Android FragmentTransaction.addToBackStack confusion - android

I was studying Fragments and got little confused on differentiating FragmentTransaction.replace(id, fragment, tag) and FragmentTransaction.addToBackStack(tag) calls. Lets say that my current fragment is FragmentA and then I loaded FragmentB. I want that in future, when I need to load FragmentA, I don't have to reload it. Just load the old one in old state. I used the following code segment:
public void loadFragment(Fragment fragmentB, String tag) {
FragmentManager fm = getSupportFragmentManager();
View fragmentContainer = findViewById(R.id.fragment_container);
FragmentTransaction ft = fm.beginTransaction();
ft.replace(fragmentContainer.getId(), fragmentB, tag);
ft.addToBackStack(tag);
ft.commit();
}
Now I am confused, where should I add the string tag? In replace() or in addToBackStack() or in both calls? Can you explain the difference between these two tag places?

Can you explain the difference between these two tag places?
The documentation for addToBackStack is pretty clear:
An optional name for this back stack state, or null.
While for replace:
Optional tag name for the fragment, to later retrieve the fragment
with FragmentManager.findFragmentByTag(String).
So these two parameters are independent, one identifies the back stack, while the other identifies the fragment within Activity's FragmentManager.
Your code seems correct from this point of view, just that I would not search the fragmentContainer view by its id, only to use then its id for replacing the fragment. Make it simpler:
public void loadFragment(Fragment fragmentB, String tag) {
FragmentManager fm = getSupportFragmentManager();
FragmentTransaction ft = fm.beginTransaction();
ft.replace(R.id.fragment_container, fragmentB, tag);
ft.addToBackStack(null);
ft.commit();
}
In case you don't need to identify this back stack later on, pass null for addToBackStack. At least I'm always doing it.

In this example you don't need to add tags as identification.
Just do:
ft.replace(R.id.fragment_container,fragmentB);
ft.addToBackStack(null);
ft.commit();
The tag as identification is commonly used when you want to add a fragment without a UI.

Passing null to addtoBackStack(null) means adding the fragment in the Fragment Stack but not adding any TAG which could be further use to identify the particular fragment in a stack before adding again.
.addToBackStack(null);
But passing TAG to addToBackStack helps in identifying the fragment in Fragment stack by TAG.
Like
.addToBackStack(FragmentName.TAG);
Now we can check the fragment before adding it to the Stack :
getFragmentManager().findFragmentByTag(SettingsFragment.TAG);
This will return null if the Fragment is not already added.

Param passed to addToBackStack() can be used to retrieve the whole BackStackEntry object, not just a single fragment. In order to set the fragment tag, consider using 3-param versions of add(int, Fragment, String) and replace(int, Fragment, String)
Prior to adding the Fragment you will be able to check if this Fragment is already in the backstack using :
getFragmentMangager().findFragmentByTag(SettingsFragment.TAG);
This will return null if the Fragment is not already added.

Related

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.

What does FragmentManager and FragmentTransaction exactly do?

I have simple code below
FragmentManager fragmentManager = getFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, mFeedFragment);
fragmentTransaction.addToBackStack(null);
fragmentTransaction.commit();
What do these lines of code do?
getFragmentManager()
Return the FragmentManager for interacting with fragments associated
with this activity.
FragmentManager which is used to create transactions for adding, removing or replacing fragments.
fragmentManager.beginTransaction();
Start a series of edit operations on the Fragments associated with
this FragmentManager.
The FragmentTransaction object which will be used.
fragmentTransaction.replace(R.id.fragment_container, mFeedFragment);
Replaces the current fragment with the mFeedFragment on the layout with the id: R.id.fragment_container
fragmentTransaction.addToBackStack(null);
Add this transaction to the back stack. This means that the
transaction will be remembered after it is committed, and will reverse
its operation when later popped off the stack.
Useful for the return button usage so the transaction can be rolled back.
The parameter name:
Is an optional name for this back stack state, or null.
See for information the other question What is the meaning of addToBackStack with null parameter?
The Last statement commits the transaction and executes all commands.
See the google documentation for more help:
http://developer.android.com/reference/android/support/v4/app/FragmentActivity.html
http://developer.android.com/reference/android/app/FragmentManager.html
http://developer.android.com/reference/android/app/FragmentTransaction.html
Android FragmentManager
A FragmentManager manages Fragments in Android, specifically it handles transactions between fragments. A transaction is a way to add, replace, or remove fragments.
Android FragmentTransaction
As said before a FragmentTransaction gives us methods to add, replace, or remove fragments in Android. It gives us an interface for interacting with fragments.
fragmentTransaction.replace(R.id.fragment_container, mFeedFragment);
The method replace(int containerViewId, Fragment fragment) replaces an existing Fragment object from the container containerViewId and adds the the Fragment fragment
fragmentTransaction.addToBackStack(null);
This method, addToBackOfStack(String name), adds this transaction to the back stack, this can be used so that Fragments are remembered and can be used again by the Activity
fragmentTransaction.commit();
The method commit() schedules this transaction, this is not instantaneous; It is scheduled on the main thread to be done when the thread is ready.
Reference
For more readability and simplified transaction You can define simple function or use simple static functions in class
FragmentTransaction :
https://github.com/mahditavakoli1312/FragmentTransaction---mahdi-tavakoli

Preloading a fragment when popBackStack

I have a problem with the backstack behaviour. That is what I am doing:
add(fragment1) + addToBackStack(null)
replace(fragment2) + addToBackStack(null)
What is happening:
Fragment 1 is added and in the backstack
Then the second fragment replaces the first one and it is added to the backstack.
Now I want to change my last backstacked fragment with a new transaction which put a new backstack fragment so:
[frag1, frag2] becomes [frag1, frag3]
but this transaction made by a popBackStack + replace is making the frag1 to load by calling its onCreateView and onActivityCreated. I know this is the expected behaviour since this is how backstack works, but I am trying to find a way to avoid this preload.
Edit
In this question I am using the concept of backstack fragment for the transaction to be more clear. Every transaction here is an add+remove (which is a replace).
The code for replace I am using is:
public int replaceFragment(BaseFragment newFragment, boolean addToBackStack, boolean animated, PopStackMode popMode) {
if (popMode != null) {
getSupportFragmentManager().popBackStack(newFragment.getFragmentTag(), popMode == PopStackMode.POP_INCLUSIVE ? FragmentManager
.POP_BACK_STACK_INCLUSIVE : 0);
}
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
if (animated) {
ft.setCustomAnimations(R.anim.slide_in_left, 0, R.anim.slide_out_right, 0);
}
ft.replace(R.id.fragment_container, newFragment, newFragment.getFragmentTag());
if (addToBackStack) {
ft.addToBackStack(newFragment.getFragmentTag());
}
return ft.commit();
}
You can see I am creating a navigation history based on the fragment backstack, as it was kind of a browser. When a "page" is added there is a fragment and a backstack transaction. In this context, I trying to:
Remove the current fragment.
Remove the transaction from the backstack.
Add a new fragment without poping and loading the previous backstack fragment.
I hope it is more clear.
Edit 2
I have filled a request feature for a flag that supports this behavior. Find it here.
First, you should understand that the backstack doesn't save fragments, but it saves transactions instead. When you call popBackStack what it actually does is revert the previous transaction. More on this here.
I think that you can do this:
Name your transactions by providing a unique name to your addToBackStack instead of null. i.e. addToBackStack("frag1").
Don't call popBackStack + replace, but instead just call replace.
Then, in your activity, override your onBackPressed and if the current fragment being displayed is Frag3 (you can check this using findFragmentByTag if you provided a tag in the replace method) you can call getSupportFragmentManager().popBackStackImmediate("frag1", FragmentManager.POP_BACK_STACK_INCLUSIVE); (otherwise call the super.onBackPressed)

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.

Android - How to resume fragment from back stack if it exist?

Hello I am working with sliding menu with fragment.
Indivisual fragment works properly. But suppose user navigates From fragment A->B , now 'B' works perfect and now if user goes from B->A than fragment 'A' is called from onAttach() .
I want such a condition if any fragment is opened , than reopening it should not load whole fragment , it should be resumed just like we handle activity with Intent.FLAG_ACTIVITY_REORDER_TO_FRONT.
Here is my code...
FragmentManager fm = MainActivity.this.getSupportFragmentManager();
FragmentTransaction ft = fm.beginTransaction();
Layout1 fragment = new Layout1();
ft.add(R.id.activity_main_content_fragment, fragment,Layout1.class.getName());
ft.addToBackStack(Layout1.class.getName());
ft.commit();
Answer updated:
Reading the documentation, there is a way to pop the back stack based on either the transaction name or the id provided by commit. Using the name may be easier since it shouldn't require keeping track of a number that may change and reinforces the "unique back stack entry" logic.
Since you want only one back stack entry per Fragment, make the back state name the Fragment's class name (via getClass().getName()). Then when replacing a Fragment, use the popBackStackImmediate() method. If it returns true, it means there is an instance of the Fragment in the back stack. If not, actually execute the Fragment replacement logic.
private void replaceFragment (Fragment fragment){
String backStateName = fragment.getClass().getName();
FragmentManager manager = getSupportFragmentManager();
boolean fragmentPopped = manager.popBackStackImmediate (backStateName, 0);
if (!fragmentPopped){ //fragment not in back stack, create it.
FragmentTransaction ft = manager.beginTransaction();
ft.replace(R.id.content_frame, fragment);
ft.addToBackStack(backStateName);
ft.commit();
}
}
If you return to a fragment from the back stack it does not re-create the fragment but re-uses the same instance and starts with onCreateView() in the fragment lifecycle, see Fragment lifecycle.
So if you want to store state you should use instance variables and not rely on onSaveInstanceState()
Check this link. it will help How to resume Fragment from BackStack if exists

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