Develop Reference

ANDROID: getActionBar() produces a null pointer exception with a tab / fragment activity

Basically I'm making a tab activity that has fragments of other activities as the tabs. However when it runs, I am recieving a null pointer exception at the line:
final ActionBar actionBar = getActionBar();
Here is the whole activity. It is taken from the google sample.
public class MainActivity extends FragmentActivity implements ActionBar.TabListener {
SectionsPagerAdapter mSectionsPagerAdapter;
ViewPager mViewPager;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.sample_main);
// Set up the action bar. The navigation mode is set to NAVIGATION_MODE_TABS, which will
// cause the ActionBar to render a set of tabs. Note that these tabs are *not* rendered
// by the ViewPager; additional logic is lower in this file to synchronize the ViewPager
// state with the tab state. (See mViewPager.setOnPageChangeListener() and onTabSelected().)
// BEGIN_INCLUDE (set_navigation_mode)
final ActionBar actionBar = getActionBar();
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// END_INCLUDE (set_navigation_mode)
// BEGIN_INCLUDE (setup_view_pager)
// Create the adapter that will return a fragment for each of the three primary sections
// of the app.
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mSectionsPagerAdapter);
// END_INCLUDE (setup_view_pager)
// 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.
// BEGIN_INCLUDE (page_change_listener)
mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
#Override
public void onPageSelected(int position) {
actionBar.setSelectedNavigationItem(position);
}
});
// END_INCLUDE (page_change_listener)
// BEGIN_INCLUDE (add_tabs)
// 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));
}
// END_INCLUDE (add_tabs)
}
/**
* Update {#link ViewPager} after a tab has been selected in the ActionBar.
*
* #param tab Tab that was selected.
* #param fragmentTransaction A {#link android.app.FragmentTransaction} for queuing fragment operations to
* execute once this method returns. This FragmentTransaction does
* not support being added to the back stack.
*/
// BEGIN_INCLUDE (on_tab_selected)
#Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
// When the given tab is selected, tell the ViewPager to switch to the corresponding page.
mViewPager.setCurrentItem(tab.getPosition());
}
// END_INCLUDE (on_tab_selected)
/**
* Unused. Required for {#link android.app.ActionBar.TabListener}.
*/
#Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
/**
* Unused. Required for {#link android.app.ActionBar.TabListener}.
*/
#Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
// BEGIN_INCLUDE (fragment_pager_adapter)
/**
* A {#link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the sections/tabs/pages. This provides the data for the {#link ViewPager}.
*/
public class SectionsPagerAdapter extends FragmentPagerAdapter {
// END_INCLUDE (fragment_pager_adapter)
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
// BEGIN_INCLUDE (fragment_pager_adapter_getitem)
/**
* Get fragment corresponding to a specific position. This will be used to populate the
* contents of the {#link ViewPager}.
*
* #param position Position to fetch fragment for.
* #return Fragment for specified position.
*/
#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.
switch (position) {
case 0:
return new SearchPlayerFragment();
case 1:
return new SearchPlayerFragment();
case 2:
return new SearchPlayerFragment();
default:
return new SearchPlayerFragment();
}
}
// END_INCLUDE (fragment_pager_adapter_getitem)
// BEGIN_INCLUDE (fragment_pager_adapter_getcount)
/**
* Get number of pages the {#link ViewPager} should render.
*
* #return Number of fragments to be rendered as pages.
*/
#Override
public int getCount() {
// Show 3 total pages.
return 3;
}
// END_INCLUDE (fragment_pager_adapter_getcount)
// BEGIN_INCLUDE (fragment_pager_adapter_getpagetitle)
/**
* Get title for each of the pages. This will be displayed on each of the tabs.
*
* #param position Page to fetch title for.
* #return Title for specified page.
*/
#Override
public CharSequence getPageTitle(int position) {
Locale l = Locale.getDefault();
switch (position) {
case 0:
return getString(R.string.title_section1).toUpperCase(l);
case 1:
return getString(R.string.title_section2).toUpperCase(l);
case 2:
return getString(R.string.title_section3).toUpperCase(l);
}
return null;
}
// END_INCLUDE (fragment_pager_adapter_getpagetitle)
}
}

getActionBar() returns null because you are extending FragmentActivity.
Extend AppCompactActivity instead.

Try calling the Support Action Bar instead
getSupportActionBar();
I suggest you inherit from an ActionBarActivity instead of a FragmentActivity

I don't want to refresh fragment on change

I have 3 fragments... with their 3 navigation tabs.
The problem is that if i'm on the first tab, and i click the third.. when I click another time the first, it reloads all the fragment. If I click the second tab (in the middle) and I click the first, it doesn't reload.
My objective is that I don't want to refresh my fragments never!
How can I do it?
My code is:
public class TabsPagerAdapter extends FragmentPagerAdapter {
public TabsPagerAdapter(FragmentManager fm) {
super(fm);
}
// Return Items
#Override
public Fragment getItem(int index) {
switch (index) {
case 0:
return new Retos();
case 1:
return new Amics();
case 2:
return new Ranking();
}
return null;
}
#Override
public int getCount() {
// get item count - equal to number of tabs
return 3;
}
}
And this :
public class Perfil extends ActionBarActivity implements ActionBar.TabListener{
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
private SearchView mSearchView;
private TextView mStatusView;
// Tab titles
private String[] tabs = { "TAB1", "TAB2", "TAB3" };
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.perfil);
// Initilization
viewPager = (ViewPager) findViewById(R.id.pager);
actionBar = getSupportActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(true);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
/** Creating fragment1 Tab */
Tab tab = actionBar.newTab()
.setText("TAB1")
.setTabListener(this);
actionBar.addTab(tab);
/** Creating fragment2 Tab */
tab = actionBar.newTab()
.setText("TAB2")
.setTabListener(this);
//.setIcon(R.drawable.ic_action_group);
actionBar.addTab(tab);
/** Creating fragment3 Tab */
tab = actionBar.newTab()
.setText("TAB3")
.setTabListener(this);
actionBar.addTab(tab);
/**
* on swiping the viewpager make respective tab selected
* */
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageSelected(int position) {
// on changing the page
// make respected tab selected
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) {
// on tab selected
// show respected fragment view
viewPager.setCurrentItem(tab.getPosition());
}
#Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
}
}

viewPager.setOffscreenPageLimit(C_FRAGMENTS_TO_KEEP_IN_MEMORY);
C_FRAGMENTS_TO_KEEP_IN_MEMORY is the number of tabs at right and left of the current selected tab to keep in memory. So in your case should be 2.
Please be sure that you are NOT creating a new fragment instance each time you call the getItem method in you viewpager adapter.
Check the #Rarw answer in this page.

Try setting your ViewPager offScreenPageLimit to cover the number of fragments you are trying to load. OffScreenPageLimist keeps fragments alive in an idle state when they are not visible. I don't see that value set in your code and based on what you're describing, that fragments keep recycling, it sounds to me like you're using the default state of 1, which will only retain one of the fragments off screen and not both.
Some caveats, this approach only really works if you know in advance how many fragments you will need since if you're dynamically adding and removing fragments its hard to know how many if any to retain.
UPDATE
This is likely why you're pages are refreshing:
#Override
public Fragment getItem(int index) {
switch (index) {
case 0:
return new Retos();
case 1:
return new Amics();
case 2:
return new Ranking();
}
return null;
}
You keep returning a new fragment each time you switch id. What you should do something like this:
case 0:
if(mRetrosFragment == null)
mRetrosFragment = new Retros();
return mRetrosFragment;
This way you stop recreating the fragment every time the tab changes and instead retain that instance.

Set Home button on each fragment

My application uses fragments and swipes to navigate different pages. I want to have an image button on each fragment activity so that when pressed first activity should be available. How can I do this. I tried viewpager.setcurrentitem option but it is only available in main activity. and in fragment activity viewpager is not available. I know there is way to by using android default property of sethome button. But I want an image button in each fragment.
This is my main activity
public class MainActivity extends FragmentActivity implements
ActionBar.TabListener {
SectionsPagerAdapter mSectionsPagerAdapter;
ViewPager mViewPager;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_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());
// 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);
}
});
Tab tab = actionBar.newTab().setIcon(R.drawable.prf).setText(getString(R.string.title_section1)).setTabListener(this);
actionBar.addTab(tab, true);
tab = actionBar.newTab().setIcon(R.drawable.pack).setText(getString(R.string.title_section2)).setIcon(R.drawable.pack).setTabListener(this);
actionBar.addTab(tab);
tab = actionBar.newTab().setIcon(R.drawable.call).setText(getString(R.string.title_section3)).setTabListener(this);
actionBar.addTab(tab);
tab = actionBar.newTab().setIcon(R.drawable.prom).setText(getString(R.string.title_section4)).setTabListener(this);
actionBar.addTab(tab);
tab = actionBar.newTab().setIcon(R.drawable.infoin).setText(getString(R.string.title_section5)).setTabListener(this);
actionBar.addTab(tab);
}
#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) {
}
/**
* A {#link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the sections/tabs/pages.
*/
public class SectionsPagerAdapter extends FragmentPagerAdapter {
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
#Override
public Fragment getItem(int position) {
switch (position) {
case 0 :
return new Home();
case 1:
return new info();
case 2:
return new Plan();
}
return null;
}
#Override
public int getCount() {
// Show 3 total pages.
return 3;
}
}

Use callback mechanism through Interface and send message back to Activity of what you want to do on click of ImageButton. Same mechanism is good for fragments communication with each other.

How to do Up navigation using ActionBar's tabs and Fragment?

I'm using the ActionBar and tab navigation with Fragments.
Each tab contain a Fragment FragmentX with a ListView, and when clicking on an item of a list FragmentX is replaced by another Fragment FragmentY with the detail of the item.
How can I set the Up button of the ActionBar to load FragmentX when I'm in FragmentY?
The doc shows that the navigation should be done between Activities, but the ActionBar's navigation tabs allow to navigate between Fragments, not Activities.
What am I missing?
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.activity_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());
// 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.
LinearLayout view = (LinearLayout) getLayoutInflater().inflate(R.layout.tab, null);
ImageView icon = (ImageView) view.findViewById(R.id.icon);
icon.setImageResource(tabIconList[i]);
TextView title = (TextView) view.findViewById(R.id.title);
title.setText(tabTitleList[i]);
actionBar.addTab(actionBar.newTab()
.setCustomView(view)
.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) {
}
/**
* A {#link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the sections/tabs/pages.
*/
public class SectionsPagerAdapter extends FragmentPagerAdapter {
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
#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 = null;
switch(position){
case 0:
fragment = new MyListFragment1();
break;
case 1:
fragment = new MyListFragment2();
break;
case 2:
fragment = new MyListFragment3();
break;
default: fragment = new MyListFragment1();
}
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 getString(R.string.title1).toUpperCase(l);
case 1:
return getString(R.string.title2).toUpperCase(l);
case 2:
return getString(R.string.title3).toUpperCase(l);
}
return null;
}
}
}

Active up navigation when you load FragmentY.
Then intercept up navigation on onOptionsItemSelected, just popup the backstack and launch your FragmentX.
I've done a rapid search on SO and i found this, wich inspired me.

I didn't use the ActionBar in the end. Too many constraints (I had another issue and I wanted to add a View above the ActionBar, which seemed complicated).
It was much simpler doing my own layout from scratch.

Strange onPause(), onResume() behaviour in fragments

The application has a main activity (MainActivity.java) with three tabs (fragments). I can navigate between them using the swipe left (riht) or clicking on a specific tab.
Upon starting the application, the 1st fragment is shown.
If I go to the 2nd fragment from the 1st fragment and then back to the 1st fragment, nothing happens (onResume() of the 1rd fragment isn't called), so it doesn't refresh it's content.
If I go to the 3rd fragment from the 1st fragment and then directly back to the 1st fragment, the onCreateView() of fragment1 is created and it's onResume() is called, which is correct.
If I go from the 3rd fragment to the 2nd fragment, the onCreateView() and onResume() of fragment1 are called, but not the onCreateView of fragment2.
I guess the logic in MainActivity isn't right, so I would kindly ask someone to take a look and tell me what could be wrong.
MainActivity.java:
public class MainActivity extends FragmentActivity implements ActionBar.TabListener {
CollectionPagerAdapter mCollectionPagerAdapter;
public TTSocket socket;
DBHandler db;
public String logged_user;
private LogedinPerson person;
ViewPager mViewPager;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Bundle extras = getIntent().getExtras();
logged_user = extras.getString("logged_user");
socket = TTSocket.getInstance();
socket.currentRef = this;
db = new DBHandler(this);
person=db.getLogedInPerson();
socket.dbHandler=db;
socket.person=person;
if(!socket.isInit){
String typeInitStr = "{\"Type\":\"Init\", \"UserId\":\""+ person.getUserId() +"\"}";
socket.Send(typeInitStr);
}
mCollectionPagerAdapter = new CollectionPagerAdapter(getSupportFragmentManager());
// Set up action bar.
final ActionBar actionBar = getActionBar();
// Specify that we will be displaying tabs in the action bar.
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// Set up the ViewPager, attaching the adapter and setting up a listener
// for when the
// user swipes between sections.
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mCollectionPagerAdapter);
mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
#Override
public void onPageSelected(int position) {
Log.d("TEST",position+"");
// the corresponding tab.
// We can also use ActionBar.Tab#select() to do this if
// we have a reference to the Tab
actionBar.setSelectedNavigationItem(position);
}
});
// For each of the sections in the app, add a tab to the action bar.
for (int i = 0; i < mCollectionPagerAdapter.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
// listener for when this tab is selected.
if(i == 0){
actionBar.addTab(actionBar.newTab()
.setIcon(R.drawable.messages)
.setTabListener(this));
}else if(i == 1){
actionBar.addTab(actionBar.newTab()
.setIcon(R.drawable.contacts)
.setTabListener(this));
}else{
actionBar.addTab(actionBar.newTab()
.setIcon(R.drawable.history)
.setTabListener(this));
}
}
}
#Override
protected void onResume() {
// TODO Auto-generated method stub
super.onResume();
socket.currentRef = this;
socket.dbHandler=db;
socket.person=person;
//mCollectionPagerAdapter.notifyDataSetChanged();
}
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
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());
}
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
/**
* A {#link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the primary sections of the app.
*/
public class CollectionPagerAdapter extends FragmentPagerAdapter {
final int NUM_ITEMS = 3; // number of tabs
public CollectionPagerAdapter(FragmentManager fm) {
super(fm);
}
/*
#Override
public Fragment getItem(int i) {
Fragment fragment = new TabFragment();
Bundle args = new Bundle();
args.putInt(TabFragment.ARG_OBJECT, i);
fragment.setArguments(args);
return fragment;
}
*/
#Override
public Fragment getItem(int position) {
Fragment fragment = new Fragment();
Bundle args = new Bundle();
args.putInt(TabFragment.ARG_OBJECT, position);
switch (position) {
case 0:
Log.i("Fragment", "0");
fragment = new Tab1Fragment();
fragment.setArguments(args);
return fragment;
case 1:
Log.i("Fragment", "1");
fragment = new Tab2Fragment();
fragment.setArguments(args);
return fragment;
case 2:
Log.i("Fragment", "2");
fragment = new Tab3Fragment();
fragment.setArguments(args);
return fragment;
default:
break;
}
return fragment;
}
#Override
public int getCount() {
return NUM_ITEMS;
}
#Override
public CharSequence getPageTitle(int position) {
String tabLabel = null;
switch (position) {
case 0:
tabLabel = getString(R.string.label1);
break;
case 1:
tabLabel = getString(R.string.label2);
break;
case 2:
tabLabel = getString(R.string.label3);
break;
}
return tabLabel;
}
}
/**
* A fragment that launches other parts of the demo application.
*/
public static class TabFragment extends Fragment {
public static final String ARG_OBJECT = "object";
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
Bundle args = getArguments();
int position = args.getInt(ARG_OBJECT);
int tabLayout = 0;
switch (position) {
case 0:
tabLayout = R.layout.tab1;
break;
case 1:
tabLayout = R.layout.tab2;
break;
case 2:
tabLayout = R.layout.tab3;
break;
}
View rootView = inflater.inflate(tabLayout, container, false);
return rootView;
}
}
}

Strange onPause(), onResume() behaviour() in fragments
It's not strange behaviour but native behaviour of ActionSherlock. This kind of behaviour is used for caching -> optimalisation for older devices with lower RAM this is reason why fragments are cached.
If you need to update content of fragment don't try to replace its layout or something similar. If you want to update fragment when scrolling between pages, you need to use method of FragmentPagerAdapter:
#Override
public int getItemPosition(Object object) {
// implementation
return super.getItemPosition(object);
}
This method is called when you will call
notifyDataSetChanged();
on your FragmentPagerAdapter. It's handy method for make updates of your fragments. There are more ways how to do it but here i'll show you how I'm doing it.
Let your fragments implement interface for example called Updateable:
interface Updateable {
public void update();
}
public class MyFragment extends SherlockFragment implements Updateable {
#Override
public void update() {
// perform Fragment updates
}
}
And in this method you will perform updates. Now back to getItemPosition() method. This method will be used for invoking update() method from Fragment i.e:
#Override
public int getItemPosition(Object object) {
Fragment f = (Fragment) object;
// determine which fragment
if (f instanceof MyFragment) {
((MyFragment) f).update(); // invokes update() method
}
return super.getItemPosition(object);
}
Now whenever you scroll page or tap on some tab (you need also call notifyDataSetChanged()) you are able to make Fragment updates. This way is more efficient against destroying and recreating fragment(s) each time you scrolling or clicking on tabs. But how i said this is not only solution there are more possible solutions.
Note: getItemPosition() can return two values: POSITION_NONE and UNCHANGED. Difference between both is that first indicates that Fragment will be always destroyed and recreated that is not very efficient and second indicates that Fragment won't be changed (is in on right place).
For more detailed explanation look here.

That is because ViewPager doesn't hide all fragments you switch.
You can control this behaviour by setOffscreenPageLimit