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Re-inflating view in handler.post when getView() is null

I recently replaced all the deprecated AsyncTask code in my apps with handlers and newSingleThreadExecutors. After retrieving response data from a remote server, I update the UI in the handler.post section of the code.
I've never personally been able to reproduce any problems with this, but on some devices (mostly oppo's, redmi's, vivo's, etc) under some real-world conditions, getView() returns null and my stop-gap attempt to re-inflate the view fails. The number of crashes has increased by a lot:
Exception java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
Rough outline of my code:
public class ResultFragment extends Fragment {
#Override
public void onAttach(#NonNull Context context) {
super.onAttach(context);
}
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
return inflater.inflate(R.layout.result, container, false);
}
public void onViewCreated(#NonNull View view, Bundle savedInstanceState) {
Bundle bundle = this.getArguments();
assert bundle != null;
String query_url = bundle.getString("query_url");
send_request(query_url);
}
void send_request(String... urls) {
Handler handler = new Handler(Looper.getMainLooper());
Executors.newSingleThreadExecutor().execute(() -> {
.....
handler.post(() -> {
context = getContext();
final TextView mTextView;
final WebView mWebView;
if (getView() != null) {
mTextView = getView().findViewById(R.id.count);
mWebView = getView().findViewById(R.id.result);
} else {
View view = LayoutInflater.from(context).inflate(R.layout.result, null); <-- crash
mTextView = view.findViewById(R.id.count);
mWebView = view.findViewById(R.id.result);
}
My understanding from the lifecycle documentation is that I should be able to get the view with this code. And I do understand that trying to re-inflate the code like this is a dangerous proposition (crashes might occur!). But how do I do so when getView() returns null?
As I say, I've never been able to replicate these crashes. So I'm open to trying anything that might work.
For general information, I'm targeting sdk version 33.

Your code will crash if the fragment view (or the entire Fragment instance) is destroyed as a result of user leaving your app or screen.
A quick fix for your issue is to cancel the handler runnable execution when the fragment view is destroyed.
#Override
public void onDestroyView() {
super.onDestroyView();
handler.removeCallbacksAndMessages(null);
}
For future development of your app you should look over more advance API to deal with this sort of issue. Ex: RxJava, Kotlin coroutines to name a few.
Small tip that will probably help you to reproduce the crash on any device/emulator - activate the developer option: "Don't keep activity", press the button that makes the network request and then immediately close the application. The thread will post a runnable that will execute after the fragment/view-fragment is destroyed -> NPE crash.

You wouldn't do this at all. If you don't have a view, reinflating it isn't going to do what you expect. It would, at best, create a new set of views that are in memory only and not displayed on the screen. In other words it would be pointless.
Also, that's not what your problem is. You problem is that the context is null. Your fragment isn't attached to any. In this case, what you probably want to do is update any persisted state (if any) and skip updating the UI.
Also, if you're inflating your UI normally on an excutor that then posts to a handler- stop. THat's not how it works. THe inflation should happen in the onCreateView function. You can fill in the views like that, but you would NEVER inflate them like that.

Android activity and fragment lifecycle issue?

I have the following stack trace for a crash caused by an NPE:
Fatal Exception: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.daybreak.my.app/com.daybreak.my.app.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ViewSwitcher.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2430)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)
at android.app.ActivityThread.access$900(ActivityThread.java:153)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1358)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5456)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:735)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
Caused by java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ViewSwitcher.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.daybreak.my.app.TimesFragment.onLocationChange(TimesFragment.java:446)
at com.daybreak.my.app.MainActivity.onLocationChange(MainActivity.java:289)
at com.daybreak.my.app.MainActivity.onCreate(MainActivity.java:112)
at android.app.Activity.performCreate(Activity.java:6302)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2383)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)
at android.app.ActivityThread.access$900(ActivityThread.java:153)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1358)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5456)
at java.lang.reflect.Method.invoke(Method.java)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:735)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
The way I have setup my app is as follows:
MainActivity
public class MainActivity extends AppCompatActivity implements LocationChangeListener {
#Override
public void onCreate(Bundle savedInstanceState) {
//...
onLocationChange(LocationManager.getSavedLocation(this)); // Manually calling onLocationChange() method
if (findViewById(R.id.fragment_container) != null) {
if (savedInstanceState != null) return;
showFragment(new TimesFragment(), TimesFragment.TAG);
}
}
#Override
public void onLocationChange(Locatin location) {
if (location == null) return;
//...
// Call attached onLocationChange() if it implements LocationChangeListener
Fragment f = getSupportFragmentManager().findFragmentById(R.id.fragment_container);
if (f instanceof LocationChangeListener)
((LocationChangeListener) f).onLocationChange(location);
}
}
TimesFragment
public class TimesFragment extends Fragment implements LocationChangeListener {
private ViewSwitcher viewSwitcher;
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
//...
viewSwitcher = (ViewSwitcher) view.findViewById(R.id.view_switcher);
}
#Override
public void onLocationChange(Location location) {
this.location = location;
viewSwitcher.setOnClickListener(null); //<-- NPE Cause here
updateContent();
}
}
MY EXPECTATION
From what I understand, the Activity.onCreate() will only be called after a fresh start or after the user navigates back to the app after the app has been killed (explicitly by the user or by memory management when other apps need memory). If this happens the fragments will also be destroyed and will need to be created, i.e., fragment's onCreateView() will be called. Therefore calling onLocationChange() from the MainActivity.onCreate() before attaching the fragment is safe as findFragmentById() within the onLocationChange() would not find any fragment.
REALITY
From the stack trace we can see that the call initiated from MainActivity.onCreate(). But what's puzzling for me is that at the time onLocationChange() is called from within MainActivity.onCreate(), findFragmentById() within the onLocationChange() finds the fragment in the view container and calls the fragments onLocationChange(). When this happens the viewSwitcher is NULL and causes the app to crash.
Obviously, fragment has already been added to the view container and the fragments onCreateView() has not been called yet.
THE QUESTION
I am not able to recreate this crash, and not sure of the lifecycle process that is causing this.
So can anyone tell me
how to reproduce this error and
the lifecycle process that is responsible for the flow that is causing the NPE?

This is being caused by device rotation. Can recreate the stack trace by rotating the device.
NOTE: This can happen even when app's orientation is locked (as in my case); if the user is in another app in an orientation that is different than the orientation your app is locked to and they switch back to the app, the orientation lifecycle for your app will be fired.
SOLUTION
Add f != null && f.isResumed() before calling methods from the fragment. isResumed() will return false if fragment hasn't been resumed after recreation.

Recursive entry to executePendingTransactions (Without fragments inside fragments)

Let me preface this post by saying that Ive searched about this issue a ton and found no answer that are targeted to my problem. Other questions about this involve Fragments inside Fragments, but that is not the case for me.
My setup is very simple. Activity hosts a ViewPager that uses a FragmentPagerAdapter with numerous fragments that are the same type.
This crash seems to be exclusive to Android 5.1.1.
Im using the support library for Fragments and call getSupportFragmentManager() for use with the adapter. The fragments display some views inside a RecyclerView.
Heres the full crash log for those interested:
java.lang.RuntimeException: Unable to destroy activity {workout.progression.lite/workout.progression.lite.ui.PerformanceTargetActivity}:
java.lang.IllegalStateException: Recursive entry to executePendingTransactions
at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:4852)
at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:4870)
at android.app.ActivityThread.access$1600(ActivityThread.java:197)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1735)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(Activity)
I dont perform anything in onPause except for unregistrering a Bus (Otto).
#Override
protected void onPause() {
BusProvider.getInstance().unregister(this);
super.onPause();
}

#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
// use below code will like U
// if(info.constellation.trim().equals("")) {
// tv_age.setText(info.age);
// } else {
// tv_age.setText(info.age + " " + info.constellation);
// }
// should be like this, no if()...else
tv_age.setText(info.age + " " + info.constellation);
}

When is fragment finally attached to activity?

I have a main fragment with a viewpager inside it. This viewpager has 2 pages (list fragments). When I start the activty, the main fragment is shown and I also show the first paged fragment. This paged fragment displays data from a db using AsyncTask.
In the main fragment I have:
#Override
public void onViewCreated(View view, Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
onPageSelected(0);
}
#Override
public void onPageSelected(int position) {
Fragment fragment = (Fragment) pagerAdapter.instantiateItem(viewPager, position);
if (fragment instanceof IPagedFragment) {
((IPagedFragment) fragment).onShown(getActivity());
}
}
And the interface is:
public interface IPagedFragment {
void onShown(FragmentActivity activity);
}
The first issue I have is that I have to pass the activity as a parameter because when onShown gets called, the activity is still null.
Furthermore, the paged fragments use progressbar logic similar to the LoginActivity sample. I also get the following exception:
IllegalStateException: Fragment PagedFragment1{4201f758} not attached to Activity
at android.support.v4.app.Fragment.getResources(Fragment.java:620)
So what is the correct stage to start retrieving data from db once the paged fragment is fully available to the UI?

Issues like yours is the reason some developers are starting to question if fragments are really that good or useful.
Also "the correct" is debatable as you can do it in a variety of places and different developers will give you different answers, But let me try to supply you some useful info.
The attach/detach callbacks:
public void onAttach(Activity activity);
public void onDetach();
between those two methods any call to getActivity() will return the non-null activity the fragments is connected to. You can override them and use a private boolean isAttached to keep track of that call.
Also useful is the:
public void onActivityCreated (Bundle savedInstanceState)
this method is called AFTER the Activity.onCreate method. That is very important if you rely on some initialisation that happened there.
Also it's important to remember that on the moment the fragment transaction happens, the Fragment.onCreate happens after the Activity.onCreate and during rotation it happens before it.
As a general rule of thumb I use the Fragment.onStart() / Fragment.onStop() for getting/listening to data. On those calls, all the UI have been created, the fragment is attached to the activity and those callbacks don't get called if there's a dialog/popup (pause/resume does)

From the documentation:
public void onActivityCreated (Bundle savedInstanceState)
[...] tells the fragment when it is fully associated with the new activity instance.
source: http://developer.android.com/reference/android/app/Fragment.html#onActivityCreated(android.os.Bundle)

To get the reference of your activity, create a local object of fragmentActivity and get your activity reference as shown below.
private FragmentActivity fragmentActivity;
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
fragmentActivity=activity;
}

Showing fragment after activity fetches data

I'm fetching data in my activity that is needed by several fragments. After the data is returned, I create the fragments. I was doing this via an AsyncTask, but it led to occasional crashes if the data returned after a screen rotation or the app is backgrounded.
I read up and thought the solution to this was instead using an AsyncTaskLoader. Supposedly it won't callback if your activity's gone, so those errors should be solved. But this now crashes every time because "Can not perform this action (add fragment) inside of onLoadFinished".
How am I supposed to handle this? I don't want my fragments to each have to fetch the data, so it seems like the activity is the right place to put the code.
Thanks!
Edit 1
Here's the relevant code. I don't think the problem is with the code per-se, but more of my whole approach. The exception is pretty clear I shouldn't be creating fragments when I am. I'm just not sure how to do this otherwise.
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getSupportLoaderManager().initLoader(BREWERY_LOADER, null, this).forceLoad();
}
//================================================================================
// Loader handlers
//================================================================================
#Override
public Loader<Brewery> onCreateLoader(int id, Bundle args) {
int breweryId = getIntent().getIntExtra(EXTRA_BREWERY_ID, -1);
return new BreweryLoader(this, breweryId);
}
#Override
public void onLoadFinished(Loader<Brewery> loader, Brewery data) {
if (data != null) {
onBreweryReceived(data);
} else {
onBreweryError();
}
}
#Override
public void onLoaderReset(Loader<Brewery> loader) {
}
...
protected void onBreweryReceived(Brewery brewery) {
...
createFragments();
}
...
protected void createFragments() {
FragmentManager fm = getSupportFragmentManager();
//beers fragment
mBeersFragment = (BreweryBeersFragment)fm.findFragmentById(R.id.beersFragmentContainer);
if (mBeersFragment == null) {
mBeersFragment = new BreweryBeersFragment();
fm.beginTransaction()
.add(R.id.beersFragmentContainer, mBeersFragment)
.commit();
Bundle beersBundle = new Bundle();
beersBundle.putInt(BreweryBeersFragment.EXTRA_BREWERY_ID, mBrewery.getId());
mBeersFragment.setArguments(beersBundle);
}
}
Edit 2
My new strategy is to use an IntentService with a ResultReceiver. I null out callbacks in onPause so there's no danger of my activity being hit when it shouldn't be. This feels a lot more heavy-handed than necessary, but AsyncTask and AsyncTaskLoader neither seemed to have everything I needed. Creating fragments in those callback methods doesn't seem to bother Android either.

From the MVC (Model -- View -- Controller) viewpoint, both the Activity and its fragments are Controller, while it is Model that should be responsible for loading data. As to the View, it is defined by the layout xml, you can define custom View classes, but usually you don't.
So create a Model class. Model is responsible for what must survive a screen turn. (Likely, it will be a static singleton; note that Android can kill and re-create the process, so the singleton may get set to null.) Note that Activities use Bundles to send data to themselves in the future.