Android如何在选项卡更改时停止刷新片段

我有以下代码：

MainActivity.java

package com.erc.library;
import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;
import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.FragmentTransaction;
import android.content.SharedPreferences;
import android.content.res.Resources;
import android.graphics.Color;
import android.os.Bundle;
import android.os.Environment;
import android.os.StrictMode;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.widget.TextView;
import android.widget.Toast;
import com.erc.sayeghlibrary.adapter.TabsPagerAdapter;
public class MainActivity extends FragmentActivity implements ActionBar.TabListener {
    private ViewPager viewPager;
    private TabsPagerAdapter mAdapter;
    private ActionBar actionBar;
    private String[] tabs = { "Stories", "Dictionaries", "eBooks"};
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        int actionBarTitleId = Resources.getSystem().getIdentifier("action_bar_title", "id", "android");
        if (actionBarTitleId > 0) {
            TextView title = (TextView) findViewById(actionBarTitleId);
            if (title != null) {
                title.setTextColor(Color.WHITE);
            }
        }       
        viewPager = (ViewPager) findViewById(R.id.pager);
        actionBar = getActionBar();
        mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
        viewPager.setAdapter(mAdapter);
        actionBar.setHomeButtonEnabled(false);
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        for (String tab_name : tabs) {
            actionBar.addTab(actionBar.newTab().setText(tab_name)
                    .setTabListener(this));
        }
        viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
                actionBar.setSelectedNavigationItem(position);
            }
            @Override
            public void onPageScrolled(int arg0, float arg1, int arg2) {
            }
            @Override
            public void onPageScrollStateChanged(int arg0) {
            }
        });
    }
    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {
    }
    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        viewPager.setCurrentItem(tab.getPosition());
    }
    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
         MenuInflater inflater = getMenuInflater();
         inflater.inflate(R.menu.main, menu);
         return true;
    } 
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
         switch (item.getItemId()) {
         case R.id.Favorites:
           Toast.makeText(this, "Favorites selected", Toast.LENGTH_SHORT)
               .show();
           break;
         default:
           break;
         }
         return true;
    } 
}

TabsPagerAdapter.java

package com.erc.library.adapter;
import com.erc.library.Dictionaries;
import com.erc.library.Ebooks;
import com.erc.library.Stories;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
public class TabsPagerAdapter extends FragmentPagerAdapter {
    public TabsPagerAdapter(FragmentManager fm) {
        super(fm);
    }
    @Override
    public Fragment getItem(int index) {
        switch (index) {
        case 0:
            return new Stories();
        case 1:
            return new Dictionaries();
        case 2:
            return new Ebooks();
        }
        return null;
    }
    @Override
    public int getCount() {
        return 3;
    }
}

我正在制作一个图书馆应用程序，其中导航是通过选项卡进行的，问题是每次我从第三个选项卡切换到第一个或从第一个切换到第三个时，选项卡内容会刷新，我想防止刷新，请帮忙！