager()
            .findFragmentByTag(myFragPages.getTag(myViewPager.getId(), WidgetDataFrag.class));