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!
Related
I have 2 fragments. My first fragment have button which leads me to second fragment. It has this code:
binding.btnGet5Days.setOnClickListener {
val forecastFragment = ForecastFrag()
val transaction: FragmentTransaction =
parentFragmentManager.beginTransaction()
transaction.replace(R.id.fragment_container, forecastFragment)
transaction.addToBackStack(null)
transaction.commit()
}
In my MainActivity i have this code:
val cityFragment = CityFrag()
val fm: FragmentManager = supportFragmentManager
fm.beginTransaction()
.add(R.id.fragment_container, cityFragment)
.commit()
Fragments are in FragmentContainer,
The problem is when i'm joining second fragment through this button and turn my phone into landscape mode, my first fragment layering to my second fragment. How can i solve it? :)
It's hard to know without seeing your full code, but it's possible the code you posted from MainActivity is adding a fragment on top of the existing stack. When you rotate the device, the Activity is destroyed and recreated, but the FragmentManager maintains its state so you don't lose everything. If your recreated activity code always adds a new fragment instance, you'll end up with what was already there, plus another CityFrag on top
The official recommendation is to use the Jetpack Navigation library, which will handle all that for you. If you don't want to go that far right now, you'll have to do your own checking and creation logic.
One thing you can do is to check if the savedInstanceState Bundle passed into your activity's onCreate is null - if it is, then this is a fresh start, and you can initialise with your first fragment. If it's not null, then your app is being recreated from some saved state, so you should probably let the FragmentManager take care of restoring itself and its back stack.
Otherwise take a look at FragmentManager - there's a bunch of useful methods like getBackStackEntryCount, findFragmentByTag etc. that you can use to work out what state your fragments are in, and if you need to add one or not. Depends on your code!
I am trying to implement a method where I want to return to the previous fragment and destroy the current one. However, when I add the fragment to the Backstack it doesn't get destroyed anymore afterwards.
Is there any way to destroy it? Or maybe to return to the previous fragment without using the Backstack?
Edit:
I want to use the backwards navigation as well.
Firstly if you are using the BackStack, it is not typical to need to specifically manually remove Fragments, which suggests you might want to have another think about your design.
That said, to specifically manually remove a Fragment, Override onBackPressed in your Activity which is showing the Fragments, manually remove the Fragment there.
To make it easy to determine which Fragment is currently showing, you can give it a Tag when you show it. For example:
fragTrans.replace(android.R.id.content, myFragment, "MY_FRAGMENT_X");
Then in the onBackPressed function of your Activity
#Override
public void onBackPressed()
{
FragmentManager fragMan = getFragmentManager();
// Check if that Fragment is currently visible
MyFragment myFragment = (MyFragment)fragMan.findFragmentByTag("MY_FRAGMENT_X");
boolean myFragXwasVisible = myFragment.isVisible();
// Let the Activity pop the BackStack as normal
super.onBackPressed();
// If it was your particular Fragment that was visible...
if (myFragXwasVisible)
{
FragmentTransaction trans = fragMan.beginTransaction();
trans.remove(myFragment).commit();
}
}
Note: When it comes to specifically destroying your Fragment object, that is what Java's garbage collection is for. You don't need to worry about that yourself, Java will take care of destroying it when it needs to. That's the whole point.
Given the application flow show in the graphic and textually described in the following.
Fragment 1 is the lowest fragment but not in the backstack by setting disallowAddToBackStack.
Fragment 2 is pushed onto the stack, using fragmentTransaction.addToBackStack().
A new instance of fragment 1 is pushed onto the stack.
The top most fragment (fragment 1) is popped from the stack.
Activity 2 becomes foreground.
Activity 1 becomes foreground.
Here is the generalized method I use to handle fragments:
private void changeContainerViewTo(int containerViewId, Fragment fragment,
Activity activity, String backStackTag) {
if (fragmentIsAlreadyPresent(containerViewId, fragment, activity)) { return; }
final FragmentTransaction fragmentTransaction =
activity.getFragmentManager().beginTransaction();
fragmentTransaction.replace(containerViewId, fragment);
fragmentTransaction.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_OPEN);
if (backStackTag == null) {
fragmentTransaction.disallowAddToBackStack();
} else {
fragmentTransaction.addToBackStack(backStackTag);
}
fragmentTransaction.commit();
}
Problem
When activity 1 resumes in the last step the lowest instance of fragment 1 also resumes. At this point in time fragment 1 returns null on getActivity().
Question
Why is a fragment which is not the top most on the stack resumed?
If resuming the fragment is correct - how should I handle a detached fragment?
When an Activity is not showing UI and then come to show UI, the FragmentManager associated is dying with all of your fragments and you need to restore its state.
As the documentation says:
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.
In your Activity onSaveInstanceState and onRestoreInstanceState, try saving you Fragment references and then restore them with something like this:
public void onSaveInstanceState(Bundle outState){
getFragmentManager().putFragment(outState,"myfragment", myfragment);
}
public void onRetoreInstanceState(Bundle inState){
myFragment = getFragmentManager().getFragment(inState, "myfragment");
}
Try this out and have luck! :-)
I don't see how this would happen, unless (based on how you described the steps) you've misunderstood how fragmentTransaction.addToBackStack() works: it manages which transactions are placed in backstack, not fragments.
From the android docs:
By calling addToBackStack(), the replace transaction is saved to the
back stack so the user can reverse the transaction and bring back the
previous fragment by pressing the Back button.
So if your step 2 looked something like this in code:
fragmentTransaction.replace(containerViewId, fragment2);
fragmentTransaction.addToBackStack();
fragmentTransaction.commit();
and your step 3:
fragmentTransaction.disallowAddToBackStack()//or just no call to addToBackStack - you do not say
fragmentTransaction.replace(containerViewId, newfragment1);
fragmentTransaction.commit();
At this point, Fragment2 will be removed from the backstack, and your backstack consists of the two Fragment1 instances. in Step 4 you pop the top one, which means you should have the bottommost Fragment1 now at the top.
This explains why it is the resumed fragment if you return to the activity. But not, i'm afraid, why it is apparently detached from its activity.
Android OS can and will create and destroy fragments when it sees fit. This is likely happening when you launch Activity 2 and return to Activity 1. I'd verify for sure that it isn't the actively displayed fragment. What is probably happening is that you are seeing it do some of the creation steps for fragment 1 before it does the creation steps for fragment 2.
As for handling the detached fragments you should take a look at this page. The gist of it is that you should only be using the getActivity in certain fragment functions(Based on the fragment life cycle). This might mean that you have to move some of your logic to other functions.
So I have enabled the setting to destroy actvities when you navigate away from an activity
Settings=>Developer Options=>Don't Keep activites
This should basically replicate an activity or fragment getting garbaged collected and then I have to restore the data via the bundle savedinstancestate.
So I understand how that works. But it seems when I navigate from fragment 1 to fragment 2 and then put the application in the background and then in the foreground(destroying the activity)
Both fragment 1 and fragment 2 show at the same time. In which only fragment 2 should be showing.
I do not know if this is something standard that I have to manage hiding and showing fragments onsavedinstance. Or if something in my code is breaking things. Below is how I push fragments which I hope is helpful:
public void pushFragmentWithAnimation(FragmentManager fm, int parentId, Fragment currentFrag, Fragment newFrag, int animEntry, int animExit) {
hideSoftKeyboard(currentFrag.getActivity());
FragmentTransaction ft = fm.beginTransaction();
// See: http://developer.android.com/reference/android/app/FragmentTransaction.html#setCustomAnimations(int, int, int, int)
ft.setCustomAnimations(animEntry, animExit, animEntry, animExit);
ft.add(parentId, newFrag, String.format("Entry%d", fm.getBackStackEntryCount())).hide(currentFrag).show(newFrag);
ft.addToBackStack(null);
ft.commit();
}
Fragment 1 is still in the backstack because when I press back I only see fragment 1. Let me know if you know why this is happening.
The lifecycle of XML added Fragments and programmatically added Fragments differ enough to make mixing them a bad idea, as explained in detail here.
The easiest way around this is to make all fragments programmatically added by replacing your XML inflated Fragment with a FrameLayout of the same ID, then in your onCreate add
FragmentManager fragMgr = getSupportFragmentManager();
if (null == fragMgr.findFragmentByTag(FRAG_TAG))
{
fragMgr.beginTransaction().
add(R.id.fragment, new Fragment1(), FRAG_TAG).commit();
}
Where FRAG_TAG is any unique string. This ensures that Fragment1 is only created if it is not already in the layout.
I am not entirely sure why this solution works. I assume its related to if the activity gets killed that it does not keep track of which fragment is currently shown and shows all of the fragments. So basically I needed to replace:
ft.add(parentId, newFrag, String.format("Entry%d", fm.getBackStackEntryCount())).hide(currentFrag).show(newFrag);
with
ft.replace(parentId, newFrag, tag);
Then when I create the initial fragment in the main activity. I only would do that when
if(savedInstanceState==null){
My updated code is below: https://github.com/CorradoDev/FragmentTest/tree/2c53f9f42e835da768f61b0233f3ab5b3adf2448
I have an activity where I dynamically replace fragments:
private void goToFragment(Fragment newFragment, String tag) {
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(R.id.fragment_container, newFragment, tag);
ft.addToBackStack(null);
ft.commit();
}
Now, I want to access the views inside the fragment so I can put data (that I have stored in my activity) into them, immediately after calling goToFragment.
The problem is, the fragment's onCreateView isn't called before the fragment is rendered completely, at least to my understanding.
I know overriding the onAttach(Activity activity) in the fragment is one way to go about it, but then I have to cast it specifically to my activity - and I just want to avoid that because I consider it bad practice for the fragment to be dependent on a specific activity.
As far as I can see, Fragment doesn't have any listeners (as a subject) implemented.
So I figure I have to make my own listener (Using the Observer Pattern to make the fragment a subject and the activity an observer), and then calling it whenever the onCreateView or onAttach is done, and then finally calling back to the fragment with the data that needs to be set. However, I need to do this for several fragments so I would have to make a listener for each fragment, which I again think is bad.
Is there any better/easier way to do this?
FragmentTransaction isn't applied instantly after calling commit(). You may force update manually:
...
mFragmentManager.executePendingTransactions();
AFAIK event callbacks' purpose is custom communication with Fragment beyond it's usual lifecycle.
The correct way to do it would be to define an interface for Activity classes wishing to display your Fragment should implement. That way, on onAttach you don't cast to a specific Activity but to your interface.
See for instance: http://developer.android.com/guide/topics/fundamentals/fragments.html#EventCallbacks
You should use onActivityCreated to set the values.
Set references in onCreateView and then set values to them in onActivityCreated.