Develop Reference

Get async response data from Application to Launcher Activity

I'd like to make an initialization request when my app starts up. I then want to use the response in my MainActivity. I don't want to make that request in the Activity and then deal with the Activity lifecycle when the phone gets rotated.
So I was thinking of deriving from Application and making the request there. But what's the best way to send the response data to the my launcher Activity?
Is there a "best practice" solution here?

You could try using a library like Event Bus in order to receive the data inside your activity once your request task is complete. By doing this you wouldn't have to worry about where the call is made from or if your activity is rotated or recreated.
If the data is specificly for your MainActivity I would recommend having the request be triggered from there for the sake of keeping things coupled.

If you're looking for best practices, you shouldn't extend an Application class for this.
There is many ways to persist your request state on screen rotation.
Consider to use a retained Fragment. This approach is deeply discussed:
Understanding Fragment's setRetainInstance(boolean)
Further understanding setRetainInstance(true)
All you need to do is this:
1. Fragment class
public class RequestFragment extends Fragment {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// This will be a guarantee that request is sent only once,
// because fragment won't be recreated on screen rotation:
setRetainInstance(true);
// Pereform sending request here.
}
}
2. Activity class
public class MainActivity extends AppCompatActivity {
private final static TAG_FRAGMENT = "persistent_fragment";
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
FragmentManager fm = getSupportFragmentManager();
// Create fragment if it doesn't exist yet.
if (fm.findFragmentByTag(TAG_FRAGMENT) == null) {
fm.beginTransaction()
.add(new RequestFragment(), TAG_FRAGMENT)
.commit();
}
}
}
But if you strongly decided to perform the request in Application onCreate() method, you have to implement an observable object that responds to activity which is subscribed to it, because you can't access an Activity from the Application class.
You can try this:
1. ResponseObservable class
public class ResponseObservale {
private MainActivity activity;
private Response response;
public void sendRequest() {
// perform your async request here.
}
/*
* Consider this method as a point where the response is delivered.
* It can be done in onPostExecute of AsyncTask or somewhere else,
* depending on your implementation.
*/
public void onResponse(Response response) {
this.response = response;
publishResponse();
}
public void onActivityCreated(MainActivity activity) {
this.activity = activity;
if (response != null) {
publishResponse();
}
}
private void publishResponse() {
if (activity != null) {
activity.obtainResponse(response);
}
}
public void onActivityDestroy() {
activity = null;
}
}
2. Application class
public class MyApplication extends Application {
private ResponseObservable observable;
#Override
public void onCreate() {
super.onCreate();
observable = new ResponseObservable();
observable.sendRequest();
}
public ResponseObservable getObservable() {
return observable;
}
}
3. Activity class
public class MainActivity extends AppCompatActivity {
private ResponseObserbale observable;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MyApplication app = (MyApplication) getApplicationContext();
observable = app.getObservable();
observable.onActivityCreated(this);
}
public void obtainResponse(Response response) {
// Manage your response here.
}
#Override
protected void onDestroy() {
observable.onActivityDestroy();
}
}
Don't forget to declare your Application class in AndroidManifest.xml:
<application
android:name="com.yournamespace.appname.MyApplication"
android:icon="#drawable/icon"
android:label="#string/app_name">

IllegalStateException fragment not attached to activity in onPostExecute after rotation but onDetach not called

I've an AppCompatActivity that uses the NavigationDrawer pattern, managing some fragments. In one of these, that has no setRetainInstance(true), I show a DialogFragment with a ProgressDialog inside and an AsyncTask with this code:
SavingLoader savingLoader = SavingLoader.newInstance(savingLoaderMaxValue);
savingLoader.show(getChildFragmentManager(), SAVING_LOADER_TAG);
new MyAsyncTask().execute();
Where the SavingLoader class is this one:
public class SavingLoader extends DialogFragment {
private static final String MAX_VALUE_TAG = "MAX_VALUE_TAG";
private static final String PROGRESS_VALUE_TAG = "PROGRESS_VALUE_TAG";
public static SavingLoader newInstance(int max_value){
SavingLoader s = new SavingLoader();
Bundle args = new Bundle();
args.putInt(MAX_VALUE_TAG, max_value);
s.setArguments(args);
return s;
}
private ProgressDialog dialog;
public SavingLoader(){}
#Override
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
setCancelable(false);
}
#NonNull
#Override
public Dialog onCreateDialog(Bundle savedInstanceState){
dialog = new ProgressDialog(getActivity(), getTheme());
dialog.setTitle(getString(R.string.dialog_title_saving));
dialog.setMessage(getString(R.string.dialog_message_saving));
dialog.setIndeterminate(false);
int max = (savedInstanceState == null ?
getArguments().getInt(MAX_VALUE_TAG) : savedInstanceState.getInt(MAX_VALUE_TAG));
if (max >= 1){
dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
dialog.setProgress((savedInstanceState == null ?
0 : savedInstanceState.getInt(PROGRESS_VALUE_TAG)));
dialog.setMax(max);
} else dialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);
return dialog;
}
#Override
public void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
outState.putInt(MAX_VALUE_TAG, dialog.getMax());
outState.putInt(PROGRESS_VALUE_TAG, dialog.getProgress());
}
public int getProgress(){
return dialog.getProgress();
}
public int getMax(){
return dialog.getMax();
}
public void incrementProgressBy(int value){
if (dialog.getProgress() + value <= dialog.getMax())
dialog.incrementProgressBy(value);
}
}
In the onPostExecute() method I need to perform some UI update so here's my problem: if I start the dialog and the AsyncTask (like above) and I don't rotate my phone, all works as expected. Same thing if I rotate phone AFTER the onPostExecute() method. But if I rotate my phone WHILE the AsyncTask is still running, when it completes and reach the onPostExecute() method it gives me the IllegalStateException saying that the fragment hosting the AsyncTask and the Dialogfragment is no longer attached to the activity. So I tried to override both the onAttach() and the onDetach() methods (with a simple System.out.println) of my fragment, to see when the onPostExecute() gets called. The result is that when I rotate my phone, I always got this output:
onDetach
onAttach
... (if I rotate more my phone)
onPostExecute
So shouldn't the fragment be attached when the AsyncTask completes? Thank you all for your time and attention.

I've finally managed to solve this problem by stop using AsyncTask and using LoaderManager + AsyncTaskLoader following this article. In short, your fragment must implement the LoaderCallbacks interface and manage the AsyncTaskLoader. A skeleton fragment could be something like this:
public class MyFragment extends Fragment implements LoaderManager.LoaderCallbacks {
#Override
public View onCreateView(LayoutInflater inflater, final ViewGroup container,
Bundle savedInstanceState) {
// Inflate here your view as you usually do and find your components
// For example imagine to have a button tha will fire the task
Button b = (Button) view.findViewById(R.id.my_button);
b.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
// Use this to start task for the first time
getLoaderManager().initLoader(0, null, this);
// .. or this for restart the task, details in
// the provided article
// getLoaderManager().restartLoader(0, null, this);
}
});
// Get fragments load manager
LoaderManager lm = getLoaderManager();
if (lm.getLoader(0) != null) {
// Reconnect to an existing loader
lm.initLoader(0, null, this);
}
// Return your view here
return view;
}
// LoaderCallbacks methods to override
#Override
public Loader onCreateLoader(int id, Bundle args) {
// Create an instance of the loader passing the context
MyTaskLoader loader = new MyTaskLoader(getActivity());
loader.forceLoad();
return loader;
}
#Override
public void onLoadFinished(Loader loader, Object data) {
// Use this callback as you would use the AsyncTask "onPostExecute"
}
#Override
public void onLoaderReset(Loader loader) {}
// Now define the loader class
private static class MyTaskLoader extends AsyncTaskLoader {
public MyTaskLoader(Context context){
super(context);
}
#Override
public Object loadInBackground() {
// Do here your async work
}
}
}

How to detect when a fragment appears on the screen?

How could some part of my code be aware of Fragment instance become visible on a screen?
The following snippet will explain my question.
public class MyApp extends Application {
public static final String TAG = MyApp.class.getSimpleName();
#Override
public void onCreate() {
super.onCreate();
registerActivityLifecycleCallbacks(new ActivityLifecycleCallbacks() {
...
#Override
public void onActivityResumed(Activity activity) {
Log.d(TAG, activity.getClass().getSimpleName() + " is on screen");
}
#Override
public void onActivityStopped(Activity activity) {
Log.d(TAG, activity.getClass().getSimpleName() + " is NOT on screen");
}
...
});
}
Here i can track when any activity within my app appears on the screen. Is there any way to extend this approach on Fragments?
Something like
Activity.getFragmentManager().registerFragmentLifecycleCallbacks();
UPD. I know nothing about activities implementations, do they use fragments at all and how do they use them (injection via xml, ViewPager etc.) The only thing I have within my class is an application context. Let's assume Activity and Fragment implementations are black boxes and i am not able to make any changes.

In your fragment, override onHiddenChanged(...) method:
#Override
public void onHiddenChanged(boolean hidden) {
super.onHiddenChanged(hidden);
if (hidden) {
Log.d(TAG, ((Object) this).getClass().getSimpleName() + " is NOT on screen");
}
else
{
Log.d(TAG, ((Object) this).getClass().getSimpleName() + " is on screen");
}
}
Hope this work for you!

Without touching the Activity or Fragment code and assuming you don't know the tag or layout it is placed in, there is very little that you can do. The best that I can see is that you could get the FragmentManager in ActivityResumed and ActivityStopped callbacks (because here you have an Activity reference) and apply a BackstackChangedListener. This assumes that you use the backstack when changing between fragments.
The issue with what you are asking is that you want lifecycle callbacks for Fragments on the Application level when you have no control over the middle men, the Activities which are already starved for Fragment callbacks. They do most everything through their FragmentManager, and propagate their own lifecycle callbacks down to the Fragments so that the fragments will behave appropriately. The onResume and onPause callbacks in fragments only occur when they are first created or when the Activity experiences those callbacks. There is only one lifecycle callback for Fragments in Activities, onAttachFragment, which if you could override, would give you references to the Fragments that are attached to the Activity. But you said you can't change the Activity or the Fragment, and you want to know when the Fragments are shown.
So if you don't use the backstack, I don't think there's a way to do what you want.

For putting Fragments inside Activity i use SlidingTabLayout which Google uses. Inside it you have ViewPager and some Adapter to populate many Fragments. First of all you have to put this and this files in your project. Then here there is good tutorial for how you can implement SlidingTabLayout.
1) After you have implemented SlidingTabLayout in your Activity, you can detect when and which Fragment becomes visible from Activity:
mSlidingTabLayout.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
//Do nothing
}
#Override
public void onPageSelected(int position) {
if (position == 0) {
//Whenever first fragment is visible, do something
} else if (position == 1) {
//Whenever second fragment is visible, do something
} else if (position == 2) {
//Whenever third fragment is visible, do something
} else if (position == 3) {
//Whenever fourth fragment is visible, do something
}
}
#Override
public void onPageScrollStateChanged(int state) {
//Do nothing
}
});
2) You can detect if Fragment is visible from Fragment itself as i answered here, however this may get called before onCreateView() of Fragment, so check answer in the link:
#Override
public void setUserVisibleHint(boolean visible){
super.setUserVisibleHint(visible);
if (visible){
//when this Fragment is active, do something
}
}
3) You can change also change colors of indicators of each Tab like this from Activity:
mSlidingTabLayout.setCustomTabColorizer(new SlidingTabLayout.TabColorizer() {
#Override
public int getIndicatorColor(int position) {
if (position == 0) {
return getResources().getColor(R.color.orange);
} else if (position == 1) {
return getResources().getColor(R.color.redDimmed);
} else if (position == 2) {
return getResources().getColor(R.color.yellow);
} else if (position == 3) {
return getResources().getColor(R.color.green);
} else {
return getResources().getColor(R.color.redLight);
}
}
#Override
public int getDividerColor(int position) {
return getResources().getColor(R.color.defaultActionBarBg);
}
});

Use same way as activity
set flag in application class to check visiblity of fragment, use below code in fragment
#Override
public void onStart() {
super.onStart();
Log.e( "Fragment is visible", "Fragment is visible");
Application Class.isFragmentShow = true;
}
#Override
public void onPause() {
super.onPause();
Log.e("Fragment is not visible", "Fragment is not visible");
Application Class.isFragmentShow = false;
}
to communicate with fragment you have to call that activity in which fragment added then use below code
MainFragment fragment = (MainFragment) fragmentManager.findFragmentByTag("MainFragment");
fragment.setFilter();

Don't exist a default way to do, but you can make your own Callbacks, I made this and works fine, first need have a BaseFragment class where we'll handle all fragment events.
public class BaseFragment extends Fragment {
private String fragmentName;
private FragmentLifecycleCallbacks listener;
public void registerCallBacks(String fragmentName){
// handle the listener that implement 'MyApp' class
try{
listener = (FragmentLifecycleCallbacks) getActivity().getApplication();
} catch (ClassCastException e) {
throw new ClassCastException("Application class must implement FragmentLifecycleCallbacks");
}
// set the current fragment Name for the log
this.fragmentName = fragmentName;
}
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
if(listener!=null) {
listener.onAttachFragment(fragmentName);
}
}
#Override
public void onResume() {
super.onResume();
if(listener!=null) {
listener.onResumeFragment(fragmentName);
}
}
#Override
public void onStop() {
super.onStop();
if(listener!=null) {
listener.onStopFragment(fragmentName);
}
}
// 'MyApp' class needs implement this interface to handle all the fragments events
public interface FragmentLifecycleCallbacks{
void onStopFragment(String fragmentName);
void onResumeFragment(String fragmentName);
void onAttachFragment(String fragmentName);
}}
On 'MyApp' class implement the interface of BaseFragment
public class MyApp extends Application implements BaseFragment.FragmentLifecycleCallbacks{
public static final String TAG = MyApp.class.getSimpleName();
#Override
public void onCreate() {
super.onCreate();
}
#Override
public void onStopFragment(String fragmentName) {
Log.d(TAG, fragmentName + " is NOT on screen");
}
#Override
public void onResumeFragment(String fragmentName) {
Log.d(TAG, fragmentName + " is on screen");
}
#Override
public void onAttachFragment(String fragmentName) {
Log.d(TAG, fragmentName + " is attached to screen");
}}
And now each Fragment that you have need extends 'BaseFragment' and register to the global listener
public class FragmentA extends BaseFragment {
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_simple, container, false);
// here register to the global listener
registerCallBacks(FragmentA.class.getName());
return rootView;
}}
Hope this helps!

Intercept onWindowFocusChanged() in the activity and propagate that to the interested fragment.

Try this
private Boolean isFragmentVisible()
{
if(getFragmentManager().findFragmentByTag("TAG") != null && getFragmentManager().findFragmentByTag("TAG").isVisible())
{
//The fragment is visible
return true;
}
return false;
}
Alternative way
private Boolean isFragmentVisible()
{
return getFragmentManager().findFragmentByTag("TAG") != null && getFragmentManager().findFragmentByTag("TAG").isVisible();
}

You can know the following with the built in method called "onActivityCreated(Bundle)" this method tells that the fragment has been created thus you get to know that the fragment appears on the screen Click here for reference
Hope it helps

I've looked through what's available without using a base Fragment or Activity class but couldn't find any. I've made an implementation that provides basic (onAdded / onRemoved) functionality for all fragments in your application. It is certainly possible to extend it to report the current state of the fragment (onAttach, onResume, onPause, onDetach, ...).
You can find the code along with a sample here: https://github.com/Nillerr/FragmentLifecycleCallbacks
It works both for non-support library Fragments and support library Fragments through different implementations. The support library class is safer to use and should perform better, because the non-support one uses Reflection to access the fragments, while the support library FragmentManager includes a getFragments() method.

If you are setting a Fragment to your View, you probably have a container where it will be shown. Given that this container is, say, a FrameLayout with id R.id.container, you can do that:
Fragment f = fragmentManager.findFragmentById(R.id.container);
if (f instanceof YourFragment) {
// TODO something when YourFragment is ready
}

Does this interface provide anything helpful to you?
https://github.com/soarcn/AndroidLifecyle/blob/master/lifecycle/src/main/java/com/cocosw/lifecycle/FragmentLifecycleCallbacks.java
It sounds like your best bet if you can't override the Fragment's own onResume() method is to create your own interface that extends ActivityLifecycleCallbacks, then put your logging code in the onFragmentResumed(Fragment yourFragment) method.
You can get a pointer to the Fragment by doing something like this:
int yourFragmentId = 0; //assign your fragment's ID to this variable; Fragment yourFragment.getId();
FragmentManager fm = activity.getFragmentManager();
Fragment f = fm.findFragmentById(yourFragmentId);

whereever u want to check if fragment is visible or not.. just check isMenuVisible() value.
this is fragment's method which i used to check visible fragment when i have to fire some http request from viewpager selected Item.
hope this helps.
in my case i was using this method in onActivityCreated().

In you fragment override method setMenuVisibility If you are using ViewPager and are swiping from left and right, this method is called when the visivility of the fragment gets changed.
Here is a sample from my project
public abstract class DemosCommonFragment extends Fragment {
protected boolean isVisible;
public DemosCommonFragment() {
}
#Override
public void setMenuVisibility(boolean menuVisible) {
super.setMenuVisibility(menuVisible);
isVisible = menuVisible;
// !!! Do Something Here !!!
}
}

Animation listener
I have NOT checked all use cases and there is an unhandled exception. You can play around with it to fit your use case. Please feel free to comment your opinions or use cases it did not solve.
NOTE: You can add fragmentWillDisappear and fragmentDidDisappear by handling for enter in onCreateAnimation.
Parent Fragment:
public class BaseFragment extends Fragment {
private Animation.AnimationListener animationListener;
private void setAnimationListener(Animation.AnimationListener animationListener) {
this.animationListener = animationListener;
}
#Override
public void onAttach(#NonNull Context context) {
super.onAttach(context);
setAnimationListener(new Animation.AnimationListener() {
#Override
public void onAnimationStart(Animation animation) {
fragmentWillAppear(animation);
}
#Override
public void onAnimationEnd(Animation animation) {
fragmentDidAppear(animation);
}
#Override
public void onAnimationRepeat(Animation animation) {
}
});
}
#Override
public Animation onCreateAnimation(int transit, boolean enter, int nextAnim) {
AnimationSet animSet = new AnimationSet(true);
Animation anim = null;
try {
anim = AnimationUtils.loadAnimation(getActivity(), nextAnim);
} catch (Exception error) {
}
if (anim != null) {
anim.setAnimationListener(animationListener);
animSet.addAnimation(anim);
}
return animSet;
}
public void fragmentDidAppear(Animation animation) {
}
public void fragmentWillAppear(Animation animation) {
}
}
Child Fragment:
class ChildFragment extends BaseFragment {
#Override
public void fragmentDidAppear(Animation animation) {
super.fragmentDidAppear(animation);
}
#Override
public void fragmentWillAppear(Animation animation) {
super.fragmentWillAppear(animation);
}
}

Android. Fragment getActivity() sometimes returns null

In developer console error reports sometimes I see reports with NPE issue. I do not understand what is wrong with my code. On emulator and my device application works good without forcecloses, however some users get NullPointerException in fragment class when the getActivity() method is called.
Activity
pulic class MyActivity extends FragmentActivity{
private ViewPager pager;
private TitlePageIndicator indicator;
private TabsAdapter adapter;
#Override
public void onCreate(Bundle savedInstanceState) {
pager = (ViewPager) findViewById(R.id.pager);
indicator = (TitlePageIndicator) findViewById(R.id.indicator);
adapter = new TabsAdapter(getSupportFragmentManager(), false);
adapter.addFragment(new FirstFragment());
adapter.addFragment(new SecondFragment());
indicator.notifyDataSetChanged();
adapter.notifyDataSetChanged();
// push first task
FirstTask firstTask = new FirstTask(MyActivity.this);
// set first fragment as listener
firstTask.setTaskListener((TaskListener) adapter.getItem(0));
firstTask.execute();
}
indicator.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageSelected(int position) {
Fragment currentFragment = adapter.getItem(position);
((Taskable) currentFragment).executeTask();
}
#Override
public void onPageScrolled(int i, float v, int i1) {}
#Override
public void onPageScrollStateChanged(int i) {}
});
}
AsyncTask class
public class FirstTask extends AsyncTask{
private TaskListener taskListener;
...
#Override
protected void onPostExecute(T result) {
...
taskListener.onTaskComplete(result);
}
}
Fragment class
public class FirstFragment extends Fragment immplements Taskable, TaskListener{
public FirstFragment() {
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
return inflater.inflate(R.layout.first_view, container, false);
}
#Override
public void executeTask() {
FirstTask firstTask = new FirstTask(MyActivity.this);
firstTask.setTaskListener(this);
firstTask.execute();
}
#Override
public void onTaskComplete(T result) {
// NPE is here
Resources res = getActivity().getResources();
...
}
}
Maybe this error happens when applications resumed from background. In this case how I should handle this situation properly?

It seems that I found a solution to my problem.
Very good explanations are given here and here.
Here is my example:
pulic class MyActivity extends FragmentActivity{
private ViewPager pager;
private TitlePageIndicator indicator;
private TabsAdapter adapter;
private Bundle savedInstanceState;
#Override
public void onCreate(Bundle savedInstanceState) {
....
this.savedInstanceState = savedInstanceState;
pager = (ViewPager) findViewById(R.id.pager);;
indicator = (TitlePageIndicator) findViewById(R.id.indicator);
adapter = new TabsAdapter(getSupportFragmentManager(), false);
if (savedInstanceState == null){
adapter.addFragment(new FirstFragment());
adapter.addFragment(new SecondFragment());
}else{
Integer count = savedInstanceState.getInt("tabsCount");
String[] titles = savedInstanceState.getStringArray("titles");
for (int i = 0; i < count; i++){
adapter.addFragment(getFragment(i), titles[i]);
}
}
indicator.notifyDataSetChanged();
adapter.notifyDataSetChanged();
// push first task
FirstTask firstTask = new FirstTask(MyActivity.this);
// set first fragment as listener
firstTask.setTaskListener((TaskListener) getFragment(0));
firstTask.execute();
}
private Fragment getFragment(int position){
return savedInstanceState == null ? adapter.getItem(position) : getSupportFragmentManager().findFragmentByTag(getFragmentTag(position));
}
private String getFragmentTag(int position) {
return "android:switcher:" + R.id.pager + ":" + position;
}
#Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
outState.putInt("tabsCount", adapter.getCount());
outState.putStringArray("titles", adapter.getTitles().toArray(new String[0]));
}
indicator.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageSelected(int position) {
Fragment currentFragment = adapter.getItem(position);
((Taskable) currentFragment).executeTask();
}
#Override
public void onPageScrolled(int i, float v, int i1) {}
#Override
public void onPageScrollStateChanged(int i) {}
});
The main idea in this code is that, while running your application normally, you create new fragments and pass them to the adapter. When you are resuming your application fragment manager already has this fragment's instance and you need to get it from fragment manager and pass it to the adapter.
UPDATE
Also, it is a good practice when using fragments to check isAdded before getActivity() is called. This helps avoid a null pointer exception when the fragment is detached from the activity. For example, an activity could contain a fragment that pushes an async task. When the task is finished, the onTaskComplete listener is called.
#Override
public void onTaskComplete(List<Feed> result) {
progress.setVisibility(View.GONE);
progress.setIndeterminate(false);
list.setVisibility(View.VISIBLE);
if (isAdded()) {
adapter = new FeedAdapter(getActivity(), R.layout.feed_item, result);
list.setAdapter(adapter);
adapter.notifyDataSetChanged();
}
}
If we open the fragment, push a task, and then quickly press back to return to a previous activity, when the task is finished, it will try to access the activity in onPostExecute() by calling the getActivity() method. If the activity is already detached and this check is not there:
if (isAdded())
then the application crashes.

Ok, I know that this question is actually solved but I decided to share my solution for this. I've created abstract parent class for my Fragment:
public abstract class ABaseFragment extends Fragment{
protected IActivityEnabledListener aeListener;
protected interface IActivityEnabledListener{
void onActivityEnabled(FragmentActivity activity);
}
protected void getAvailableActivity(IActivityEnabledListener listener){
if (getActivity() == null){
aeListener = listener;
} else {
listener.onActivityEnabled(getActivity());
}
}
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
if (aeListener != null){
aeListener.onActivityEnabled((FragmentActivity) activity);
aeListener = null;
}
}
#Override
public void onAttach(Context context) {
super.onAttach(context);
if (aeListener != null){
aeListener.onActivityEnabled((FragmentActivity) context);
aeListener = null;
}
}
}
As you can see, I've added a listener so, whenever I'll need to get Fragments Activity instead of standard getActivity(), I'll need to call
getAvailableActivity(new IActivityEnabledListener() {
#Override
public void onActivityEnabled(FragmentActivity activity) {
// Do manipulations with your activity
}
});

The best to get rid of this is to keep activity reference when onAttach is called and use the activity reference wherever needed, for e.g.
#Override
public void onAttach(Context context) {
super.onAttach(context);
mContext = context;
}
#Override
public void onDetach() {
super.onDetach();
mContext = null;
}
Edited, since onAttach(Activity) is depreciated & now onAttach(Context) is being used

Don't call methods within the Fragment that require getActivity() until onStart in the parent Activity.
private MyFragment myFragment;
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
myFragment = new MyFragment();
ft.add(android.R.id.content, youtubeListFragment).commit();
//Other init calls
//...
}
#Override
public void onStart()
{
super.onStart();
//Call your Fragment functions that uses getActivity()
myFragment.onPageSelected();
}

I've been battling this kind of problem for a while, and I think I've come up with a reliable solution.
It's pretty difficult to know for sure that this.getActivity() isn't going to return null for a Fragment, especially if you're dealing with any kind of network behaviour which gives your code ample time to withdraw Activity references.
In the solution below, I declare a small management class called the ActivityBuffer. Essentially, this class deals with maintaining a reliable reference to an owning Activity, and promising to execute Runnables within a valid Activity context whenever there's a valid reference available. The Runnables are scheduled for execution on the UI Thread immediately if the Context is available, otherwise execution is deferred until that Context is ready.
/** A class which maintains a list of transactions to occur when Context becomes available. */
public final class ActivityBuffer {
/** A class which defines operations to execute once there's an available Context. */
public interface IRunnable {
/** Executes when there's an available Context. Ideally, will it operate immediately. */
void run(final Activity pActivity);
}
/* Member Variables. */
private Activity mActivity;
private final List<IRunnable> mRunnables;
/** Constructor. */
public ActivityBuffer() {
// Initialize Member Variables.
this.mActivity = null;
this.mRunnables = new ArrayList<IRunnable>();
}
/** Executes the Runnable if there's an available Context. Otherwise, defers execution until it becomes available. */
public final void safely(final IRunnable pRunnable) {
// Synchronize along the current instance.
synchronized(this) {
// Do we have a context available?
if(this.isContextAvailable()) {
// Fetch the Activity.
final Activity lActivity = this.getActivity();
// Execute the Runnable along the Activity.
lActivity.runOnUiThread(new Runnable() { #Override public final void run() { pRunnable.run(lActivity); } });
}
else {
// Buffer the Runnable so that it's ready to receive a valid reference.
this.getRunnables().add(pRunnable);
}
}
}
/** Called to inform the ActivityBuffer that there's an available Activity reference. */
public final void onContextGained(final Activity pActivity) {
// Synchronize along ourself.
synchronized(this) {
// Update the Activity reference.
this.setActivity(pActivity);
// Are there any Runnables awaiting execution?
if(!this.getRunnables().isEmpty()) {
// Iterate the Runnables.
for(final IRunnable lRunnable : this.getRunnables()) {
// Execute the Runnable on the UI Thread.
pActivity.runOnUiThread(new Runnable() { #Override public final void run() {
// Execute the Runnable.
lRunnable.run(pActivity);
} });
}
// Empty the Runnables.
this.getRunnables().clear();
}
}
}
/** Called to inform the ActivityBuffer that the Context has been lost. */
public final void onContextLost() {
// Synchronize along ourself.
synchronized(this) {
// Remove the Context reference.
this.setActivity(null);
}
}
/** Defines whether there's a safe Context available for the ActivityBuffer. */
public final boolean isContextAvailable() {
// Synchronize upon ourself.
synchronized(this) {
// Return the state of the Activity reference.
return (this.getActivity() != null);
}
}
/* Getters and Setters. */
private final void setActivity(final Activity pActivity) {
this.mActivity = pActivity;
}
private final Activity getActivity() {
return this.mActivity;
}
private final List<IRunnable> getRunnables() {
return this.mRunnables;
}
}
In terms of its implementation, we must take care to apply the life cycle methods to coincide with the behaviour described above by Pawan M:
public class BaseFragment extends Fragment {
/* Member Variables. */
private ActivityBuffer mActivityBuffer;
public BaseFragment() {
// Implement the Parent.
super();
// Allocate the ActivityBuffer.
this.mActivityBuffer = new ActivityBuffer();
}
#Override
public final void onAttach(final Context pContext) {
// Handle as usual.
super.onAttach(pContext);
// Is the Context an Activity?
if(pContext instanceof Activity) {
// Cast Accordingly.
final Activity lActivity = (Activity)pContext;
// Inform the ActivityBuffer.
this.getActivityBuffer().onContextGained(lActivity);
}
}
#Deprecated #Override
public final void onAttach(final Activity pActivity) {
// Handle as usual.
super.onAttach(pActivity);
// Inform the ActivityBuffer.
this.getActivityBuffer().onContextGained(pActivity);
}
#Override
public final void onDetach() {
// Handle as usual.
super.onDetach();
// Inform the ActivityBuffer.
this.getActivityBuffer().onContextLost();
}
/* Getters. */
public final ActivityBuffer getActivityBuffer() {
return this.mActivityBuffer;
}
}
Finally, in any areas within your Fragment that extends BaseFragment that you're untrustworthy about a call to getActivity(), simply make a call to this.getActivityBuffer().safely(...) and declare an ActivityBuffer.IRunnable for the task!
The contents of your void run(final Activity pActivity) are then guaranteed to execute along the UI Thread.
The ActivityBuffer can then be used as follows:
this.getActivityBuffer().safely(
new ActivityBuffer.IRunnable() {
#Override public final void run(final Activity pActivity) {
// Do something with guaranteed Context.
}
}
);

#Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
// run the code making use of getActivity() from here
}

I know this is a old question but i think i must provide my answer to it because my problem was not solved by others.
first of all : i was dynamically adding fragments using fragmentTransactions.
Second: my fragments were modified using AsyncTasks (DB queries on a server).
Third: my fragment was not instantiated at activity start
Fourth: i used a custom fragment instantiation "create or load it" in order to get the fragment variable.
Fourth: activity was recreated because of orientation change
The problem was that i wanted to "remove" the fragment because of the query answer, but the fragment was incorrectly created just before. I don't know why, probably because of the "commit" be done later, the fragment was not added yet when it was time to remove it. Therefore getActivity() was returning null.
Solution :
1)I had to check that i was correctly trying to find the first instance of the fragment before creating a new one
2)I had to put serRetainInstance(true) on that fragment in order to keep it through orientation change (no backstack needed therefore no problem)
3)Instead of "recreating or getting old fragment" just before "remove it", I directly put the fragment at activity start.
Instantiating it at activity start instead of "loading" (or instantiating) the fragment variable before removing it prevented getActivity problems.

In Kotlin you can try this way to handle getActivity() null condition.
activity?.let { // activity == getActivity() in java
//your code here
}
It will check activity is null or not and if not null then execute inner code.

Android Update Current Activity From Background Thread

My application has a refresh button on the main activity. When the user presses that button, a new thread is created which starts updating the SQLite database. When this thread started, user could possibly get into another activies of the application.
The problem is these other activities(ListActivity) should be updated according to the DB when that background thread is completed. How could I provide that. I tried getting current task with ActivityManager but It requires extra permission which I dont want.

Edit:
Sorry seems I misunderstood you. Please take a look at the following code, it is similar to Chinaski's (you just use an interface for the callback methods) but I added a bit more to ensure you know how to use it in a way that will avoid memory leaks.
Note how the activity detaches during onDestroy -- alternatively you could use a WeakReference, however these days you'd use a Fragment with setRetainInstance(true) and completely avoid the detaching/attaching as the fragment would be retained.
MyAsyncTask
public class MyAsyncTask extends AsyncTask<Void, Void, Void> {
private Callback mCallback;
private boolean mIsComplete = false;
private boolean mHasCallbackBeenCalled = false;
public MyBackgroundTask(Callback callback) {
mCallback = callback;
}
/** Only safe to call this from the UI thread */
public void attach(Callback callback) {
mCallback = callback;
if (mIsComplete && !mHasCallbackBeenCalled) {
fireCallback();
}
}
/** Only safe to call this from the UI thread */
public void detach() {
mCallback = callback;
}
#Override
public void doInBackground() {
// do the heavy stuff here
return null;
}
#Override
public void onPostExecute(Void result) {
mIsComplete = true;
fireCallback();
}
private void fireCallback() {
if (mCallback != null) {
mCallback.callbackMethod();
mHasCallbackBeenCalled = true;
}
}
public static interface Callback {
public void callbackMethod();
}
}
MyActivity
public class MyActivity extends Activity implements MyAsyncTask.Callback {
private MyAsyncTask mTask;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// Check for a retained task after a configuration change
// e.g. a rotation
if (getLastNonConfigurationInstance() != null) {
mTask = (MyAsyncTask) getLastNonConfigurationInstance();
// Re-attach the task
mTask.attach(this);
}
}
#Override
public void onDestroy() {
// Detach from task to avoid memory leak
if (mTask != null) {
mTask.detach();
}
super.onDestroy();
}
#Override
public Object onRetainNonConfigurationInstance() {
// Retain the async task duration a rotation
return mTask;
}
/** Callback method */
#Override
public void callbackMethod() {
// Do something here
}
}

You could make a singleton in which you will have your thread and a queue of "tasks". When a task is finished, you check / launch the next task, and when you add a task, you launch it, or add it in the queue if a task is already running.
I don't say this is the best solution, but it's one.