Develop Reference

Managing Fragments on Android Foldable

I am creating a new Android Project and soon android foldable devices will be launched. I have an Activity which has fragment called first fragment.
First Fragment has a button called first button which open second fragment which has a button called second and on click of second, third fragment opens.
Suppose user is in third fragment and user decides to unfold his device, will the user go back to fragment one or will he stay in fragment three. As far as I have understood from the Developer Summit, the activity will be destroyed and recreated when user unfolds his device so technically user goes backs to first fragment leading to poor user experience.
So my question is should I consider even using fragments?, If yes how to manage state so that user goes to the same fragment he was when he folds or unfolds his device.
Following is my code if I am changing fragments
private fun displayView(fragment: Fragment?, title: String) {
if (fragment != null) {
supportFragmentManager.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE)
supportFragmentManager.beginTransaction()
.replace(R.id.framelayout_activity_main, fragment, title).commit()
}
}

In onCreate(), you only want to execute a FragmentTransaction if this activity is being
newly created, instead of being recreated from a configuration change. Or, more accurately,
you only want to execute a FragmentTransaction if you do not already have fragments in the state that you want them.
So, a typical approach is to see if you already have a fragment in your container:
override fun onCreate(state: Bundle) {
super.onCreate(state)
if (supportFragmentManager.findFragmentById(R.id.framelayout_activity_main) == null) {
// do something to show your fragment
}
// other good stuff goes here
}
On the first onCreate() invocation, findFragmentById() will return null, so you execute your code to display your first fragment. On a subsequent onCreate() invocation after a configuration change, Android will have already set up your fragment(s) for you by the time onCreate() is called. So, in that case, findFragmentById() will return something other than null, so you know that you already have a fragment in your container and do not need to do anything more.

Fragment does not call lifecycle methods

I've a fragment A. I add() it with tag like this:
fragmentTransaction.addToBackStack(special_tag);
Then I simply add() fragment B on top of fragment A. After that, I decide to remove fragment B and go back to fragment A using:
activity.fragmentManager.popBackStackImmediate(special_tag, 0)
When I reach the fragment A, it seems that fragment doesn't re-run it's lifecycle methods: onAttach(), onResume(), onCreate() ect.
Can someone explain this behavior and maybe suggest an alternative?
(I need to "refresh" the data when I come back to fragment A second time)
What is causing this result?
Is there a clean solution/work-around?
Update
Fragment B is GuidedStepFragment and does not have a .replace() function. I found that it has finishGuidedStepFragments(), but it behaves the same (it does not call fragment life cycle functions)
Situation (again):
Fragment A (Simple fragment) -> .add(Fragment B) (GuidedStepFragment) -> popBackStackImmediate() or finishGuidedStepFragments()
I add Fragment B like this:
GuidedStepFragment.add(activity.fragmentManager, fragmentB.createInstance())

Using fragmentTransaction.add(Fragment) doesn't remove Fragment A. What is actually happening is that Fragment A is still running behind Fragment B. Since Fragment A never stopped running, it's lifecycle has no need to retrigger.
Consider using fragmentTransaction.replace(Fragment) and replace the fragment in the container (fragment A) with fragment B. If you pop that transaction from the back stack, then Fragment A will reattach and follow your expected lifecycle.
Update
Since you seem to be using GuidedStepFragments from the leanback library, this is a little tricky. GuidedStepFragment actually performs replace(...) under the hood, but you're adding fragment B to a different container so the original behavior I mentioned doesn't apply.
I'm not super familiar with leanback (since it's usually only used for android tv), but I do know that you can at least do the following. If you keep track of your backstack size, when all of the GuidedStepFragments have been popped, you will have returned to your original fragment. For example, let's assume your backstack starts at zero:
activity.fragmentManager.addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener() {
#Override
public void onBackStackChanged() {
if (activity.fragmentManager.getBackStackEntryCount() == 0){
// handle your updates
}
}
});
// the next line of code will add an entry to the backstack
GuidedStepFragment.add(activity.fragmentManager, fragmentB.createInstance());
// eventually when back is pressed and the guided fragment is removed, the backstack listener should trigger

How to avoid multiple instances of fragments in Activity after app is killed and resumed?

I have an app with a Home screen that has 2 fragments (for now) and a navigation drawer. Currently I load the fragment A (Explore) on startup and load fragment B when clicked. From then on, I show and hide fragments. It's faster than recreating fragments on every click and my fragment A takes some time to load.
I've noticed that when I go to fragment B and go to another activity (let's call it activity 2) from there and leave the app and wait for it to be killed (or do something crazy like change the device language), and then come back to the same activity, it's still there. When I press back to go back to fragment B, sometimes (50% of times) the fragment B is drawn over fragment A. On clicking fragment A in the drawer, fragment A appears fine, but on clicking fragment B, there's another instance of fragment A and on top of that fragment B.
I've spent more than 2 days on this problem and got nowhere.
Here's my code for selecting the fragment:
private void selectItem(int position, boolean addExploreFragment) {
Log.d(tag, "selectItem: " + position);
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
fragmentTransaction.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
//add explore fragment - this is called on app startup, but also when the app is killed and resumed which results in 2 explore fragments
if (addExploreFragment){
fragmentTransaction.replace(R.id.content_frame, mExploreFragment, EXPLORE_FRAGMENT_TAG);
Log.d(tag, "Replaced frame and added "+ mFragmentTags[position]);
} else {
//add fragment for the first time
if (getSupportFragmentManager().findFragmentByTag(mFragmentTags[position]) == null && position != 0) {
fragmentTransaction.add(R.id.content_frame, mFragments[position], mFragmentTags[position]);
Log.d(tag, "Added Fragment: "+ mFragmentTags[position]);
}
//shows and hides fragments
for (int i = 0; i < mFragments.length; i++) {
if (i == position) {
fragmentTransaction.show(mFragments[i]);
Log.d(tag, "Showing Fragment: "+ mFragmentTags[i]);
} else {
if (getSupportFragmentManager().findFragmentByTag(mFragmentTags[i]) != null) {
fragmentTransaction.hide(mFragments[i]);
Log.d(tag, "Hid Fragment: "+ mFragmentTags[i]);
}
}
}
}
fragmentTransaction.commit();
//not null check for calling selectItem(0) before loading the drawer
if (mDrawerList != null){
mDrawerList.setItemChecked(position, true);
}
}
I know for sure, the explore fragment is getting created twice and the two instances behave independently of each other (just sharing).
I'm lost what to do next. This is an issue which can be reproduced very easily on low end devices but on a device like Nexus 4 (my test device), the issue can be reproduced by changing the device language.
Has anyone got any ideas about this? Basically if the addExploreFragment block doesn't get called when there is already an exploreFragment, this issue could be solved, I think, but I've been unable to do so. Also, I tried removing all the fragments and then adding the exploreFragment but same thing happens (50% of times).
Thanks! and sorry for the long post, I felt I should share all the details.
Update: When I change the device language and come back to the app on Activity 2 and go back to Home activity, it has the fragment B open which is good, but fragment A get recreated because it's a heavy fragment and the system probably removed it from memory. Again, that's ok that it gets recreated IF it got removed by the system but why does it get recreated when it's not removed. I believe it's something with my code, on every 2nd attempt (without closing the app) this happens, 2 instances of the heavy fragment A. Out of ideas.
But shouldn't fragmentTransaction.replace remove all the previously added fragments and then add exploreFragment. It's not working like that. Neither fragment A nor Fragment B are getting removed.

I found out something new and rather odd to me. When you use fragmentTransaction.add, the listeners you have, like DrawerItemClickListener, on the previous fragment, are still active. And this is even if you use fragmentTransaction.commit.
So...I suspect when the add method is used, you actually clicked on another hidden button or hidden UI that has an event listener on the previous fragment. I don't like this of course and the effect may be very confusing. Yes, this happened to me and I didn't understand why for a while.
For now, I think the easiest code fix would be to use the replace method instead of add. The replace() makes listeners inactive. If it works, then you can make a better/elegant fix.
Let me know what happens....

I started to notice your post
when I go to fragment B and go to another activity
When you interact or start another Activity, you start a new set of Fragments. Look at this Google webpage # Fragments Lifecycle.
For clarification of my claim, there is a quote saying
A fragment must always be embedded in an activity and the fragment's
lifecycle is directly affected by the host activity's lifecycle.
You might as well read few paragraphs of it, at least.
I am not sure what your solution should be. Perhaps make the fragments distinctive, different and clear between the two Activities you have.

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.

How to pop back stack for Activity with multiple Fragments?

Assume I have an Activity which contains two FrameLayouts (let's call them FrameA and FrameB) which in turn each contain a Fragment (let's call them FragmentA1 and FragmentB1 respectively). Now, I commit a series of individual fragment transactions using code similar to the following...
getFragmentManager()
.beginTransaction()
.replace(frameId, fragment)
.addToBackStack(null)
.commit();
... such that I replace FragmentA1 in FrameA with FragmentA2, then I replace FragmentB1 in FrameB with FragmentB2, then I replace FragmentA2 in FrameA with FragmentA3, then I replace FragmentB2 in Frame2 with FragmentB3, and the final state looks like the picture above (where only FragmentA3 and FragmentB3 are visible).
If I understood correctly how the back stack works, pressing 'back' will interleave popping of the Fragments between FrameA and FrameB (reflecting how I added them).
Does anyone know if it is possible to pop the last transaction on FrameA or FrameB selectively? (i.e. if I pressed 'Pop FrameA' then FrameA would be transitioned back from FragmentA3 to FragmentA2 and, instead, if I pressed 'Pop FrameB' then FrameB would be transitioned back from FragmentB3 to FragmentB2)
Supplement: I know I can get the Fragment last added to a given FrameLayout using the FragmentManager.findFragmentById(int framelayoutId) method, but calling FragmentTransaction.remove(fragment).commit() only removes the Fragment from the View and does not transition the View back to the Fragment it previously displayed.

Basically, no, there is only one back stack for an activity.
You will just need to implement your own separate back stacks.
As of Android 4.0 (and the associated support library) there are APIs that should make this relatively easy -- FragmentTransaction.detach(Fragment) lets you put a fragment into the same state it is when in the back stack, and FragmentManager.saveFragmentInstanceState(Fragment) lets you go further and completely throw away the Fragment object. Not coincidentally, these are used to implement ViewPager's FragmentPagerAdapter and FragmentStatePagerAdapter, respectively, so you could look at the code for these as an example of how to use them.

FragmentManager.popBackStack(String name, FragmentManager.POP_BACK_STACK_INCLUSIVE)
Here is the simplest answer, and the explanation is very clear: Well there are a few ways to go about this depending on the intended behavior, but this link should give you all the best solutions and not surprisingly is from Dianne Hackborn...