Retain the Fragment object while rotating

问题:

I have developed an app in Honeycomb and I am using fragments.
This is my app

• I have an Activity (Say A1) and in that there is a fragment
• Initially this fragment hold the object one fragment object say (F1)
• Then depending on the user actions it may change to other objects F2,F3 ....

What my problem is

When The user rotate the device the activity is recreated and which make F1 as the fragment object even though before rotating it wasn't
What is the way to retain the fragment object while rotating?
I used setRetainInstance(true); but it didn't work for me
And I have added the fragment by code in my onCreate function like this

@Override
public void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.main);

   FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction();

   Fragment homeFragment = new Home();
   fragmentTransaction.add(R.id.mainFragement, homeFragment);
   fragmentTransaction.commit();
}

回答1:

By default Android will retain the fragment objects. In your code you are setting the homeFragment in your onCreate function. That is why it is allways some homeFragment or fl what ever that you set in onCreate.

Because whenever you rotate, the onCreate will execute and set your fragment object to the first one

So the easy solution for you is check whether savedInstanceState bundle is null or not and set the fragment object

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

    if(null == savedInstanceState) {
        // set you initial fragment object 
    }
 }

回答2:

You need to give your Fragment a unique tag, and check whether this Fragment is already added to your Activity already or not.

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    String tag = "my_fragment";
    FragmentManager fragmentManager = getFragmentManager();
    if(fragmentManager.findFragmentByTag(tag) == null) {
        FragmentTransaction fragmentTransaction = getFragmentManager().beginTransaction();
        Fragment homeFragment = new Home();
        fragmentTransaction.add(R.id.mainFragement, homeFragment, tag);
        fragmentTransaction.commit();
    }
}

Checking whether savedInstanceState is null is not a safe way to check whether your fragment is already set - it will work in most cases, but in some cases (such as when the device is on low memory), Android may kill your Activity, which could break your application.

To see this in action, tick "Don't keep activities" in the device's development options (the setting is available in Android 4.0+, not sure about earlier versions). When you open a new activity, your first activity is destroyed. When you return to it (by pressing back), it is created again, and savedInstanceState is not null. However, your fragment is not in the activity anymore, and you have to add it again.

EDIT - Showing the original principle but with SupportFragmentManager

public class ActivityAwesome extends AppCompatActivity
{
    private final String TAG = getClass().getSimpleName();
    private FragmentHome mHomeFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_layout);

        FragmentManager fragmentManager = getSupportFragmentManager();
        Fragment fragment = fragmentManager.findFragmentByTag(TAG);
        if(fragment == null)
        {
            // Create the detail fragment and add it to the activity using a fragment transaction.
            mHomeFragment = new FragmentHome();
            fragmentManager.beginTransaction()
                    .add(R.id.fragment_container, mHomeFragment, TAG)
                    .commit();
        }
        else
        {
            // get our old fragment back !
            mHomeFragment = (FragmentHome)fragment;
        }
    }
}

this comes in especially useful if you want to manipulate the fragment (in this case mHomeFragment) after rotating your device

回答3:

Use onAttachFragment() in your Activity to reassign the object:

@Override
public void onAttachFragment(Fragment fragment) {
    if (fragment instanceof MyFragment)
        this.myFragment = (MyFragment) fragment;
}

回答4:

I defined a Fragment in activity's layout, onSaveInstanceState in the Fragment does get called, but the savedInstanceState Bundle in the Fragment's onCreatView comes as null.

The reason was that my Fragment did not have a ID in XML:

android:id="@+id/compass_fragment" ...

回答5:

You can simply set the RetainInstance property inside OnCreate of the fragment class.

public override void OnCreate(Bundle savedInstanceState)
{
    base.OnCreate(savedInstanceState);
    RetainInstance = true;
}

Retain the Fragment object while rotating