life cycle callbacks of fragments are not called accordingly

Issue

I have a main activity with view pager and three action tabs "fragment". and i implemented the life cycle callbacks for each of the main activity and the three fragments. and in each of the life cycle call back i placed a log statements indicates which life cycle callback being called to know how the action bars with view pager behaves. at run time i perceived a strange behaviour which i cant understand or attribute it to any reason.

first behaviour

when the App fisrt starts I receive:

02-08 15:16:14.771 32243-32243/com.example.com.vpager_00 W/MainActivity: onCreate()
02-08 15:16:14.901 32243-32243/com.example.com.vpager_00 W/MainActivity: onStart()
02-08 15:16:14.901 32243-32243/com.example.com.vpager_00 W/MainActivity: onResume()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onAttach()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onCreate()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onCreateView()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onStart()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_1: onResume()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onAttach()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onCreate()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onCreateView()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onStart()
02-08 15:16:14.941 32243-32243/com.example.com.vpager_00 W/Frag_2: onResume()

and i cant find any reason why only the life cycle callbacks of the mainactivty, frag_1 and frag_2 were called BUT NOT frag_3? any explaination?

2nd behaviour

occured when i touched tab3 "frag_3", i received:

02-08 15:16:36.031 32243-32243/com.example.com.vpager_00 D/ViewRootImpl: ViewPostImeInputStage ACTION_DOWN
02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onAttach()
02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onCreate()
02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onCreateView()
02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onStart()
02-08 15:16:36.121 32243-32243/com.example.com.vpager_00 W/Frag_3: onResume()
02-08 15:16:36.461 32243-32243/com.example.com.vpager_00 W/Frag_1: onPause()
02-08 15:16:36.461 32243-32243/com.example.com.vpager_00 W/Frag_1: onStop()
02-08 15:16:36.471 32243-32243/com.example.com.vpager_00 W/Frag_1: onDestroy()

i think being the life cycle callbackes were called this is understandable, but why ONLY the life cycle callbacks of frag_1 also called, i think, beside the callbacks of frag_3 and frag_1, aslo frag_2 callbacks must have been called because frag_2 was in its onResume() state lately.

any explaination?

mainactivity

public class MainActivity extends AppCompatActivity {

private final String TAG = this.getClass().getSimpleName();

private Toolbar mTB = null;
private TabLayout mTL = null;
private ViewPager mVP = null;
private VPagerAdapter mVPAdapter = null;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.w(TAG, "onCreate()");

    this.initViews(R.layout.act_main);
    this.initObjs();
}

private void initObjs() {
    this.mVPAdapter = new VPagerAdapter(getSupportFragmentManager(), this.mTL.getTabCount());
    this.mVP.setAdapter(this.mVPAdapter);

    this.mVP.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(this.mTL));
    this.mTL.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            mVP.setCurrentItem(tab.getPosition());
        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) {

        }

        @Override
        public void onTabReselected(TabLayout.Tab tab) {

        }
    });
}

private void initViews(int rootView) {
    setContentView(rootView);

    this.mTB = (Toolbar) findViewById(R.id.toolbar);
    this.mTL = (TabLayout) findViewById(R.id.tab_layout);
    this.mVP = (ViewPager) findViewById(R.id.pager);

    setSupportActionBar(this.mTB);
    this.mTL.addTab(this.mTL.newTab().setText("Tab 1"));
    this.mTL.addTab(this.mTL.newTab().setText("Tab 2"));
    this.mTL.addTab(this.mTL.newTab().setText("Tab 3"));
    this.mTL.setTabGravity(TabLayout.GRAVITY_FILL);
}

@Override
protected void onStart() {
    super.onStart();
    Log.w(TAG, "onStart()");
}

@Override
protected void onResume() {
    super.onResume();
    Log.w(TAG, "onResume()");
}

@Override
protected void onPause() {
    super.onPause();
    Log.w(TAG, "onPause()");
}

@Override
protected void onStop() {
    super.onStop();
    Log.w(TAG, "onStop()");
}

@Override
protected void onDestroy() {
    super.onDestroy();
    Log.w(TAG, "onDestroy()");
}

}

frag_1

public class Frag_1 extends Fragment {

private final String TAG = this.getClass().getSimpleName();

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    Log.w(TAG, "onAttach()");
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.w(TAG, "onCreate()");
}

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    Log.w(TAG, "onCreateView()");

    return inflater.inflate(R.layout.frag_1, container, false);
}

@Override
public void onStart() {
    super.onStart();
    Log.w(TAG, "onStart()");
}

@Override
public void onResume() {
    super.onResume();
    Log.w(TAG, "onResume()");
}

@Override
public void onPause() {
    super.onPause();
    Log.w(TAG, "onPause()");
}

@Override
public void onStop() {
    super.onStop();
    Log.w(TAG, "onStop()");
}

@Override
public void onDestroy() {
    super.onDestroy();
    Log.w(TAG, "onDestroy()");
}

}

frag_2:

public class Frag_2 extends Fragment {

private final String TAG = this.getClass().getSimpleName();

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    Log.w(TAG, "onAttach()");
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.w(TAG, "onCreate()");
}

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    Log.w(TAG, "onCreateView()");

    return inflater.inflate(R.layout.frag_2, container, false);
}

@Override
public void onStart() {
    super.onStart();
    Log.w(TAG, "onStart()");
}

@Override
public void onResume() {
    super.onResume();
    Log.w(TAG, "onResume()");
}

@Override
public void onPause() {
    super.onPause();
    Log.w(TAG, "onPause()");
}

@Override
public void onStop() {
    super.onStop();
    Log.w(TAG, "onStop()");
}

@Override
public void onDestroy() {
    super.onDestroy();
    Log.w(TAG, "onDestroy()");
}

}

frag_3:

public class Frag_3 extends Fragment {

private final String TAG = this.getClass().getSimpleName();

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    Log.w(TAG, "onAttach()");
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.w(TAG, "onCreate()");
}

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    Log.w(TAG, "onCreateView()");

    return inflater.inflate(R.layout.frag_3, container, false);
}

@Override
public void onStart() {
    super.onStart();
    Log.w(TAG, "onStart()");
}

@Override
public void onResume() {
    super.onResume();
    Log.w(TAG, "onResume()");
}

@Override
public void onPause() {
    super.onPause();
    Log.w(TAG, "onPause()");
}

@Override
public void onStop() {
    super.onStop();
    Log.w(TAG, "onStop()");
}

@Override
public void onDestroy() {
    super.onDestroy();
    Log.w(TAG, "onDestroy()");
}

}

Solution

In the ViewPager, fragments are being created ahead of time.

That means, that your Frag_2 is not visible yet (because Frag_1 covers the entire screen), but it still creates the view beforehand, so scrolling to Frag_2 will be smooth.

By default, the ViewPager preloads one fragment on each side. So if you would set the start position of the ViewPager to Frag_2, it would load Frag_1 and Frag_2 as well, because they are the neighbors.

You'll notice that Frag_3 will be created when you swipe to Frag_2

You can increase the number of preloaded fragments by calling ViewPager.setOffscreenPageLimit(int limit).

Note that the min. number is 1, so this can not be disabled.

Answered By - Daniel Zolnai