为什么 Fragment 的 onCreate() 有时会先于 Activity 的 onCreate() 被调用？

Question

Recently I came across a hard to reproduce issue. The NPE occurs when a fragment tries to initialize ArrayAdapter with the data from Activity. The default list initialized in Activity's onCreate method:

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    // ...
    mAccounts = new ArrayList<>();
    // ...
}

@Override
public List<Account> getAccounts(){
    return mAccounts;
}

The fragment creates a list adapter also in its onCreate() :

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setRetainInstance(true);
    setHasOptionsMenu(true);

    //mAccountProvider is an interface implemented by the activity
    mAccounts = mAccountProvider.getAccounts();

    mAccountAdapter = new AccountAdapter(getActivity(), R.layout.account_list_item, mAccounts);
}

The NPE occurs inside the AccountAdapter when default getCount() method is called. The reason is that mAccounts is null. The issue appears seldom and I wasn't able to reproduce it.

When is it possible that fragment's onCreate() is called before activity's onCreate() ? According to the source code, Fragment's onCreate() is dispatched in the Activity's onCreate() . Why is is it then called after Activity's onCreate() has finished its execution?

Answer 1

The Activities callback isn't called after the Fragments; the Fragment's is called during the Activity's.

Initialise mAccounts before calling super.onCreate() in Activity.onCreate()

Answer 2

onCreate() of your fragment can be called before onCreate() method of your activity has been finished. You have onActivityCreated() callback in your fragment and onCreateView() . You can use any of it - it executes after onCreate method of activity.

Answer 3

This bug can be reproduce by turning on "Dont't keep activities" in developer options. I've encountered similar issue as well and solved using solution provided by FunkTheMonk.

Answer 4

Recently ran in to the problem. See stack trace in the bottom.

As FunkTheMonk have stated, the crash is due to the fact that your customized onCreate() code in your Activity (right after super.onCreate()) is executed after your customized onCreate() code in your Fragment (right after super.onCreate())

If you really need your callback be promisingly executed after the whole Activity's onCreate(), put your code fragments in onActivityCreated() rather than onCreate()

Otherwise the aforementioned condition will happen when your memory is low and the activity is recycled.

Per my experience, when my MyActivity sends an intent, the App crashes once I returned to MyFragment by pressing back key.

Caused by: java.lang.NullPointerException
   at MyFragment.onCreate(MyFragment.java:20)
   at android.support.v4.app.Fragment.performCreate(Fragment.java:1939)
   at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1029)
   at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1248)
   at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1230)
   at android.support.v4.app.FragmentManagerImpl.dispatchCreate(FragmentManager.java:2037)
   at android.support.v4.app.FragmentController.dispatchCreate(FragmentController.java:154)
   at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:289)
   at android.support.v7.app.AppCompatActivity.onCreate(AppCompatActivity.java:58)
   at MyActivity.onCreate(MyActivity.java:129)
   at android.app.Activity.performCreate(Activity.java:5248)
   at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1110)
   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2162)
   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257) 
   at android.app.ActivityThread.access$800(ActivityThread.java:139) 
   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210) 
   at android.os.Handler.dispatchMessage(Handler.java:102) 
   at android.os.Looper.loop(Looper.java:136) 
   at android.app.ActivityThread.main(ActivityThread.java:5086) 
   at java.lang.reflect.Method.invokeNative(Native Method) 
   at java.lang.reflect.Method.invoke(Method.java:515) 
   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785) 
   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601) 
   at dalvik.system.NativeStart.main(Native Method) 

Answer 5

I may be too late to answer but still. I encountered this a few days ago. It happens when you are using dynamic fragments and in the layout you use the fragment tag to display the fragment. Instead use a Layout (preferably FrameLayout). Assuming you were using the support fragment library.

It is worth a try.

Answer 6

As many stated this happens when using <fragment/> tag in xml. I used it for navigation component at first but later Android Studio suggested to change it to <androidx.fragment.app.FragmentContainerView/> .
After applied the change activity was called first and afterwards the fragments.