Develop Reference

How to handle fragment backstack

I'm having problem with fragment. Lets try to understand my issue, I have two fragment A and B. When app start with main activity,i start fragment A as you can see :
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,new MusicFragment())
.commit();
When i click on a button, it starts fragment B
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container,new BarFragment())
.addToBackStack(null)
.commit();
Main problem is after starting fragment B,when i pressed back to go back to fragment A , Fragment A Recreated with new state.
I don' want to recreate fragment A. I only want to start fragment from old state where i left. How to fix it ?

Instead of calling the replace method you should be calling the add method with a subsequent call to addToBackStack with an argument of null. The add method adds the fragement to the stack with a null tag and calling the addToBackStack with an argument of null then the current fragment is stopped upon commit. If the method is not called then the current fragment is destroyed and recreated when coming back.
getSupportFragmentManager().beginTransaction()
.add(R.id.fragment_container,new BarFragment())
.addToBackStack(null)
.commit();
You can clearly find it in the documentation quote saying this:
If you don't call addToBackStack() when you perform a transaction that
removes a fragment, then that fragment is destroyed when the
transaction is committed and the user cannot navigate back to it.
Whereas, if you do call addToBackStack() when removing a fragment,
then the fragment is stopped and is later resumed if the user
navigates back.

Here are some points that need to be taken care while creating fragments.
Check the backstack if the fragment is already created.
If it was created previously pop it from the backstack and put it on top so that it is visible to the user.
If fragment is not present in backstack crate it and store it in backstack.
Create a method like below which will handle such situation.
private void openFragment(Fragment fragment_to_be_opened){
String fragment_to_be_opened_name = fragment_to_be_opened.getClass().getName();
FragmentManager manager = getSupportFragmentManager();
// fetching the fragment if it is present in backstack
boolean fragment_allready_present = manager.popBackStackImmediate (fragment_to_be_opened_name, 0);
if (!fragment_allready_present){ //fragment is not present in backstack so create it and save the name in //backstack
FragmentTransaction fragment_trasition = manager.beginTransaction();
fragment_trasition.replace(R.id.fragment_container, fragment_to_be_opened);
fragment_trasition.addToBackStack(fragment_to_be_opened_name);
fragment_trasition.commit();
}
}
Now call this method from the activity to open a new fragment like
// create instance of fragment and pass it to the open fragment method
Fragment myFragment = new myFragment();
openFragment(myFragment);

How to get current fragment from MainActivity

I've done some research but I really couldn't find the answer.
I have single activity with side menu, and holder. I have many (non support) fragments, and all of them are using same holder (one at a time).
When user uses menu (it's in main activity), and goes another page, I want to add name of the current fragment to backstack (using .addToBackStack(Fragment1.class.getName())), but I couldn't find how to get current fragment.
I don't want to implement interface etc to keep track of current fragment. There is a super simple way using fragmentManger isn't there?

You can get your current fragment like this:
if (getFragmentManager().getBackStackEntryCount() > 1) {
Fragment f = getFragmentManager().findFragmentById(R.id.content_frame);
if (f instanceof BlankFragment) {
// Do something
}
}

OK,
If you want to get latest entry from backstack(thanks to #AndroidGeek);
fragmentManager.getBackStackEntryAt(fragmentManager.getBackStackEntryCount()-1);
and, if you want to get currently active fragment (thanks to #Salman500 #AndroidGeek);
Fragment f = getFragmentManager().findFragmentById(R.id.fragment_holder);

you can use this to get fragment id for non support fragment
Fragment fragment = getFragmentManager().findFragmentById(R.id.fragment_id);
if(fragment!=null)
{
getFragmentManager()
.beginTransaction()
.addToBackStack(null)
.commit();
}

You can keep track of fragments in the main activity (with variables) and access them there. Example:
FragmentManager manager = getSupportFragmentManager();
FragmentTransaction transaction= manager.beginTransaction();
MyFragment myFragment = new MyFragment();
myFragment.doSomething();
Adding to the back-stack:
FragmentTransaction fragment = getSupportFragmentManager().beginTransaction();
fragment.addToBackStack(fragment);
fragment.commit();
This is answered here: get currently displayed fragment

Just use addToBackStack() before you commit() the fragment trancsaction. See it here
So your code will look like :
...
fragmentTransaction.replace(R.id.holder, newFragmentToShow, newFragmentTag);
fragmentTransaction.addToBackStack();
fragmentTransaction.commit();
...
EDIT : after OP was edited
You do not need the fragment class to call addToBackStack() as you have mentioned in the OP. The String argument is an optional string just to keep the state for the backstack state. You should see the documentation
It is all internally managed and the current active fragment is automatically added to the backStack, you may call it from where ever you want, it will always use current active fragment.

How to save state with navigation drawer and fragments

I have an application with a navigation drawer, one activity and many fragments. When I change the device orientation my main activity is recreate and it's my main fragment who appears, not the current fragment. Just like when I launch the app.
How can I restore the current fragment and maybe the data associated with the selected fragment ?
I Have seen this post but it doesn't work : Save fragment state with navigation drawer
Thanks in advance to those who can help me.

I've found that the fragment states are already persisted for you.
This is what I do: I check if savedInstanceState is null. If it is, I execute the FragmentManager transaction to display the main fragment. If it is not null, it means the fragment state is persisted. I don't do any kind of transaction, and the fragment is recreated for me from the persisted state.

if (savedInstanceState == null){
//Launch HomeFragment onStart...
Fragment fragment = new HomeFragment();
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.replace(R.id.frame, fragment).commit();
}
I added this to onCreate in my Main Activity

Why am I getting Fragment not Attached to Activity Error?

I have a thread which triggers a callback method in my Activity which has a fragment. The Activity then calls a method within the Fragment to refresh the UI with new info. In that method, getString() is called. When getstring() is called, I get a 'Fragment not Attached to Activity error'. This UI refreshing error is guarded runOnUIThread. The fragment is added like so:
FragmentManager fm = getSupportFragmentManager();
fm.beginTransaction()
.add(fragmentContainerID, fragment, tag)
.commit();
fm.executePendingTransactions();
The activity uses two different layout files for horizontal and vertical orientation. Their are two fragment containers in both layouts. They have the same names in both horizontal and vertical layouts, but they do not match each other. The fragments are added using the above code the first time the Activity is created. On orientation changes, the fragments are automatically added into the layouts because the fragment containers have the same ids in both vertical and horizontal orientation.
When the savedInstanceState is not null, I get a reference to the fragments by using
fm.findFragmentByTag
Can anyone please tell me what I am doing wrong ? The fragments are visibly added to the Activity perfectly fine, and several minutes go by before this thread callback occurs, so I don't see how the Fragment is not "attached".
EDIT: Sigh, after some debugging Ive found that adding fragments in this way is perfectly fine. Code elsewhere in the Activity ( I registered for a callback twice) caused my fragment not attached error. If anyone reading this is getting this error, keep in mind that this issue can be triggered from elsewhere. Also, you have to setRetainInstance to true in the fragment to avoid it being recreated. Tags alone don't do that.

Fragments are added one-per-container so if you are using .add(fragmentContainerID, fragment, tag) ensure firstly that fragmentContainerID is different for each of the fragments added.
However, these sorts of issue are most often caused by a misunderstanding in how orientation changes are handled. When you change orientation and let the oncreate handle the adding: you get a new fragment of the same type what this means is that if your background task has a reference to your fragment - it has a reference to the old (now detached) fragment. What you need to do is to make sure the same fragment gets added to the tag (not a new instance). To do that just don't (re)create a fragment instance when we are only restoring state
public class MyActivity ... {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
FragmentManager fm = getSupportFragmentManager();
if (savedInstanceState == null) {
MyFragment frag = new MyFragment();
frag.setArguments( getIntent().getExtras() ); //if it takes params
fm.beginTransaction().add(fragmentContainerID, frag, tag).commit();
}
//after this point a call to MyFragment frag = (MyFragment) fm.findFragmentByTag(tag); should always return the correct fragment
}
It's probably also a wise idea for your task to look for the fragment right when it does its callback to update the UI (and NOT to hold on to a reference to it). Depending on how you've set up your task it just means changing from FragmentManager fm = getSupportFragmentManager(); then the task is created, to calling it as part of the onPostExecute

When change orientation, fragment automatically added to FragmentManager and forget own tag!!
Use this code to find your fragment:
FragmentManager fragmentManager= getSupportFragmentManager();
for (Fragment fragment : fragmentManager.getFragments())
{
if(fragment instanceof MyFragment)
myFragment = (MyFragment)fragment;
}

After you find the fragments on orientation change, try detaching them and then reattaching them before any operation. Or if that does not work remove them and add them. then continue..... I have even had to sometimes put a time delay of a few millisec after detach and before attach to ensure all OK.

FragmentTransaction add() behavior

Working with fragments I've always used replace() for my transactions, but I wish I didn't have to save instance states anymore to restore a fragment's view and prevent reloading when coming back to that fragment. So, I've decided to work with add(). The thing is when I add another fragment, the previous fragment view remains in the background and that's fine (that's the behavior I expected), but the problem is I can actually interact with the views in the background. Example:
Fragment A has a Button
Fragment B has a TextView
When I add Fragment A and later add Fragment B, I'm able to click on Fragment A's Button, even staying on Fragment B's view.
I'm using:
FragmentTransaction fragmentTransaction =
getSupportFragmentManager().beginTransaction().
add(getRootViewContainer(),fragment,fragment.getClass().getSimpleName());
if (shouldGoBack)
fragmentTransaction.addToBackStack(fragment.getClass().getSimpleName());
where getRootViewContainer() returns the id of the FrameLayout I'm using as my activity main container.
Now, is it really the default behavior of add()?
If so, is there a proper way to avoid this or one just has to use replace()?

What you can do here is just hide previous fragment at the time of transaction of current fragment.
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
Fragment newFragment= new MyFragment ();
ft.hide(CurrentFragment.this);
ft.show(newFragment);
ft.commit();
It worked for me just try it.

FragmentTransaction.hide(fragmentBehind); //works for me!
example :
//I have it globally available
FragmentTransaction trans = MainActivity.getManager().beginTransaction();
//not globally
FragmentTransaction trans = getFragmentManager().beginTransaction();
MapFragment newFragment = new newFragment();
trans.add(R.id.fragmentContainer, newFragment, tag);
trans.hide(this);
trans.addToBackStack(tag);
trans.commit();

Yes, this is a default behaviour of add().
If you really don't want to user replace(), you can try to disable views which are inside "old" fragment.