Develop Reference

getSupportFragmentManager().popBackStackImmediate() works but getSupportFragmentManager().getBackStackEntryAt(index) doesn't

So I've got a main activity that hosts all of the fragments in my app. Let me just say beforehand that every time I open a new fragment, I do it like this:
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(vg.getId(), new MyFragment()); //obviously MyFragment varies from usage to usage but nothing else
ft.addToBackStack("My Fragment's Name");
ft.commit();
MyFragment in this case extends androidx.fragment.app.Fragment, which only has a method getFragmentManager(). It does NOT have getSupportFragmentManager().
If I want to go back to a previous fragment, from the currently shown fragment I would do getFragmentManager().popBackStackImmediate(). However, if I wanted to pop the backstack from the activity, I have to use getSupportFragmentManager().popBackStackImmediate() or else nothing happens. This led me to assume that calling getFragmentManager() from a fragment returned the same reference as calling getSupportFragmentManager() from the activity. However, if I try to run getSupportFragmentManager().getBackStackEntryAt(/* an int */) from the main activity I get a NullPointerException. If I run getSupportFragmentManager().getBackStackEntryCount() from the main activity it always returns zero.
So why is it then that getSupportFragmentManager() in the main activity simultaneously works and doesn't work? Why can I use it to pop the backstack, yet I can't access the backstack itself from the main activity? I'm totally clueless. Help would be appreciated.
EDIT: This is my onBackPressed():
#Override
public void onBackPressed() {
getSupportFragmentManager().popBackStackImmediate();
getWindow().setSoftInputMode(defaultSoftInputMode);
try {
String fragmentName = getSupportFragmentManager().getBackStackEntryAt(getSupportFragmentManager().getBackStackEntryCount() - 1).getName();
Toast.makeText(getApplicationContext(), fragmentName, Toast.LENGTH_SHORT).show(); // Debug to see if the correct name is being shown
} catch (NullPointerException npe) {
// Print to console
// It always catches an error here and I don't know why
}
}
The specific method which throws the exception is getSupportFragmentManager().getBackStackEntryAt(int index). Further inspection shows that this method queries an ArrayList<BackStackRecord> for a value, but the exception occurs because this ArrayList is null.

Resuming a dead app from background is making it behave weirdly

I am building an application which has the following structure.
MainActivity -> Fragment1
-> Fragment2 -> Fragment3
Here Fragment1 is hosted on MainActivity in FrameLayout. when user click any option on Fragment1, I am switching to Fragment2 which hosts Fragment3 on ViewPager.
Problem is, if user moves to android home without closing the app, and in a duration of time the app is killed by Android, and user tries to resume the app from recent app list, Fragment3 is displaying the blank screen.
here is the method which I am using for Fragment transaction
public void requestChangeFragment(Fragment newFragment,String tag,Fragment oldFragment, int anim_in, int anim_out){
if(isStopped()){
return;
}
mFragmentManager = getSupportFragmentManager();
android.support.v4.app.FragmentTransaction fragmentTransaction = mFragmentManager.beginTransaction();
fragmentTransaction.setCustomAnimations(anim_in,anim_out,anim_in,anim_out);
fragmentTransaction.add(R.id.main_frame_layout, newFragment, tag);
Log.i("FragMngr","Animations set in "+anim_in+" out "+anim_out);
fragmentTransaction.addToBackStack(tag);
int count = mFragmentManager.getBackStackEntryCount();
String log="";
fragmentTransaction.commit();
for(int i=0;i<count;i++){
android.support.v4.app.FragmentManager.BackStackEntry backStackEntry = mFragmentManager.getBackStackEntryAt(i);
log += "count "+i+" name "+backStackEntry.getName()+"\n";
}
Log.d("FragMngr","BackStack trace after adding is\n "+log);
if(oldFragment!=null) {
if(oldFragment.isResumed()) {
oldFragment.onPause();
}
}
topFragment = newFragment;
}
requestChangeFragment() is called from running fragments with the help of interface implemented in MainActivity.

After doing some research I identified the problem to be the saved instances which stores the current state of the fragment whenever android garbage collector kills the app when app is not in use. So when I tried to resume the dead app, it moves the fragment that was selected before pausing the app. But the fragment do not have any data which it must have received from the it Main class.
Solution : Clear the saved instances in your MainActivity in onCreate method. Or if you are storing any values in your saved instance state, then perform the operation related to the saved Instance State, clear the value and then call super.onCreate(savedInstanceState)

Understanding FragmentManager and FragmentTransaction lifecycle with regards to Activity

I am trying to get a better understanding of FragmentManager and FragmentTransactions to properly develop my application. It is specifically in regards to their lifecycle, and the long-term effect of committing a FragmentTransaction(add). The reason I have a confusion over it is when I ran a sample Activity, listed at the end of the post.
I purposely created a static FragmentManager variable called fragMan and initially set it to null. It is then checked against in onCreate() if it is null and when null value is seen, the fragMan variable is set to the getFragmentManager() return value. During a configuration change, the Log.d showed that fragma was not null, but the Fragment "CameraFragment" previously added was not found in fragman and the fragman.isDestroyed() returned true. This to me meant that the Activity.getFragmentManager() returns a new instance of a FragmentManager, and that the old FragmentManager instance in fragMan had its data wiped(?)
Here is where the confusion comes in.
1) How is "CameraFragment" still associated in the Activity on a configuration change and is found in
the new instance of FragmentManager?
2) When I hit the back button on my phone to exit the Activity, I then relaunched the sample
Activity using the Apps menu. The CameraFragment was not visible anymore, and the
onCreate() check revealed that fragMan was still not null. I thought that hitting the back button
called the default finish() command, clearing the Activity from memory and that restarting it
would produce the same result as the initial launch of the sample Activity?
Thank you for any and all help you can provide!
public class MainActivity extends Activity
{
static FragmentManager fragMan = null;
FragmentTransaction fragTransaction;
#Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (fragMan != null)
{
Log.d("log", Boolean.toString(fragMan.isDestroyed()));
if(fragMan.findFragmentByTag("Camera Preview") == null)
{
Log.d("log", "Camera Preview not found.");
}
}
else
{
fragMan = getFragmentManager();
fragTransaction = fragMan.beginTransaction();
Fragment cameraFragment = new CameraFragment();
ViewGroup root_view = (ViewGroup) findViewById(android.R.id.content);
fragTransaction.add(root_view.getId(), cameraFragment, "Camera Preview");
fragTransaction.commit();
}

Static variables in Java are kept across Activity creation/destruction - they are associated with the class itself but not a particular instance of the class.
See the official documentation here:
http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html
Your application doesn't end when the user returns to the home screen, it just gets put in a background state. If you force stopped the application and restarted it, then the static FragmentManager will be null.
With regards to CameraFragment, unless you've set setRetainInstance(true), it will get destroyed on an orientation change.
==== EDIT
Here's a more detailed flow of what's happening...
You open the application up for the first time
Activity, say instance A1, gets created and its corresponding FragmentManager instance, FM1, also gets created
You store FM1 as a static variable
You go back to home
Activity A1 and FM1 gets destroyed because of the normal Activity lifecycle, although FM1's reference is still held onto by the static variable. At this point, FM1 loses all the fragments it contains and isDestroyed() will return true.
Starting the app again
New Activity instance A2 gets created along with its new FragmentManager instance FM2

Fragment getActivity() returns null in Activity JUnit test

I wrote Android JUnit test for Activity that instantiates fragments (actually tabs). During the test, when I try to do anything with these tabs, they crash because getActivity() method in them returns null. The actual application (not a test) never shows this behavior and fragment getActivity() always returns the right parent activity there. My test case looks like:
public class SetupPanelTest extends ActivityUnitTestCase<MyAct> {
FSetup s;
public SetupPanelTest() {
super(MyAct.class);
}
protected void setUp() throws Exception {
super.setUp();
startActivity(new Intent(), null, null);
final MyAct act = getActivity();
AllTabs tabs = act.getTabs();
String tabname = act.getResources().getString(R.string.configuration);
// This method instantiates the activity as said below
s = (FSetup) tabs.showTab(tabname);
FragmentManager m = act.getFragmentManager();
// m.beginTransaction().attach(s).commit();
// ... and even this does not help when commented out
assertTrue(s instanceof FSetup); // Ok
assertEquals(act, s.getActivity()); // Failure
}
public void testOnPause() {
// this crashes because s.getActivity == null;
s.onPause();
}
}
The AllTabs creates a fragment, then required, in this way:
FragmentManager manager = getFragmentManager();
Fragment fragment = manager.findFragmentByTag(tabname);
if (fragment == null || fragment.getActivity() == null) {
Log.v(TAG, "Instantiating ");
fragment = new MyFragment();
manager.beginTransaction().replace(R.id.setup_tab, fragment, tabname).commit();
....
Here, all fragments are initially placeholders that are later replaced by the actual fragments:
<FrameLayout
android:id="#+id/setup_tab"
android:layout_width="fill_parent"
android:layout_height="fill_parent" />
The logcat shows that the new fragment has been instantiated. In the same layout, there is also the previously mentioned AllTabs fragment that seems not having this problem (where and how it gets FragmentManager otherwise):
<TabWidget
android:id="#android:id/alltabs"
...
Most impressively, when I call attach directly on the fragment manager obtained on the right activity, this still has no effect. I tried to put five seconds delay (I have read that transaction may be delayed), I tried to call the rest of the test through runOnUiThread - nothing helps.
The question is that is need to do so to attach my fragments to the activity also during the test. I have fragment and I have activity, I cannot attach one to another.

Even if you call .commit() on transaction, it is still not done, fragments are attached only lazily.
FragmentManager m = activity.getFragmentManager();
m.executePendingTransactions();
This finally attaches all fragments to the activity. Seems redundant when running the application itself but required in JUnit test case.

Got exception: fragment already active

I have a fragment;
MyFragment myFrag = new MyFragment();
I put bundle data to this fragment:
Bundle bundle = new Bundle();
bundle.putString("TEST", "test");
myFrag.setArguments(bundle);
Then, I replace old fragment with this one and put on backstack:
//replace old fragment
fragmentTransaction.replace(R.id.fragment_placeholder, myFrag, "MyTag");
//put on backstack
fragmentTransaction.addToBackStack(null);
//commit & get transaction ID
int transId = fragmentTransaction.commit();
Later, I pop backstack with the above transaction ID(transId):
//pop the transaction from backstack
fragmentManager.popBackStack(transId,FragmentManager.POP_BACK_STACK_INCLUSIVE);
Later, I set bundle data as argument again to my fragment(myFrag):
//Got Java.lang.IllegalStateException: fragment already active
myFrag.setArguments(bundle);
As you see, my above code got exception Java.lang.IllegalStateException: fragment already active . I don't understand why myFrag is still active though I have popped the transaction of it from backstack., anyhow, since I got the exception I thought I have no choice but de-active the fragment, So, I did:
Fragment activeFragment = fragMgr.findFragmentByTag("MyTag");
fragmentTransaction.remove(activeFragment);
I am not sure if my above code really can de-active the fragment, since I didn't find how to de-active an fragment. :(
After that, when I try to set bundle data to my fragment myFrag again, I still got the same error:
Java.lang.IllegalStateException: fragment already active
Seems even I removed the fragment, it is still active...Why? How to de-active a fragment?

Reading the setArguments(Bundle args) source will help you understand:
/**
* Supply the construction arguments for this fragment. This can only
* be called before the fragment has been attached to its activity; that
* is, you should call it immediately after constructing the fragment. The
* arguments supplied here will be retained across fragment destroy and
* creation.
*/
public void setArguments(Bundle args) {
if (mIndex >= 0) {
throw new IllegalStateException("Fragment already active");
}
mArguments = args;
}
You cannot use setArguments(Bundle args) again in your code on the same Fragment. What you want to do I guess is either create a new Fragment and set the arguments again. Or you can use getArguments() and then use the put method of the bundle to change its values.

Try removing the previous fragment before adding the new one: https://stackoverflow.com/a/6266144/969325

remove() change fragment status to de-actiive. In your case, you just didn't call commit() after remove(..).
fragmentTransaction.remove(activeFragment);
You would do commit() after remove(), too.
fragmentTransaction.remove(activeFragment).commit();

Had the same issue. I was adding the fragment to backstack. And the error was because I didn't call popbackstack(). Using popbackstack helped me

I'm running into the same issue on Xamarin.android. Here's what the documentation says.
This can only be called before the fragment has been attached to its activity

Just call public method from fragment
if(userFragment==null){
userFragment = new UserFragment();
Bundle bundle = new Bundle();
bundle.putString(Constants.EXTRA_CUSTOMER, result);
userFragment.setArguments(bundle);
}else{
try {
Customer customer = new Customer();
customer.parseCustomer(new JSONObject(result));
userFragment.updateVeiw(customer);
} catch (JSONException e) {
e.printStackTrace();
}
}

First I start with describing why this happens and then I'll come up with the solution I found working... .
This issue happens when Android is removing the fragment from the stack but is not yet finished with removing. In order to check this, you can use the isRemoving() method of the fragment. If false, i.e. the fragment is not active, you can go on with setting the arguments using setArguments(bundle). Otherwise, you can't set arguments to an already active fragment and can only override it by addressing the same arguments using getArguments().putAll(bundle).
To summarize,
if (myFrag.isRemoving()) {
myFrag.getArguments().putAll(bundle);
} else {
myFrag.setArguments(bundle);
}
If you want to avoid this, i.e. removing the fragment at once so there is no active fragment, you might want to use onBackPressed() in onBackStackChangedListener(), which will set the isRemoving() to false.

Check whether your layout current one or old one for example
setContentView(R.layout.activity_main);
Delete old .gradle file in your project file and rebuild gradle file for project.