Develop Reference

If I add a new transaction(adding fragment, no removing) to the fragment backstack, do the fragment objects in previous transaction get copied or used

So let's say for my first transaction I add Fragment A, add to backstack, and commit.
For my second transaction, I add Fragment B, add to backstack, and commit.
At this point after my second transaction, I have both Fragment A and Fragment B existing. Is Fragment A still referring to the same Fragment A object I created in the first transaction or a copy of it?
I read somewhere saying that after each transaction, a snapshot of the fragments gets created and it only makes sense to me if existing fragments get copied so old snapshots don't get messed up by new ones.
How does it work exactly?

A FragmentTransaction that uses addToBackStack records just the minimum information around the operation - in your case, the Fragment that is added and the fact that it was an 'add' operation. The FragmentManager absolutely does not do any snapshot type of behavior.
This is used in two ways:
To execute the FragmentTransaction. An 'add' operation does not affect other Fragments (so Fragment A is not changed when you add Fragment B).
When you pop the back stack (i.e., hit the system back button), the operation is reversed - an 'add' operation becomes a 'remove' operation. For a 'remove' operation, no other Fragment is changed (so Fragment A still exists when you pop Fragment B)
Of course, other operations, such as 'replace' affect two Fragments (you can think of it as a 'remove' of the old Fragment and an 'add' of the new Fragment).

What Fragments am I hosting and displaying?

Is there a way to know which Fragment is currently displayed in a given <fragment> container of an Activity without keeping track of all the changes via the onAttachFragment callback?
Is it even possible to know which fragments are displayed when fragment transactions can take place when the user presses the back key? In this latter case, i.e. when a Fragment is re-displayed due to a back, the onAttach is not called.

In my experience, the only way to know for sure which fragment is being displayed is to keep track of that carefully yourself.
For example, you could make a variable in your Activity:
Fragment mCurrentDisplayedFragment;
and then whenever the user requests a different fragment do:
mCurrentFragment = (Fragment) userRequestedFragment;
fragmentManager.replace(container, mCurrentFragment, tag);
Then, whenever you needed to do things to the currently displayed fragment, you could triage it by try/catching a cast or with instanceof.
You could also handle the back pressed behavior by overriding that method in the activity:
#Override
public void onBackPressed() {
int stackSize = fragmentManager.getBackStackEntryCount();
// This counts up from the bottom so the most recent fragment is the biggest number/size
backFragId = fragmentManager.getBackStackEntryAt(stackSize);
// Get a handle on the fragment that is about to be popped
mCurrentFragment = fragmentManager.findFragmentById(backFragId);
super.onBackPressed();
}
Also, are you sure that onAttach is not called when a fragment is popped off the stack? I seem to remember that it will be, and you can call through the interface created there (if you have one and the activity implements it) to register the fragment as the current fragment in the activity at the time.
But to directly answer your question, there isn't a built in way to just know what fragment is currently displayed (and there could be more than one!). The implementation details of that are up to you. Hopefully I've given you some ideas of how it could be handled though. You might also find the FragmentManager documentation helpful.

Each time when you add/replace fragment to the container, use tag for it:
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.add(R.id.container, fragment, tag).commit();
then you can find out the fragment is current visible or not:
Fragment fg = getFragmentManger().findFragmentByTag(tag);
if(fg.isVisible())
//fg is the current visible fragment
Hope this help!

Android Fragment View State Loss When Using FragmentTransaction.replace()

I am having a pretty big issue and I am not quite understanding what is happening. I am developing an application that uses Fragments (from the support library) and am using FragmentTransaction.replace() to place new Fragments on to the back stack and replace the old one. The code looks as follows:
FragmentManager fm = getSupportFragmentManager();
FragmentTransaction ft = ft.beginTransaction();
// Animations in my res/anim folder
ft.setCustomAnimations(R.anim.slide_in_right, R.anim.slide_out_left, R.anim.slide_in_left, R.anim.slide_out_right);
ft.replace(R.id.fragment_container, newFragment, tag);
ft.addToBackStack(null);
ft.commit();
This is successful in replacing my fragment. My issue is the following. In one Fragment, I have a list of items that is built from user input. Now, when the user clicks next and then clicks the back button (to return to the list), the list is empty because the view is destroyed. Now, I have noted the following:
onSaveInstanceState is not called. I believe this is because that is only called when the parent Activity tells it to. Based on the docs: " There are many situations where a fragment may be mostly torn down (such as when placed on the back stack with no UI showing), but its state will not be saved until its owning activity actually needs to save its state.". Apparently, performing a replace on the FragmentTransaction is not one of those times. Does anyone have confirmation on this or a better explanation?
setOnRetainInstanceState(true) is not helpful in this situation. Again, I believe this has to do with info from the docs: "Control whether a fragment instance is retained across Activity re-creation (such as from a configuration change)". I am not performing any action in re-creating the activity so this is of no use.
So, I guess my main question is: is there a way to preserve the View state (simply retain the Fragment) when using replace? There is FragmentTransaction.add(), but there are a few issues with this as well. One being that the exit animation is not performed, thus the animation is not correct. Another is that the new Fragment that the old fragment (the one that is being put into a non-visible state) is still clickable. For example, if I have a ListFragment, and I place a content fragment on top of that by using add, I can still click the list items in the ListFragment.

Without being able to see the code of your fragments this is a bit of a guess, but in the past I've run into this same issue and I've found that resetting the adapter in your ListFragment in onViewStateRestored seems to do the trick.
public void onViewStateRestored (Bundle savedInstanceState)
{
super.onViewStateRestored (savedInstanceState);
setListAdapter(new ArrayAdapter(Activity, R.layout.nav_item, objects));
}
Which is weird considering the documentation states that this method is called after onActivityCreated but before onStart. But it seems that it is also called at other times because when the most recent fragment transaction is popped off the back stack this method is called before the previously replaced fragment is displayed. The activity that owns the fragments has not been paused or obscured in any way, so according to the docs onViewStateRestored should not be called since just the fragments were modified. But this seems to work anyway.

It sounds like you simply need to make sure you have properly implemented onCreateView and onDestroyView. The situation you are describing seems to indicate that when the list fragment is put on the back stack (as a result of the replace transaction) Android is calling onDestroyView to free up some resources. However, it apparently has not destroyed the list fragment because when you tap back you are getting back the same instance of the fragment.
Assuming this is all true then, when the user taps back Android will call onCreateView. Any state that you have stored in the fragment's instance variables should still be there and all you need to do is repopulate the view...perhaps set the adapter on the ListView or whatever.
Also make sure your onSaveInstanceState() callback actually does save any instance state that you need to rebuild the view. That way if the fragment actually does get completely destroyed the FragmentManager can restore the state when it needs to recrete the fragment later.

How to keep only first added Fragment in back stack (fragment overlapping)?

Scenario what i'm trying to achieve:
Loading activity with two frame containers (for list of items and for details).
At the app launch time add listFragment in listFrame and some initial infoFragment in detailsFrame containers.
Navigating through list items without adding each detail transaction to back stack (want to keep only infoFragment in stack).
As soon as user hit back button (navigate back) he falls back to intial infoFragment what was added in launch time.
If sequential back navigation fallows then apps exit.
My code:
protected override void OnCreate(Bundle savedInstanceState)
{
...
var listFrag = new ListFragment();
var infoFrag = new InfoFragment();
var trans = FragmentManager.BeginTransaction();
trans.Add(Resource.Id.listFrame, listFrag);
trans.Add(Resource.Id.detailsFrame, infoFrag);
trans.Commit();
...
}
public void OnItemSelected(int id)
{
var detailsFrag = DetailFragment.NewInstance(id);
var trans = FragmentManager.BeginTransaction();
trans.Replace(Resource.Id.detailsFrame, detailsFrag);
if (FragmentManager.BackStackEntryCount == 0)
{
trans.AddToBackStack(null);
}
trans.Commit();
}
My problem:
After back button has been hit, infoFrag is overlapped with previous detailFrag! Why?

You can do this:
if (getSupportFragmentManager().getBackStackEntryCount() > 0) {
getSupportFragmentManager().popBackStack(getSupportFragmentManager().getBackStackEntryAt(0).getId(), getSupportFragmentManager().POP_BACK_STACK_INCLUSIVE);
} else {
super.onBackPressed();}
In your activity, so you to keep first fragment.
You shouldn't have, in your first fragment, the addToBackStack. But, in the rest, yes.

Very nice explanation by Budius. I read his advice and implemented similar navigation, which I would like to share with others.
Instead of replacing fragments like this:
Transaction.remove(detail1).add(detail2)
Transaction.remove(detail2).add(detail3)
Transaction.remove(detail3).add(detail4)
I added a fragment container layout in the activity layout file. It can be either LinearLayout, RelativeLayot or FrameLayout etc.. So in the activity on create I had this:
transaction.replace(R.id.HomeInputFragment, mainHomeFragment).commit();
mainHomeFragment is the fragment I want to get back to when pressing the back button, like infoFrag. Then, before EVERY NEXT transaction I put:
fragmentManager.popBackStackImmediate();
transaction.replace(R.id.HomeInputFragment, frag2).addToBackStack(null).commit();
or
fragmentManager.popBackStackImmediate();
transaction.replace(R.id.HomeInputFragment, frag3).addToBackStack(null).commit();
That way you don't have to keep track of which fragment is currenty showing.

The problem is that the transaction that you're backing from have two steps:
remove infoFrag
add detailsFrag (that is the first1 detail container that was added)
(we know that because the documentation This is essentially the same as calling remove(Fragment) for all currently added fragments that were added with the same containerViewId and then add(int, Fragment, String) with the same arguments given here. )
So whenever the system is reverting that one transaction is reverting exactly those 2 steps, and it say nothing about the last detailFrag that was added to it, so it doesn't do anything with it.
There're two possible work arounds I can think on your case:
Keep a reference on your activity to the last detailsFrag used and use the BackStackChange listener to whenever the value change from 1 to 0 (you'll have to keep track of previous values) you also remove that one remaining fragment
on every click listener you'll have to popBackStackImmediatly() (to remove the previous transaction) and addToBackStack() on all transactions. On this workaround you can also use some setCustomAnimation magic to make sure it all looks nice on the screen (e.g. use a alpha animation from 0 to 0 duration 1 to avoid previous fragment appearing and disappearing again.
ps. I agree that the fragment manager/transaction should be a bit more clever to the way it handles back stack on .replace() actions, but that's the way it does it.
edit:
what is happening is like this (I'm adding numbers to the details to make it more clear).
Remember that .replace() = .remove().add()
Transaction.remove(info).add(detail1).addToBackStack(null) // 1st time
Transaction.remove(detail1).add(detail2) // 2nd time
Transaction.remove(detail2).add(detail3) // 3rd time
Transaction.remove(detail3).add(detail4) // 4th time
so now we have detail4 on the layout:
< Press back button >
System pops the back stack and find the following back entry to be reversed
remove(info).add(detail1);
so the system makes that transaction backward.
tries to remove detail1 (is not there, so it ignores)
re-add(info) // OVERLAP !!!
so the problem is that the system doesn't realise that there's a detail4 and that the transaction was .replace() that it was supposed to replace whatever is in there.

You could just override onBackPressed and commit a transaction to the initial fragment.

I'm guessing but:
You've added the transaction to replace infoFrag with 1st detailsFrag into the backstack.
But then you replace 1st detailsFrag with 2nd detailsFrag.
At this point when you click back, the fragment manager cannot cleanly replace 1st detailsFrag with infoFrag as 1st detailsFrag has already been removed and replaced.
Whether the overlapping behaviour is expected or not I don't know.
I would suggest debugging the Android core code to see what it is doing.
I'm not sure whether you can achieve without say overriding Activity::onBackPressed() and doing the pops yourself having added all transactions to the backstack.

FragmentManager.popBackStack() does not simply loads the previous Fragment

I have a main TabActivity which has two tabs, A and B (for now). Tab A loads a FragmentActivity (code given below) which just conatains a FrameLayout, so I can load my Fragments for that specific Tab in it.
The first Fragment has some TextViews and one ListView. Data is pulled from a web service. When I click on an ListView's item, I load that item's detail in another Fragment (this also comes from a web service) and replace the current Fragment (with ListView and other controls) with another detail fragment.
To achieve this, I am using android-support-v4.jar library to use Fragments as they were preferred.
Tab A's FragmentActivity's XML:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<FrameLayout
android:id="#+id/updates_frame"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="#drawable/background"/>
</LinearLayout>
Tab A's FragmentActivity Java code:
public class UpdatesFragmentActivity extends FragmentActivity implements
IUpdateNotifier {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.updates);
//Load Initial Fragment into FrameLayout
//I am adding this Fragment to BackStack
Fragment newFragment = new UpdatesFragment();
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.addToBackStack(null);
ft.add(R.id.updates_frame, newFragment);
ft.commit();
}
//This is an Interface method which I call with the clicked "FEED" object to load its detail in another Fragment
#Override
public void onFeedSelected(Feed feed) {
// Instantiate a new fragment.
Fragment newFragment = new FeedDetailFragment(feed);
// Add the fragment to the activity, pushing this transaction
// on to the back stack.
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.replace(R.id.updates_frame, newFragment);
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
ft.addToBackStack(null);
ft.commit();
}
//This is another Interface Method which I call when the user presses "BACK".
//I am trying to load the previously loaded Fragment, which I added to BackStack.
//But this causes reconstruction of the previously loaded fragment. LIST in this case
//which call the web service. I DONT WANT TO CALL SERVICE AGAIN.
#Override
public void onBackPressed() {
FragmentManager fm = getSupportFragmentManager();
if (fm.getBackStackEntryCount() > 0) {
fm.popBackStack();
}
}
}
I have created an interface IUpdateNotifier, which contains two methods:
public void onFeedSelected(Feed feed);
public void onBackPressed();
Parent UpdatesFragmentActivity implements these methods. I call these methods from children Fragments upon following actions.
I call onFeedSelected(Feed feed) from the Fragment which has a ListView. I send the clicked feed item to parent FragmentActivity, so it loads another Fragment which would contain that feed detail.
I call onBackPressed() from the second feed detail Fragment when the user presses a button that is supposed to bring back the first fragment which contained ListView with other controls. As you can see, I try to call FragmentManager's popBackStack() method to bring back that first Fragment...
But the first Fragment gets refreshed and loads all the data from web service.
Actually I cannot get and store data only once nor the updates are frequent on some time intervals. The user can update the list when he wants. Initially, the list loads the top 10 items from the service, and then user can click the "More" button at the end of list if he wants to load more items.
It will load the next 10 items and so on. But I think I can store the retrieved ArrayList in some variable in UpdatesFragmentActivity and then just reassign that ArrayList to the list's adapter instead of loading the data from service, but I don't know how to make Fragment not to call service again.
I want it to behave like when I click on tab 2 and then on tab 1 again. It simply shows the loaded data as if was hidden and does not call the service.
How can I achieve this?

Your design pattern is flawed due to a poor separation of concerns. The updating of data should be decoupled from the UI, therfore when a user goes back to the previous Fragment it should have nothing to do with loading data from a web service.
There are a couple of easy fixes but I do not know what will work best as you have given little context to the problem.
First option would be to introduce a Splash Screen on start up. This Activity would make use of an AsyncTask to download the data you need from the web service. This works well if you only want the data to be downloaded once during the runtime of the app. You would make sure not to add this Activity to the history so when back is pressed from the next activity, the app would then exit.
Another option, which I have used in many apps and the one I prefer, is the use of Alarms via the AlarmManager. You can set a periodic updates at specific time intervals, the AlarmManager even helps you to the point where it contains enumerations of time. The Alarm will trigger a broadcast receiver which will execute your custom code, that will download the data you need from the web service and store it.
There is a tutorial on this approach, which can be found here http://android.arnodenhond.com/tutorials/alarm-notification.
Finally; you should not need to pop the back stack to get around this problem, although you might be doing this for entirely different reasons but it is hard to tell without more info.

Your question is not clear enough, ask more simple and precise questions... And as you say above
popBackStack does not load the last fragment, it's commonly used to pop the entire stack :
fragmentManager.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
before loading another fragment
beginTransaction()
replace() Or add()
commit()
That's all i can think about with that question
Good luck