android恢复在堆栈中的片段的实例状态

Question

Hi I have an activity that has two fragments.

1. Fragment A

2. Fragment B

   • Fragment A has an EditText and a ListView.
   • Once I enter something in the EditText and hit Enter , I populate the ListView.
   • Now Fragment A's ListView is populated with data.
   • Clicking any item on the ListView of fragment A will send the user to fragment B
   • At this point I am replacing fragment A with fragment B
   • So when the user hits back button he comes back to fragment A

Now the problem is if the user is in fragment B and config changes occur like screen rotation etc more than once then my that my ListView is empty because my arraylist is null.

Note that I am using onSavedInstanceState in fragment A and fragment B. If my current fragment is fragment A and config changes occur then there is no problem of restoring the state since in onCreateView I am getting the arraylist from bundle.

I know the reason why my arraylist is null when I comeback from fragment B to fragment A which is in backstack previously.When fragment A is in backstack the only method that is being called is onSaveInstanceState so after the first config change my arraylist field is null as I couldnt assign my arraylist stored in savedInstateState bundle to the arraylist field.

I do not want to use android:configchanges attribute in my manifest.

My Question is how can I restore the state of a fragment that is in backstack.

Answer 1

I achieved the goal of retaining the state of fragment which is in backstack by passing all the details of that fragment to main activity before it is added to backstack and saving the state in main activity's savedInstance state and whenever backstack changes and previous fragment is visible I am passing the saved instance state of that fragment to it from main activity and this way I am retaining the state. I am not sure if this the right approach.

Answer 2

I have gone through your scenario. Basically you are doing right job when you are adding arraylist in savedinstanceState of your fragment but according to me you are doing little mistake in your activity.

As when orientation changes Activity call onDestroy then onCreate in that scenario if you are not checking that either your fragment is already added or not then it will replace it with new one. Here is code you should put in onCreate()

 FragmentManager fragmentManager = getFragmentManager();
    Fragment fragment = fragmentManager.findFragmentByTag("FragmentA");
    if(fragment==null){
        fragmentManager.beginTransaction().add(R.id.container, new FrgamentA(), "FragmentA").commit();
    }

Another point is when you want your first fragment to be retained add

setRetainInstance(true);

in onCreate of fragment.

Please find this demo at GitHub inpulse-fragment

In this demo at first fragment when you will increase the counter it'll appear in text view above and if you click on that text view you will be navigate to Fragment B. Now you can try rotating. Let me know in case of any query.

Answer 3

您可以在onAttach使用setRetaininstance(true) ，并在活动清单文件中使用ConfigChanges = "screenSize|keyBoardHidden|orientation"这行。