Sincronizar ViewPager con BottomNavigationView

readme.md

Sincronizar un ViewPager con BottomNavigationView

Establecer los controles

BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
mViewPager = (ViewPager) findViewById(R.id.view_pager);

Asignar el listener del BottomNavigationView

navigation.setOnNavigationItemSelectedListener(this);

Cargar el adaptador y asignarlo al ViewPager

DemoFragmentAdapter adapterViewPager = new DemoFragmentAdapter(getSupportFragmentManager());
mViewPager.setAdapter(adapterViewPager);

Sincronizar el ViewPager con el BottomNavigationView se hace con el OnPageChangeListener

mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
    public void onPageScrollStateChanged(int state) {}
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {}

    public void onPageSelected(int position) {
        navigation.getMenu().getItem(position).setChecked(true);
    }
});

Para seleccionar un item de la BottomNavigationView mediante su posición navigation.getMenu().getItem(position).setChecked(true);

DemoFragmentAdapter.java

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.view.ViewGroup;

//FragmentStatePagerAdapter
//FragmentPagerAdapter

public class DemoFragmentAdapter extends FragmentPagerAdapter {
    public DemoFragmentAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                return FirstFragment.newInstance();
            case 1:
                return SecondFragment.newInstance();
            case 2:
                return ThirdFragment.newInstance();
            default:
                return null;
        }
    }

    @Override
    public int getCount() {
        return 3;
    }

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


}

FirstFragment.java

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.CheckBox;
import android.widget.CompoundButton;


public class FirstFragment extends Fragment {
    private static final String TAG = FirstFragment.class.getSimpleName();

    private final String SAVED_BUNDLE_TAG = "saved_bundle";
    private boolean isChecked;

    public static FirstFragment newInstance() {
        FirstFragment fragment = new FirstFragment();
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        Log.i(TAG, "onSaveInstanceState: ");
        outState.putBoolean(SAVED_BUNDLE_TAG, isChecked);
        super.onViewStateRestored(outState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        Log.i(TAG, "onCreateView: FirstFragment");
        View view = inflater.inflate(R.layout.fragment_one, container, false);
        final CheckBox cbCheckBox = (CheckBox) view.findViewById(R.id.checkBox);

        if (savedInstanceState != null) {
            Log.d(TAG, "savedInstanceState: true");
            isChecked = savedInstanceState.getBoolean(SAVED_BUNDLE_TAG, false);
            cbCheckBox.setChecked(isChecked);
        }
        cbCheckBox.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton compoundButton, boolean b) {
                Log.d(TAG, "onCheckedChanged: " + b);
                isChecked = b;
            }
        });

        return view;
    }

}

fragment_one.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Primer fragmento" />

    <CheckBox
        android:id="@+id/checkBox"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="CheckBox" />

</LinearLayout>

MainActivity.java

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.view.MenuItem;

public class MainActivity extends AppCompatActivity
        implements BottomNavigationView.OnNavigationItemSelectedListener {

    private ViewPager mViewPager;

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

        //mTextMessage = (TextView) findViewById(R.id.message);
        final BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
        navigation.setOnNavigationItemSelectedListener(this);

        mViewPager = (ViewPager) findViewById(R.id.view_pager);
        //mViewPager.setOffscreenPageLimit(1);
        DemoFragmentAdapter adapterViewPager = new DemoFragmentAdapter(getSupportFragmentManager());
        mViewPager.setAdapter(adapterViewPager);
        //mViewPager.setCurrentItem(0);

        mViewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            public void onPageScrollStateChanged(int state) {
            }

            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            }

            public void onPageSelected(int position) {
                navigation.getMenu().getItem(position).setChecked(true);
            }
        });

        if (savedInstanceState == null) {
            onNavigationItemSelected(navigation.getMenu().findItem(R.id.navigation_home));
        }

    }


    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {

        switch (item.getItemId()) {
            case R.id.navigation_home:
                mViewPager.setCurrentItem(0);
                return true;
            case R.id.navigation_dashboard:
                mViewPager.setCurrentItem(1);
                return true;
            case R.id.navigation_notifications:
                mViewPager.setCurrentItem(2);
                return true;
            default:
                return false;
        }

    }

}

Hi, the code looks great. Do you know why it's necessary to use mViewPager.setCurrentItem(ID) rather than getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,SELECTED_Fragment).commit();
in this case?