Fragments and ViewPager about alligator HOT 3 CLOSED

/**
 * Fragment state pager adapter that gets the fragments from NavigationFactory, the fragments
 * to be loaded are defined in a Screen list.
 */
public class ScreenFragmentPagerAdater extends FragmentPagerAdapter {

    protected final List<Screen> screens;
    private final NavigationFactory navigationFactory;
    private final Map<Screen, WeakReference<Fragment>> fragmentMap;

    public ScreenFragmentPagerAdater(FragmentManager fm, NavigationFactory navigationFactory) {
        super(fm);
        Preconditions.checkNotNull(navigationFactory);
        this.navigationFactory = navigationFactory;
        fragmentMap = new HashMap<>();
        this.screens = new ArrayList<>();
    }

    public void setScreens(@NonNull List<Screen> screens) {
        Preconditions.checkNotNull(screens);
        this.screens.clear();
        this.screens.addAll(screens);
        notifyDataSetChanged();
    }

    @NonNull
    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Fragment fragment = (Fragment) super.instantiateItem(container, position);
        fragmentMap.put(screens.get(position), new WeakReference<>(fragment));
        return fragment;
    }

    @Override
    public Fragment getItem(int position) {
        try {
            return getOrCreateFragment(screens.get(position));
        } catch (ScreenSwitchingException e) {
            throw new RuntimeException(e);
        }
    }

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

    private Fragment getOrCreateFragment(Screen screen) throws ScreenSwitchingException {
        Fragment fragment = fragmentMap.get(screen) != null ? fragmentMap.get(screen).get() : null;
        if (fragment == null) {
            fragment = navigationFactory.createFragment(screen);
            try {
                navigationFactory.getScreen(fragment);  // Check that the screen has a valid screen getting function
            } catch (Exception e) {
                throw new ScreenSwitchingException(e.getMessage());
            }
            fragmentMap.put(screen, new WeakReference<>(fragment));
        }
        return fragment;
    }

}