I know this question is very common and I have read so many different answers but none fits in my problem. In my application, I have an activity and in rhye activity I load a fragment. I also send some data(in the form of Bundle) to the fragment. So my Problem is when the screen is rotated, I save the fragment in onSaveInstanceState Activity method and check in onCreate Method weather savedInstance is null or not and on that basis I load the fragment.
Activity code :
#Override
public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
super.onSaveInstanceState(outState, outPersistentState);
outState.putParcelable(Const.TAG_REQ_CUSTOM,DetailsItems);
outState.putString(Const.TAG_FLOW, Const.TAG_MAIN_FLOW);
getSupportFragmentManager().putFragment(outState,"current_fragment",fragment);
}
onCreate Method :
if (findViewById(R.id.fragment_frame) != null) {
if (savedInstanceState != null) {
// this invoke when screen rotate but the app crash
DetailsItems = savedInstanceState.getParcelable(Const.TAG_REQ_CUSTOM);
String flow = savedInstanceState.getString(Const.TAG_FLOW);
ft = getSupportFragmentManager().getFragment(savedInstanceState,"current_fragment");
mFragmentManager=getSupportFragmentManager();
mFragmentTransaction = mFragmentManager.beginTransaction();
bundle= new Bundle();
bundle.putString(Const.TAG_FLOW, flow);
bundle.putParcelable(Const.TAG_REQ_BOOKING_DETAILS, bookingDetailsItems);
ft.setArguments(bundle);
mFragmentTransaction.replace(R.id.fragment_frame, ft).commit();
}else{
// load fragment on first time
}
}
So my Question is: Where do I have to save the custom Object(in parent Activity or in fragment) ?
When my saved Instance is not null than app crashesh and logs is :
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
You should use ViewModel. ViewModel is specifically made for this purpose.
From the docs:
ViewModel is a class that is responsible for preparing and managing the data for an Activity or a Fragment. It also handles the communication of the Activity / Fragment with the rest of the application (e.g. calling the business logic classes).
use this code in Activity :
if (findViewById(R.id.fragment_frame) != null) {
if (savedInstanceState != null) {
fragment =getSupportFragmentManager().getFragment(savedInstanceState,"current_fragment");
}else{
// load fragment on first time
}
}
and in fragment :
//save custom object
#Override
public void onSaveInstanceState(Bundle outState){
super.onSaveInstanceState(outState);
outState.putParcelable("key",customObject);
}
//now retrieve here
#Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
if (savedInstanceState != null)
customObject= savedInstanceState.getParcelable("key");
}
Take a look at onRetainNonConfigurationInstance() and getLastNonConfigurationInstance()
From docs:
Called by the system, as part of destroying an activity due to a configuration change, when it is known that a new instance will immediately be created for the new configuration. You can return any object you like here, including the activity instance itself, which can later be retrieved by calling getLastNonConfigurationInstance() in the new activity instance.
Related
When my activity containing viewpager is killed by system in background and then restores its state, fragments are correctly created and viewpager adapter can also point to them correctly.
But when I get a fragment reference and try to access its fields, they are all null (checked by using breakpoint).
I checked this by placing breakpoints in fragment onCreateView() and in my activity button's clickListener.
((WelcomeFragment)homeActivityFragmentPageAdapter.getItem(POSITION_HOME)).setdata(myData);
Now this method will through null pointer exception since setdata(data) is internally accessing arraylist field of fragment.
This creates a problem for me since, my activity has to continuously feed network data to the fragment by calling its public method (as suggested by documentation).
How to insure that after state restored; correct instance is pointed in my activity.
Try to use instantiateItem adapter method instead getItem.
((WelcomeFragment)homeActivityFragmentPageAdapter.instantiateItem(mViewPager, POSITION_HOME)).setdata(myData);
Method getItem is overrided method, and common use is creation of child fragments.
EDIT:
In case of the question's scenario, you also need to store the state of FragmentStatePagerAdapter manually:
#Override
public void onSaveInstanceState(Bundle savedInstanceState) {
savedInstanceState
.putParcelable("pages",homeActivityFragmentPageAdapter.saveState());
super.onSaveInstanceState(savedInstanceState);
}
Then you can retrieve the state in oncreate:
#Override
protected void onCreate(final Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (savedInstanceState != null) {
homeActivityFragmentPageAdapter.restoreState(savedInstanceState.getParcelable("pages"),this.getClassLoader());
welcomeFragment = (WelcomeFragment) homeActivityFragmentPageAdapter.instantiateItem(mViewPager, POSITION_HOME);
}
else { //simply create a new instance here}
homeActivityFragmentPageAdapter.addFragmentToAdapter(welcomeFragment);
homeActivityFragmentPageAdapter.notifyDataSetChanged();
}
I'm working on the project initially created by another developer. There is a root Activity (let's call it CustomActivity) with code below.
private static SomeOtherClass instance = null;
#Override
public void onCreate(final Bundle savedInstanceState)
{
super.onCreate(null);
if (!(Thread.getDefaultUncaughtExceptionHandler() instanceof CustomExceptionHandler)) {
Thread.setDefaultUncaughtExceptionHandler(new CustomExceptionHandler());
}
// #see http://stackoverflow.com/questions/19545889/app-restarts-rather-than-resumes)
if (!isTaskRoot() && getIntent().hasCategory(Intent.CATEGORY_LAUNCHER) && null != getIntent().getAction()
&& getIntent().getAction().equals(Intent.ACTION_MAIN)) {
finish();
return;
}
instance = new SomeOtherClass();
this.screen = new CustomFragment();
this.getFragmentManager.beginTransaction()
.replace(R.id.content_frame, screen).commit();
...
}
#Override
protected void onDestroy()
{
this.instance = null;
...
}
static public SomeOtherClass getInstance()
{
return instance;
}
this.screen has a button with CustomActivity.getInstance().methodCall() on tap. And one of beta testers said he just tapped that button and got crash with this method in stacktrace: Attempt to invoke virtual method '...CustomActivity.getInstance().methodCall()' on a null object reference.
I don't understand - how it's possible due to Activity lifecycle.
According to stacktrace, onCreate shouldn't be called after some previous onDestroy. Even if the last one can happen when we were in another Activity (and yep, this.screen is not nullified anywhere), but this.screen fragment can't be rendered without Activity recreation. Am I right?
P.S.: there is no more instance variable management at all and SomeOtherClass has no custom parent class (just default object).
P.P.S.: nope, device wasn't locked / app just launched. Tester worked with it, rotated phone to remove sim card, removed, rotated back and saw crash alert.
P.P.P.S: don't know why null in super.onCreate() but anyhow this Activity has no code to support saved states.
As an alternative to an Intent, i'm saving data in a retained headless Fragment during Activity re-creation (my saved object can be pretty large and it wouldn't fit the size limit of an Intent, and i think this is a faster approach than serializing-deserializing into JSON for example).
I've got the idea from this Google documentation, although my implementation is a bit different.
The Fragment:
public class DataFragment extends Fragment {
private Data data;
#Override
public void onCreate(#Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setRetainInstance(true);
}
public void setData(Data data) {
this.data = data;
}
public Data getData() {
return data;
}
}
I save my data to this Fragment in the onSaveInstanceState() method of my Activity:
#Override
protected void onSaveInstanceState(Bundle outState) {
FragmentManager fm = getSupportFragmentManager();
dataFragment = (DataFragment) fm.findFragmentByTag(TAG_DATA);
if (dataFragment == null) {
dataFragment = new DataFragment();
fm.beginTransaction().add(dataFragment, TAG_DATA).commitNow();
}
dataFragment.setData(myData);
super.onSaveInstanceState(outState);
}
And the relevant part of onCreate():
Data data;
FragmentManager fm = getSupportFragmentManager();
dataFragment = (DataFragment) fm.findFragmentByTag(TAG_DATA);
if (dataFragment == null) {
// the Fragment is not attached, fetching data from DB
DatabaseManager dbm = DatabaseManager.getInstance(this);
data = dbm.getData();
} else {
// the Fragment is attached, fetching the data from it
data = dataFragment.getData();
fm.beginTransaction().remove(dataFragment).commitNow();
}
This works flawlessly on orientation changes.
The problem is, sometimes, when my app is in the background and i'm returning to it, dataFragment.getData() returns null.
In other words, in the following line in onCreate() sometimes data is null:
data = dataFragment.getData();
How is this possible?
It does not throw a NullPointerException, so dataFragment is not null for sure.
Why did its initialized instance variable became null?
What you experience is PROCESS DEATH.
Technically it's also called "low memory condition".
The retained fragment is killed along with the application, but the FragmentActivity recreates your retained fragment in super.onCreate(), so you'll find it by its tag but the data in it won't be initialized.
Put the app in background then press the red X in the bottom left in Android Studio to kill the process. That recreates this phenomenon.
NOTE: After AS 4.0, if you launch your app from AS, then "Terminate" will trigger Force Stop (which does not produce this phenomenon). But if you launch your app from LAUNCHER on the phone after that, then you'll get this phenomenon.
if activity is recreated after it was previously destroyed, you are able to saved your state from the Bundle that the system passes your activity. Both the onCreate() and onRestoreInstanceState() callback methods receive the same Bundle that contains the instance state information.
Obviously yours onCreate() method is called whether the system is creating a new instance of your activity or recreating a previous one, you need to check whether the state Bundle is null before you attempt to read it. If it is null, then the system is creating a new instance of the activity, instead of restoring a previous one that was destroyed.
in oncreate():
Data data;
FragmentManager fm = getSupportFragmentManager();
dataFragment = (DataFragment) fm.findFragmentByTag(TAG_DATA);
**if (dataFragment.getData()== null) {**
// the Fragment is not attached, fetching data from DB
DatabaseManager dbm = DatabaseManager.getInstance(this);
data = dbm.getData();
} else {
// the Fragment is attached, fetching the data from it
data = dataFragment.getData();
fm.beginTransaction().remove(dataFragment).commitNow();
}
I have a Fragment Activity with a FragmentTabHost. I add the fragments to the tab using the following code:
mTabHost.addTab(mTabHost.newTabSpec(tab1Name).setIndicator(tabIndicator1),
EventSettingsStep1Fragment.class, null);
mTabHost.addTab(mTabHost.newTabSpec(tab2Name).setIndicator(tabIndicator2),
EventSettingsStep2Fragment.class, null);
When I switch to different tabs, I'd like to retain all the values (view state, etc) so that I have the same data when I switch back to the tab.
I overrode the onSaveInstanceState method & in there, I added values that I want retained to the bundle.
I ran through the methods being called and I have the following:
Switching from Tab1 to Tab2: Tab1:onPause then Tab2:onCreateView, Tab2:onResume
Switching from Tab2 to Tab1: Tab2:onPause then Tab1:onCreateView, Tab1:onResume
onSaveInstanceState is not being called.
Here is the code for one of my fragments:
public class EventSettingsStep1Fragment extends Fragment implements View.OnClickListener {
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
Log.d(TAG, "onCreateView");
if (savedInstanceState != null) {
Log.d(TAG, "restoring onSavedInstanceState");
Gson gson = new Gson();
event = gson.fromJson(savedInstanceState.getString("event"), Event.class);
}
if (event != null) {
//set views
}
return v;
}
#Override
public void onResume() {
super.onResume();
Log.d(TAG, "onResume");
}
#Override
public void onPause() {
super.onPause();
Log.d(TAG, "onPause");
}
#Override
public void onSaveInstanceState(Bundle outState) {
Log.d(TAG, "onSaveInstanceState");
super.onSaveInstanceState(outState);
Gson gson = new Gson();
outState.putString("event", gson.toJson(event));
}
}
Why is onSaveInstanceState not being called? Is it only triggered through the FragmentActivity?
onSaveInstanceState is not being called because the framework simply reuses the already-existing instance of the fragment. onSaveInstanceState only gets called when the instance is about to be destroyed and then recreated. This happens for example when you rotate the display and force the hosting activity to be recreated.
onSaveInstanceState is also not called when you push a fragment on the backstack of a FragmentManager. You will have to restore the state from the already existing instance, which can be very annoying. See SO questions How can I maintain fragment state when added to the back stack? and Once for all, how to correctly save instance state of Fragments in back stack? for example.
Basically you will have to do what the answers to these questions suggest: continue using the values of your instance variables and do not rely on a saved instance state.
I have two fragments FragmentA and FragmentB.
From Fragment A i update Views in Fragment B by passing custom object as a parameter to a method in Fragment B.
For example below is the Fragment B:
public class FragmentB extends Fragment {
private ArrayList<String> customObj = new ArrayList<String>();
public void updateViews(ArrayList<String> obj) {
customObj = obj;
}
#Override
public void onSaveInstanceState(Bundle outState) {
System.out.println("Custom Object : "+customObj);//custom object is always empty
}
Now, whenever i rotate the screen, the custom object is al;ways empty.
Note
Here I am just updating the views of Fragement B. The object is passed as a parameter from Fragment A to the method updateViews of Fragment B.
Also, i am not looking forward to define static for custom object.
You can do two things
1. Store that custom object somewhere which is not created every time orientation is changed i.e. Application class, Service etc.
2. Store that object inside the Bundle which you get as a parameter inside OnSaveInstanceState() and use that Bundle to set the Object in OnRestoreInstanceState() or OnCreate()...
For Example,
#Override
protected void onCreate(Bundle savedInstanceState) {
Logger.d("Dash onCreate");
super.onCreate(savedInstanceState);
...
// create fragments to use
if (savedInstanceState != null) {
//get your Object
}
if (yourObject != null)
//restore the View using yourObject
...
}
#Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
}
Now in your Custom Class you will have to either implement Serializable or Parcelable for storing that inside a Bundle. Serialization is simple but if you implement Parcellable you have more control on it.
Some other similar threads on SO related to this topic are
Similar Example
Another good example
You can get a similar Example in Android Documentations for Fragments.