I am trying to redesign some fragments to remove dependencies from the onAttach and onActivityCreated overrides and instead look up the Activity later on in the onViewCreated override.
Are there any cases in the Android application lifecycle where onViewCreated for the fragment is called before Activity onCreate finishes. For example I know that:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
will usually not result in Fragments onAttach being called (assuming the fragment is added to the fragment manager programmatically later on), however in rare cases with configuration updates the fragment manager can recreate the fragments in the super.onCreate which causes the onAttach to be called before onCreate for the activity has finished.
onCreate()
The system calls this when creating the fragment. Within your implementation, you should initialize essential components of the fragment that you want to retain when the fragment is paused or stopped, then resumed.
onCreateView()
Called to create the view hierarchy associated with the fragment.
The system calls this when it's time for the fragment to draw its user interface for the first time. To draw a UI for your fragment, you must return a View from this method that is the root of your fragment's layout. You can return null if the fragment does not provide a UI.
Refer Fragments Life Cycle
Related
I need to recreate a fragment when user press a toggle. I'm using this function:
public void refreshFragment(){
FragmentTransaction ft = getFragmentManager().beginTransaction();
if (Build.VERSION.SDK_INT >= 26) {
ft.setReorderingAllowed(false);
}
ft.detach(this).attach(this).commit();
}
It works fine, but when calling it, onCreate isn't called, this is the sequence of methods called after recreate:
onCreateView()
onViewCreated()
onResume()
And this this is the sequence of methods called when creating it for the first time:
onCreate()
onCreateView()
onViewCreated()
onResume()
Appearently, onCreate is never called again when refreshing the fragment with that function. This will be as is for ever? Or in some circumstances can change? I'm asking it because it's perfect for me that onCreate only gets called the first time, because then I can put there code that I want to execute only one item and not when refreshing. But I need to know if it's safe or onCreate can be called when refreshing.
onCreate() will only be called when your fragment is being, well, created. Detaching and reattaching don't create new fragments, but onCreate() will also be called if your activity is destroyed and recreated (e.g., on a configuration change like rotation or if your app's process is killed).
I'm using an Activity which has a ViewPager holding 2 fragments, the pager handler is some implementation of FragmentPagerAdapter.
As I understand, pager adapter handles the lifecycle of the fragments inside it.
I found out that my Activity onResume() method already gets called but the fragment onStart() method didn't even started.
how in the world can I fix that? it destroy the whole point of lifecycle interactions between activity an fragments...
Since pager adapter handles the lifecycle of the Fragment, does this means I can no longer depend on interaction with the Activity? I mean, if I want the Activity to do something in the onResume() but after the Fragment onStart() is called, I just can't do it...
Edited:
To make things clear:
Google says lifecycle of activity and fragment are going together, once one gets called, the other also gets called, e.g
Activiy -> onCreate() , and then, Fragment -> onCreate()
Activiy -> onResume() , and then, Fragment -> onResume()
BUT! in my case I get:
Activity -> onCreate() -> onStart() -> onResume() -> onPostResume()
And then:
Fragment -> onAttach() -> onCreateView() -> ... ->onResume().
and to be clear, I am using a pager adapter (not "state" pager) and I have an abstract base activiy in my app which all activities should extend.
public abstract class AbsLoginAppCompatActivity extends AppCompatActivity {
.............
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.d(TAG, "*******************onCreate");
//do some general stuff like check for updates on server
}
And in my extend activity:
public class A extends AbsLoginAppCompatActivity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.e(TAG, "*******************onCreate");
setContentView(R.layout.activity_a);
//also set pager + adapter + give it getSupportFragmentManager()
}
I am using:
android.support.v4.view.ViewPager
android.support.v4.app.FragmentPagerAdapter
android.support.v4.app.Fragment
android.support.v7.app.AppCompatActivity (for abs activity)
The Fragment[State]PagerAdapter uses the activities FragmentManager - or in case of a nested ViewPager in a parent fragment - that fragment's child FragmentManager to manage the fragments, just like normal fragments would do. Really, the only thing that these adapter implementations do is that they hide the nasty FragmentTransaction stuff for you.
I had never problems that particular lifecycle callbacks weren't called for me in my fragments, so I cannot say anything about that. One thing however that is important to understand and that many people get wrong is that the adapter's getItem() method is called only when a fragment is freshly created; if it is restored from a saved state this method is not called again and people tend to do all fancy things there to initialize their just "created" fragment, while they should really look into instantiateItem(), which either returns the instance you give the adapter via getItem() or returns the reference of the fragment that was automatically re-created for you.
Another thing that is good to know about fragments in pager is the method setUserVisibleHint(boolean). Since fragments are usually recreated and resumed all at once (non-state adapter) or on demand (state adapter), its usually important to know when one instance is actually visible to the user. This can be achieved by overriding the aforementioned method in a custom fragment.
In a Fragment's Lifecycle, the onAttach() method is called before the onCreate() method. I can't wrap my head around this. Why would you attach a Fragment first?
TL;DR:
In order to not break the design consistency amongst different UI components in android,the onCreate() method will have similar functionality across all of them.
When linking Containers to Contents like Window to Activity and Activity to Fragment a preliminary check needs to be done to determine the state of container.
And that explains the use and position of onAttach() in the fragment lifecycle.
Too short;Need longer:
The answer is in the archetype code itself,
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
try {
mListener = (OnFragmentInteractionListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString()
+ " must implement OnFragmentInteractionListener");
}
}
Another example would be in Jake Wharton's ActionBarSherlock library.
Why would you want to use a method like onCreate() which is has the same purpose in an activity ,service.
The onCreate() is meant to handle issues with respect to that particular context creation.It does not make sense if onCreate() is used to check the state of its container.
The second reason that I can come determine is that a fragment is designed to be activity independent.The onAttach() provides an interface to determine the state/type/(other detail that matters to the fragment) of the containing activity with reference to the fragment before you initialize a fragment.
EDIT:
An activity exists independently and therefore has a self sustaining lifecycle.
for a fragment :
The independent lifecycle components(same as any other components):
onCreate()
onStart()
onResume()
onPause()
onStop()
onDestroy()
The interaction based components:
onAttach()
onCreateView()
onActivityCreated()
onDestroyView()
onDetach()
from the documentation:
The flow of a fragment's lifecycle, as it is affected by its host
activity, (...) each successive state of the activity determines which
callback methods a fragment may receive. For example, when the
activity has received its onCreate() callback, a fragment in the
activity receives no more than the onActivityCreated() callback.
Once the activity reaches the resumed state, you can freely add and
remove fragments to the activity. Thus, only while the activity is in
the resumed state can the lifecycle of a fragment change
independently.
However, when the activity leaves the resumed state, the fragment
again is pushed through its lifecycle by the activity.
answering another question which came up in the comments:
Caution: If you need a Context object within your Fragment, you can call getActivity(). However, be careful to call getActivity() only when the fragment is attached to an activity. When the fragment is not yet attached, or was detached during the end of its lifecycle, getActivity() will return null.
The design philosophy states that a Fragment is designed for reuse. A fragment (by design) could(and should) be used across multiple activities.
The onCreate by definition is responsible to create a fragment.
Consider the case of orientation,your fragment could be:
- using different layouts in different orientations.
- applicable only in portrait orientation and not landscape
- To be used only on tables and mobile phones.
All these situations would require a check before the fragment is initialized from the android perspective(onCreate()) and the view inflated(onCreateView()).
Also consider the situation of a headless fragment.The onAttach() provides you the interface required for preliminary checks.
Because onAttach() assigns hosting activity to the Fragment. If it had been called after onCreate() then there would be no context for your fragment (getActivity() would return null) and you would not be able to do anything in onCreate() method without that context anyway.
Another fitting reason is that Fragment's lifecycle is similar to Activity's lifecycle. In Activity.onAttach() activity gets attached to its parent (a window). Similarly in Fragment.onAttach() fragment gets attached to its parent (an activity), before any other initialization is done.
This is related to retained fragments. Following Fragment setRetainInstance(boolean retain) documentation:
If set, the fragment lifecycle will be slightly different when an activity is recreated:
onDestroy() will not be called (but onDetach() still will be, because the fragment is being detached from its current activity).
onCreate(Bundle) will not be called since the fragment is not being re-created.
onAttach(Activity) and onActivityCreated(Bundle) will still be called.
Take a look at the source code (android.support.v4.app.FragmentManager, v21):
void moveToState(Fragment f,
int newState,
int transit,
int transitionStyle,
boolean keepActive) {
...
f.onAttach(mActivity);
if (!f.mCalled) {
throw new SuperNotCalledException("Fragment " + f
+ " did not call through to super.onAttach()");
}
if (f.mParentFragment == null) {
mActivity.onAttachFragment(f);
}
if (!f.mRetaining) {
f.performCreate(f.mSavedFragmentState); // <- Here onCreate() will be called
}
...
}
Example
Case 1: not retained fragment or setRetainInstanceState(false)
Application is started. Fragment is added dynamically using FragmentManager or inflated from XML via setContentView().
onAttach() called after Activity super.onCreate() call - Activity is already initialised.
MainActivity﹕ call super.onCreate() before
MainActivity﹕ super.onCreate() returned
MainFragment﹕ onAttach() getActivity=com.example.MainActivity#1be4f2dd
MainFragment﹕ onCreate() getActivity=com.example.MainActivity#1be4f2dd
Configuration changed. Activity recreates fragments from saved state, fragments are added/attached from inside Activity super.onCreate() call:
MainActivity﹕ call super.onCreate() before
MainFragment﹕ onAttach() getActivity=com.example.MainActivity#2443d905
MainFragment﹕ onCreate() getActivity=com.example.MainActivity#2443d905
MainActivity﹕ super.onCreate() returned
Case 2: setRetainsInstanceState(true)
Application is started. Fragment is added dynamically using FragmentManager or inflated from XML via setContentView(). Same as above:
onAttach() called after Activity super.onCreate() call - Activity is already initialised.
MainActivity﹕ call super.onCreate() before
MainActivity﹕ super.onCreate() returned
MainFragment﹕ onAttach() getActivity=com.example.MainActivity#3d54a168
MainFragment﹕ onCreate() getActivity=com.example.MainActivity#3d54a168
Configuration changed.
Fragment onCreate() not called, but onAttach() still called - you need to know, that hosting Activity has changed. But still fragment is already created, so no onCreate() called.
MainActivity﹕ call super.onCreate() before
MainFragment﹕ onAttach() getActivity=com.example.MainActivity#d7b283e
MainActivity﹕ super.onCreate() returned
Two Points from Android developer Site hints at why onAttach() is called before onCreate() in case of Fragment Life cycle.
A fragment must always be embedded in an activity. Now this means for Fragment to EXIST, there has to be a "living" Activity.
Also, When you add a fragment as a part of your activity layout, it lives in a ViewGroup inside the activity's view hierarchy.
So Fragment must FIRST "attach" itself to activity to defines its own view layout
onCreateis Called to do initial creation of a fragment.
It is obvious that you will create something only when its pre-condition of creation is in place (and the pre-condition is A fragment must always be embedded in an activity, and it must be attached to its Activity)
I have been asked an interview question: Can a fragment exist without activity? I searched for answers but didn't get a proper answer and explanation. Can someone help?
Yes, you can do this anywhere:
new YourFragment();
As fragments must have a parameter-less constructor.
However its lifecycle doesn't kick in until it is attached. So onAttach, onCreate, onCreateView, etc. are only called when it is attached. So most fragments do nothing until they are attached.
It can exist as an object in memory (by creating it with new), but it needs to be attached to an Activity in order to appear on the screen, assuming it has any UI (fragments don't have to have UI).
A Fragment can exist independently, but in order to display it, you need the help of an Activity. The Activity will act like a container for the Fragment(s).
A fragment is not required to be a part of the Activity layout; you may also use a fragment without its own UI as an invisible worker for the Activity but it needs to be attached to an Activity in order to appear on the screen.
As soon as you create an instance of the Fragment class, it exists, but in order for it to appear on the UI, you must attach that fragment to an activity because a fragment's lifecycle runs parallel to an activity's lifecycle. Without any call to Activity's onCreate(), there will be no call for onAttach(), onCreate(), onCreateView() and onActivityCreated() of fragment and so it can't be started.
I read above top rated answer , i am not disagreeing but android already provides to make independent fragment without activity DialogFragment , which extends fragment . if you want show in full screen first extends DialogFragment then
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setStyle(STYLE_NO_FRAME, android.R.style.Theme_Holo_Light);
}
Android app must have an Activity or FragmentActivity that handles the fragment.
Fragment can't be initiated without Activity or FragmentActivity.
I really need your help here.
I have two Fragments:
1. Fragment A
2. Fragment B
One Interface
1. onSkillsSelectedListener ( method onDoneClicked )
Fragment A implements onSkillsSelectedListener and when "DONE" button is called in Fragment B, i call onDoneClicked() , and Fragment B is destroyed.
Now method onDoneClicked in Fragment A is called before even OnResume in Fragment A is called so i cannot make any changes in my Layout.
How to fix this issue?
The fragments onResume() or onPause() will be called only when the Activities onResume() or onPause() is called. They are tightly coupled to the Activity.
http://developer.android.com/guide/components/fragments.html#Lifecycle
You need to find another way to implement this.
I FIX IT
Just changed the way i called my Fragment B
from replace to add
So now Fragment A is not even Paused and callback is working!
Cheers!