TabLayout scrolls to unkown position after calling notifyDataSetChanged on PagerAdapter

Question

I have sample project with TabLayout and PagerAdapter. Strange things happens with TabLayout when I call pagerAdapter.notifyDataSetChanged(); after tabLayout.setupWithViewPager(viewPager);

TabLayout is scrolling to unknown x position so the current tab is not visible. However if I scroll to left to expecting tab, this tab has indicator.

What is going on? Could anyone help me? I have spent on it too many time.

Below the code.

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Get the ViewPager and set it's PagerAdapter so that it can display items
        ViewPager viewPager = (ViewPager) findViewById(R.id.viewpager);
        final SampleFragmentPagerAdapter pagerAdapter = new SampleFragmentPagerAdapter(getSupportFragmentManager(), MainActivity.this);
        viewPager.setAdapter(pagerAdapter);

        // Give the TabLayout the ViewPager
        TabLayout tabLayout = (TabLayout) findViewById(R.id.sliding_tabs);
        tabLayout.setupWithViewPager(viewPager);

        findViewById(R.id.fab).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                pagerAdapter.notifyDataSetChanged();
            }
        });
    }

}

I tested on nexus emulators and nexus real devices (api 21+)

Gradle settings:

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.2"
    defaultConfig {
        applicationId "xx.xxx.myapplication4"
        minSdkVersion 21
        targetSdkVersion 25
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

Link to reported issue and ready to test project as attachment here

Answer 1

Every time setup view pager with the use of setupWithViewPager might be costly. Because notifyDataSetChanged() on your ViewPager Adapter causing TabLayout to redraw its all views. So for redrawing, TabLayout remove all its associated Views and re add them.

Please check below thread execution steps which happen after notifyDataSetChanged on pager adapter.

  at android.support.design.widget.TabLayout.removeAllTabs(TabLayout.java:654)
  at android.support.design.widget.TabLayout.populateFromPagerAdapter(TabLayout.java:904)
  at android.support.design.widget.TabLayout$PagerAdapterObserver.onChanged(TabLayout.java:2211)
  at android.database.DataSetObservable.notifyChanged(DataSetObservable.java:37)
  - locked <0x129f> (a java.util.ArrayList)
  at android.support.v4.view.PagerAdapter.notifyDataSetChanged(PagerAdapter.java:287)
  at com.sample.testtablayout.MainActivity$1.onClick(MainActivity.java:45)

According to thread execution -

Clicked on Action Button > Pager Adapter Notified for data set changed > TabLayout got notification of data change through observers > It try to populate new data from adapter > Removed all tabs.

Below is a code of TabLayout class, from which you can check whenever all tabs are going to remove then last selected tab is lost(Intentionally marked null).

/**
 * Remove all tabs from the action bar and deselect the current tab.
 */
public void removeAllTabs() {
    // Remove all the views
    for (int i = mTabStrip.getChildCount() - 1; i >= 0; i--) {
        removeTabViewAt(i);
    }

    for (final Iterator<Tab> i = mTabs.iterator(); i.hasNext();) {
        final Tab tab = i.next();
        i.remove();
        tab.reset();
        sTabPool.release(tab);
    }

    mSelectedTab = null; // Thats a cause for your issue.
}

To retain last selected tab I have created my own CustomTabLayout class and retained last selected position.

public class RetainableTabLayout extends TabLayout {

   /*
   * Variable to store invalid position.
   */
   private static final int INVALID_TAB_POS = -1;

   /*
   * Variable to store last selected position, init it with invalid tab position.
   */
   private int mLastSelectedTabPosition = INVALID_TAB_POS;

   public RetainableTabLayout(Context context) {
       super(context);
   }

   public RetainableTabLayout(Context context, AttributeSet attrs) {
       super(context, attrs);
   }

   public RetainableTabLayout(Context context, AttributeSet attrs, int defStyleAttr) {
      super(context, attrs, defStyleAttr);
  }

   @Override
    public void removeAllTabs() {
       // Retain last selected position before removing all tabs
       mLastSelectedTabPosition = getSelectedTabPosition();
       super.removeAllTabs();
   }

   @Override
   public int getSelectedTabPosition() {
       // Override selected tab position to return your last selected tab position
       final int selectedTabPositionAtParent = super.getSelectedTabPosition();
       return selectedTabPositionAtParent == INVALID_TAB_POS ? 
              mLastSelectedTabPosition : selectedTabPositionAtParent;
   }
}

At the end make sure to reselect your tab after recreation of your TabLayout.

findViewById(R.id.fab).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            pagerAdapter.notifyDataSetChanged();
            // At the end make sure to reselect your last item.
            new Handler().postDelayed(
                    new Runnable() {
                        @Override
                        public void run() {
                            final TabLayout.Tab selectedTab = tabLayout.getTabAt(
                                 tabLayout.getSelectedTabPosition());
                            if (selectedTab != null) {
                                selectedTab.select();
                            }
                        }
                    }, 100);
        }
    });

This issue will resolve your problem but what I believe is TabLayout must have to retain last selected position in case of data set change. This is what I understand, any comments or more understanding is welcome.

Answer 2

You can call:

setupWithViewPager(@Nullable final ViewPager viewPager, boolean autoRefresh)

and set param autoRefresh to false.

See also: TabLayout(ViewPager,boolean)

Answer 3

I just added notifyDataSetChanged method inside of TabLayout implementation of Rasi.

It works for me.

public class CustomTabLayout extends TabLayout {

/*
   * Variable to store invalid position.
   */
private static final int INVALID_TAB_POS = -1;

/*
* Variable to store last selected position, init it with invalid tab position.
*/
private int mLastSelectedTabPosition = INVALID_TAB_POS;

public CustomTabLayout(Context context) {
    super(context);
}

public CustomTabLayout(Context context, AttributeSet attrs) {
    super(context, attrs);
}

public CustomTabLayout(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
}

@Override
public void removeAllTabs() {
    // Retain last selected position before removing all tabs
    mLastSelectedTabPosition = getSelectedTabPosition();
    super.removeAllTabs();
}

@Override
public int getSelectedTabPosition() {
    // Override selected tab position to return your last selected tab position
    final int selectedTabPositionAtParent = super.getSelectedTabPosition();
    return selectedTabPositionAtParent == INVALID_TAB_POS ?
            mLastSelectedTabPosition : selectedTabPositionAtParent;
}

public void notifyDataSetChanged() {
    post(new Runnable() {
        @Override
        public void run() {
            TabLayout.Tab selectedTab = getTabAt(getSelectedTabPosition());
            if (selectedTab != null) {
                selectedTab.select();
            }
        }
    });
}
}

and to notify call

mPagerAdapter.notifyDataSetChanged();
mCustomTabLayout.notifyDataSetChanged();