Restoring fragment state when changing fragments through bottom navigation bar

Question

I have bottom navigation bar on click of item in navigation bar i am replacing fragments. I have 3 fragments A,B,C so on click of b item B fragment is loaded and in B i am calling 3-4 APIs. So now if i go to C and then again come to B a new instance of B Fragment is created and again those APIs are called how can i save the fragment instance state and not call APIs again while changing fragments. This is my code.

mBottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            int id = item.getItemId();
            Fragment currentLoaded = fgMan.findFragmentById(R.id.container_body);
            switch (id) {
                case R.id.nearby_fragment:
                    if (!(currentLoaded instanceof SpotFeedMapFragment)) {
                        removeScroll();
                        mNearByFragment = fgMan.findFragmentByTag(NEARBY_FRAGMENT_TAG) != null ? fgMan.findFragmentByTag(NEARBY_FRAGMENT_TAG) : mNearByFragment;
                        fgMan.beginTransaction().setCustomAnimations(R.anim.abc_fade_in, R.anim.abc_fade_out);
                        fgMan.beginTransaction().replace(R.id.container_body, mNearByFragment, NEARBY_FRAGMENT_TAG).commit();
                        fgMan.executePendingTransactions();
                        getSupportActionBar().setTitle(getString(R.string.nearby_fragment));
                    }
                    break;
                case R.id.route_fragment:
                    if (!(currentLoaded instanceof BusLocationsFragment)) {
                        if (!inParent) {
                            mRl.removeView(fixLayout);
                            p.addRule(RelativeLayout.BELOW, toolbar.getId());
                            scrollView.setLayoutParams(p);
                            scrollView.addView(fixLayout);
                            mRl.addView(scrollView);
                            inParent = true;
                        }
                        //mFragment = new BusLocationsFragment();
                        mBusLocFragment = fgMan.findFragmentByTag(BUS_LOC_FRAGMENT_TAG) != null ? fgMan.findFragmentByTag(BUS_LOC_FRAGMENT_TAG) : mBusLocFragment;
                        fgMan.beginTransaction().setCustomAnimations(R.anim.abc_fade_in, R.anim.abc_fade_out);
                        fgMan.beginTransaction().replace(R.id.container_body, mBusLocFragment, BUS_LOC_FRAGMENT_TAG).commit();
                        fgMan.executePendingTransactions();
                        getSupportActionBar().setTitle(getString(R.string.app_name));
                    }
                    break;
                case R.id.newsfeed_activity:
                    if (!(currentLoaded instanceof NewsFeedActivity)) {
                        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.LOLLIPOP_MR1) {
                            removeScroll();
                        }
                        mNewsFeedFragment = fgMan.findFragmentByTag(NEWSFEED_FRAGMENT_TAG) != null ? fgMan.findFragmentByTag(NEWSFEED_FRAGMENT_TAG) : mNewsFeedFragment;
                        fgMan.beginTransaction().setCustomAnimations(R.anim.abc_fade_in, R.anim.abc_fade_out);
                        fgMan.beginTransaction().replace(R.id.container_body, mNewsFeedFragment, NEWSFEED_FRAGMENT_TAG).commit();
                        fgMan.executePendingTransactions();
                        getSupportActionBar().setTitle(getString(R.string.news));
                    }
                    break;
            }
            return true;
        }
    });

I have already initialized fragments member variables above in onCreateof MainActivity

Answer 1

You should use a FragmentPagerAdapter to initiate the fragments so when you want to switch in between them, the state of the fragments will be saved.

CutomViewPager viewPager = (CustomViewPager) findViewById(R.id.viewpager1);
ViewPagerAdapter adapter = new ViewPagerAdapter (MainActivity.this.getSupportFragmentManager());
adapter.addFragment(new SpotFeedMapFragment(), "title");
adapter.addFragment(new BusLocationsFragment(), "title");
adapter.addFragment(new NewsFeedActivity(), "title");
viewPager.setAdapter(adapter);

then in the bottom navigation selected you can set fragment by simple command

viewPager.setCurrentItem(n);

my viewpager class is as follows:

public class CustomViewPager extends ViewPager {

private boolean isPagingEnabled;

public CustomViewPager(Context context) {
    super(context);
    this.isPagingEnabled = true;
}

public CustomViewPager(Context context, AttributeSet attrs) {
    super(context, attrs);
    this.isPagingEnabled = true;
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    return this.isPagingEnabled && super.onTouchEvent(event);
}

//for samsung phones to prevent tab switching keys to show on keyboard
@Override
public boolean executeKeyEvent(KeyEvent event) {
    return isPagingEnabled && super.executeKeyEvent(event);
}

@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
    return this.isPagingEnabled && super.onInterceptTouchEvent(event);
}

public void setPagingEnabled(boolean enabled) {
    this.isPagingEnabled = enabled;
}
}

in the xml instead of a empty layout for fragemnt u need:

<com.package.util.CustomViewPager
    android:id="@+id/viewpager1"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

Code for custom FragmentPagerAdapter:

private class ViewPagerAdapter extends FragmentPagerAdapter {
    private final SparseArray<WeakReference<Fragment>> instantiatedFragments = new SparseArray<>();
    private final List<Fragment> mFragmentList = new ArrayList<>();
    private final List<String> mFragmentTitleList = new ArrayList<>();

    ViewPagerAdapter(FragmentManager manager) {
        super(manager);
    }

    @Override
    public Fragment getItem(int position) {
        return mFragmentList.get(position);
    }

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

    void addFragment(Fragment fragment, String title) {
        mFragmentList.add(fragment);
        mFragmentTitleList.add(title);
    }

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

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        instantiatedFragments.remove(position);
        super.destroyItem(container, position, object);
    }

    @Nullable
    Fragment getFragment(final int position) {
        final WeakReference<Fragment> wr = instantiatedFragments.get(position);
        if (wr != null) {
            return wr.get();
        } else {
            return null;
        }
    }

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