Can't retain nested fragments

问题:

Is there another way of saving the state of the nested fragment ? Or if we shouldn't do this, why ? Thanks !

02-13 11:42:43.258: E/AndroidRuntime(7167): java.lang.IllegalStateException: Can't retain fragements that are nested in other fragments
02-13 11:42:43.258: E/AndroidRuntime(7167):     at android.support.v4.app.Fragment.setRetainInstance(Fragment.java:742)

回答1:

You can use FragmentManager.saveFragmentInstanceState(Fragment) to retrieve a fragment state. The return value implements Parcelable, so you can put it in a Bundle.

For restoration, you can provide the state after creating the fragment using Fragment.setInitialSavedState(Fragment.SavedState).

回答2:

Since support library 20+ (https://code.google.com/p/android/issues/detail?id=74222), there is a bug with instance recreation for child Fragments, there is a fix for it - http://ideaventure.blogspot.com.au/2014/10/nested-retained-fragment-lost-state.html

Code from the webpage(add this to your parent Fragment) -

private FragmentManager childFragmentManager() {//!!!Use this instead of getFragmentManager, support library from 20+, has a bug that doesn't retain instance of nested fragments!!!!
        if(mRetainedChildFragmentManager == null) {
            mRetainedChildFragmentManager = getChildFragmentManager();
        }
        return mRetainedChildFragmentManager;
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        if (mRetainedChildFragmentManager != null) {
            //restore the last retained child fragment manager to the new
            //created fragment
            try {
                Field childFMField = Fragment.class.getDeclaredField("mChildFragmentManager");
                childFMField.setAccessible(true);
                childFMField.set(this, mRetainedChildFragmentManager);
            } catch (NoSuchFieldException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        }
    }
 @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setRetainInstance(true);

    }

回答3:

Problem: mChildFrgamentManager is being recreated (https://code.google.com/p/android/issues/detail?id=74222)
Workaround: Retaining mChildFrgamentManager if fragment has setRetainInstance(true):

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);

    if (getRetainInstance()) {
        if (mRetainedChildFragmentManager != null) {
            try {
                Field childFMField = Fragment.class.getDeclaredField("mChildFragmentManager");
                childFMField.setAccessible(true);
                childFMField.set(this, mRetainedChildFragmentManager);
            } catch (NoSuchFieldException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        } else {
            mRetainedChildFragmentManager = getChildFragmentManager();
        }
    }
}

Warning: With this code, setRetainInstace should be called before onAttach.

P.S: This is a bit improved version of @attels answer.

回答4:

This is no longer a limitation with latest support library, after this AOSP commit.

Below is commit message:

Permit setRetainInstance(true) on nested fragments

Save arbitrarily nested fragments across config changes as nonconfiguration objects. This permits the use of retain-instance child fragments as arbitrary opaque dependencies within other fragments.