Question

I have a FragmentPagerAdapter that pages through views generated by a custom SurveyPage class. The final page in the pager is generated by SurveyFinishPage, which tries to access the Survey associated with the FragmentPagerAdapter and pulls all of the survey answers together to display them. The Survey object is grabbed by SurveyFinishPage via a getSurvey() method in the main Activity.

Normally this works fine and the SurveyFinishPage is able to access the same Survey object that the FragmentPagerAdapter is using. If the Activity has been killed and restored, though, the SurveyFinishPage seems to still be accessing the old Survey (or maybe a copy of it) – it displays the answers from before the Activity was discarded regardless of changes to answers to the new, rebuilt Survey.

It seems like restoring the value of aSurvey in onRestoreInstanceState() in my MainActivity should establish the survey object for everything else, since MainActivity passes its Survey object to the FragmentPagerAdapter, which uses that object to make Fragments… but instead it seems that the FragmentPagerAdapter and the SurveyFinishPage are looking at two different things.

How can I restore the state of the fragments in my FragmentPagerAdapter properly so that they all reference the same Survey object?

MainActivity.java

public class MainActivity extends FragmentActivity {
    SurveyPagerAdapter mSurveyPagerAdapter;
    ViewPager mViewPager;
    Survey aSurvey;

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

        if(savedInstanceState != null) {
            testSurvey = savedInstanceState.getParcelable("survey");
        }
        else {
            testSurvey = new Survey();
        }
        mSurveyPagerAdapter = new SurveyPagerAdapter(getSupportFragmentManager(), aSurvey);
        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSurveyPagerAdapter);
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putParcelable("survey", aSurvey);
    }

    @Override
    protected void onRestoreInstanceState(Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        aSurvey = (Survey)savedInstanceState.getParcelable("survey");
    }
    [...]
}

SurveyPagerAdapter.java

public class SurveyPagerAdapter extends FragmentPagerAdapter {
    private Survey survey;
    [...]

    @Override
    public Fragment getItem(int i) {
        Fragment fragment;
        Bundle args = new Bundle();

        // index 0 is the title page
        if(i == 0) {
            fragment = new SurveyTitlePage();
        }
        // final index is the finish page
        else if(i == getCount()-1) {
            fragment = new SurveyFinishPage();
        }
        // all other indices are regular poll pages
        else {
            args.putParcelable("question", survey.getQuestion(i-1));
            fragment = new SurveyPage();
            fragment.setArguments(args);
        }

        return fragment;
    }
    [...]
}

SurveyFinishPage.java

public class SurveyFinishPage extends Fragment {
    private Survey survey;
    [...]

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    setSurvey(((MainPoll)getActivity()).getSurvey());
    LinearLayout view = new LinearLayout(getActivity());
    view.setOrientation(LinearLayout.VERTICAL);

    Button finishButton = new Button(getActivity());
    finishButton.setText("FINISH");

        final TextView textView = new TextView(getActivity());
        finishButton.setOnClickListener(new Button.OnClickListener() {
            @Override
            public void onClick(View v) {
                String output = "";

                for(int i=0; i<survey.getSize(); i++) {
                    try {
                        output += survey.getQuestion(i).getPrompt() + " - " + survey.getQuestion(i).getAnswer() + "\n";
                    } catch (Exception e) {
                        output += survey.getQuestion(i).getPrompt() + " - " + "no answer\n";
                    }
                }
                textView.setText(output);
            }
        });

        view.addView(finishButton);
        view.addView(textView);

        return view;
    }
}

SurveyPage.java

public class SurveyPage extends Fragment {
        [...]

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            Bundle args = getArguments();
            Question question = args.getParcelable("question");
            View pageView = question.getQuestionView(getActivity(), container);
        return pageView;
    }
}

Answer 1

You could lift the Survey object to the Android application class and reference it as a singleton. That answer will likely create some debate, but it is a reasonable solution to your problem. Android guarantees there will only ever be 1 instance of your application so you can be certain (within reason) that you’ll have only 1 instance of Survey.

http://developer.android.com/guide/faq/framework.html

Singleton class

You can take advantage of the fact that your application components run in the same process through the use of a singleton. This is a class that is designed to have only one instance. It has a static method with a name such as getInstance() that returns the instance; the first time this method is called, it creates the global instance. Because all callers get the same instance, they can use this as a point of interaction. For example activity A may retrieve the instance and call setValue(3); later activity B may retrieve the instance and call getValue() to retrieve the last set value.