当从堆栈弹出片段时，应用程序在后台崩溃

Question

Application crashes, when I hit a server RPC, and when the RPC is in progress, I put the application in background. Meanwhile, when the RPC gets the response from server, it pops a fragment from stack. While popping the fragment, the application crashes. I have read about creating WeakReference , which will be null if the activity is destroyed. But not sure how to implement it this case.

Following is my code :

private void showFragment(SherlockFragment fragment) {
    FragmentManager fm = getSupportFragmentManager();
    fm.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
    FragmentTransaction ft = fm.beginTransaction();
    ft.replace(R.id.content, fragment);
    ft.commit();
}

I get a crash on executing the following line:

fm.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);

StackTrace :

01-15 16:37:44.435: E/AndroidRuntime(28049): FATAL EXCEPTION: main
01-15 16:37:44.435: E/AndroidRuntime(28049): java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState
01-15 16:37:44.435: E/AndroidRuntime(28049):    at android.support.v4.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1299)
01-15 16:37:44.435: E/AndroidRuntime(28049):    at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:1310)
01-15 16:37:44.435: E/AndroidRuntime(28049):    at android.support.v4.app.FragmentManagerImpl.popBackStack(FragmentManager.java:452)
01-15 16:37:44.435: E/AndroidRuntime(28049):    at com.druva.inSync.ValidationActivity$2.run(ValidationActivity.java:93)

Answer 1

I had the exact same problem, which I resolved using a flag. It may seem a little "hacky", but it does the job

public abstract class PopActivity extends Activity {

    private boolean mVisible; 

   @Override
    public void onResume() {
        super.onResume();
        mVisible = true;
    }

    @Override
    protected void onPause() {
        super.onPause();
        mVisible = false;
    }

    private void popFragment() {
        if (!mVisible) {
            return;
        }

        FragmentManager fm = getSupportFragmentManager();
        fm.popBackStack(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
    }
}

So when you implement the above code alone when you resume the app you will find yourself in a fragment that you actually want to be popped. You can use the following snipped to fix this issue:

public abstract class PopFragment extends Fragment {

    private static final String KEY_IS_POPPED = "KEY_IS_POPPED";
    private boolean mPopped;

    @Override
    public void onSaveInstanceState(Bundle outState) {
        outState.putBoolean(KEY_IS_POPPED, mPopped);
        super.onSaveInstanceState(outState);
    }

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (savedInstanceState != null) {
            mPopped = savedInstanceState.getBoolean(KEY_IS_POPPED);
        }
    }

    @Override
    public void onResume() {
        super.onResume();
        if (mPopped) {
            popFragment();
        }
    }

    protected void popFragment() {
        mPopped = true;
        // null check and interface check advised
        ((PopActivity) getActivity()).popFragment();
    }
}