Q:

Viewpager not visible when it has any other layout as its sibling

Here is main activity layout XML
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="fill_parent">

    <LinearLayout
        android:id="@+id/left"
        android:layout_width="100dp"
        android:layout_height="fill_parent"
        android:layout_alignParentLeft="true"
        android:background="@color/red"
        android:orientation="horizontal" >
    </LinearLayout>

    <android.support.v4.view.ViewPager
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/pager"
        android:layout_width="0dip"
        android:layout_height="fill_parent"
        android:layout_toLeftOf="@+id/left"
        android:layout_toRightOf="@+id/right"
        tools:context=".MainActivity" />

    <LinearLayout
        android:id="@+id/right"
        android:layout_width="100dp"
        android:layout_height="fill_parent"
        android:layout_alignParentRight="true"
        android:background="@color/red"
        android:orientation="horizontal" >
    </LinearLayout>

</RelativeLayout>

Fragment Layout XML :
<?xml version="1.0" encoding="utf-8"?>    
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent" android:orientation="horizontal"
        tools:context=".DummySectionFragment" android:background="@color/gray">

        <TextView
            android:id="@+id/section_label"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />

    </LinearLayout>

Main Activity :
package com.example.androidtest;

import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.view.Menu;

public class MainActivity extends FragmentActivity implements
        ActionBar.TabListener {

    /**
     * The {@link android.support.v4.view.PagerAdapter} that will provide
     * fragments for each of the sections. We use a
     * {@link android.support.v4.app.FragmentPagerAdapter} derivative, which
     * will keep every loaded fragment in memory. If this becomes too memory
     * intensive, it may be best to switch to a
     * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    ViewPager mViewPager;

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

        // Set up the action bar.
        final ActionBar actionBar = getActionBar();
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        // Create the adapter that will return a fragment for each of the three
        // primary sections of the app.
        mSectionsPagerAdapter = new SectionsPagerAdapter(
                getSupportFragmentManager(), getApplicationContext());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        // When swiping between different sections, select the corresponding
        // tab. We can also use ActionBar.Tab#select() to do this if we have
        // a reference to the Tab.
        mViewPager
                .setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
                    @Override
                    public void onPageSelected(int position) {
                        actionBar.setSelectedNavigationItem(position);
                    }
                });

        // For each of the sections in the app, add a tab to the action bar.
        for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
            // Create a tab with text corresponding to the page title defined by
            // the adapter. Also specify this Activity object, which implements
            // the TabListener interface, as the callback (listener) for when
            // this tab is selected.
            actionBar.addTab(actionBar.newTab()
                    .setText(mSectionsPagerAdapter.getPageTitle(i))
                    .setTabListener(this));
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public void onTabSelected(ActionBar.Tab tab,
            FragmentTransaction fragmentTransaction) {
        // When the given tab is selected, switch to the corresponding page in
        // the ViewPager.
        mViewPager.setCurrentItem(tab.getPosition());
    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab,
            FragmentTransaction fragmentTransaction) {
    }

    @Override
    public void onTabReselected(ActionBar.Tab tab,
            FragmentTransaction fragmentTransaction) {
    }
}

Fragment :
package com.example.androidtest;

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.view.ViewGroup.LayoutParams;
import android.widget.TextView;

/**
 * A dummy fragment representing a section of the app, but that simply
 * displays dummy text.
 */
public class DummySectionFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    public static final String ARG_SECTION_NUMBER = "section_number";

    public DummySectionFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main_dummy,
                container, false);
        TextView dummyTextView = (TextView) rootView
                .findViewById(R.id.section_label);
        dummyTextView.setText(Integer.toString(getArguments().getInt(
                ARG_SECTION_NUMBER)));
        return rootView;
    }
}

Pager Adapter :
package com.example.androidtest;

import java.util.Locale;

import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;

/**
 * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
 * one of the sections/tabs/pages.
 */
public class SectionsPagerAdapter extends FragmentPagerAdapter {

    private Context context;

    public SectionsPagerAdapter(FragmentManager fm, Context context) {
        super(fm);
        this.context = context;
    }

    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a DummySectionFragment (defined as a static inner class
        // below) with the page number as its lone argument.
        Fragment fragment = new DummySectionFragment();
        Bundle args = new Bundle();
        args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, position + 1);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        Locale l = Locale.getDefault();
        switch (position) {
        case 0:
            return context.getString(R.string.title_section1).toUpperCase(l);
        case 1:
            return context.getString(R.string.title_section2).toUpperCase(l);
        case 2:
            return context.getString(R.string.title_section3).toUpperCase(l);
        }
        return null;
    }
}

Expected : 2 red bars on both sides. ViewPager in the middle.
Whats happening : 2 red bars on both sides. Blank in the middle.
Question : Why ?
EDIT :
I modified the main activity layout XML to the following to get this to work.
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="fill_parent">

    <LinearLayout
        android:id="@+id/left"
        android:layout_width="100dp"
        android:layout_height="fill_parent"
        android:layout_alignParentLeft="true"
        android:background="@color/red"
        android:orientation="horizontal" >
    </LinearLayout>

    <android.support.v4.view.ViewPager
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/pager"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        tools:context=".MainActivity" android:layout_toRightOf="@+id/left"/>

    <LinearLayout
        android:id="@+id/right"
        android:layout_width="100dp"
        android:layout_height="fill_parent"
        android:layout_alignParentRight="true"
        android:background="@color/red"
        android:orientation="horizontal" >
    </LinearLayout>

</RelativeLayout>

Only change is I removed  android:layout_toRightOf="@+id/right" and changed layout_width="fill_parent". Fill_parent I understand. By why does removing the other make a difference ?

A:

You have to do two things in main.xml:

Use LinearLayout instead of RelativeLayout for your main.xml with horizontal orientation.
Add android:layout_weight="1" to your View Pager.

