android - 从 backstack 恢复后保持 ui 状态

Question

我对在使用 backstack 时保留我的 ui 状态有疑问。

场景
两个片段与 setRetainInstance(true) 一起使用；他们的 ui 是在 onActivityCreated() 中创建的。一个片段有一个 GridView，当通过 backstack 返回时，它应该保持其滚动位置，更改为另一个 App 等。

问题：
我有一个 GridView 有几个项目，如果我按下 home 并返回我的应用程序，它会保留它的滚动位置。
但是，如果我滚动，转到另一个片段并通过后退按钮返回，则重新创建 ui（不保留滚动位置）。

我该怎么做才能在使用 backstack 时保留 ui 状态而不手动保存和恢复每个状态？

更新

我认为我的代码在这里并不重要，因为我只是想知道原理。我的代码已经可以很好地处理活动，只有片段不起作用，因为我通过 backstack 返回到另一个活动。如果我按主页然后再次启动我的应用程序，那么所有内容都会按预期保留，并且这些片段也将保留。

主要片段：

公共类 ThumbnailFragment 扩展 Fragment 实现 IView {

GlobalModel model;
ThumbnailController controller;


final ApplicationDataHolder appDataHolder = new ApplicationDataHolder().getInstance();

PreferencesProvider preferences;
Button datePicker;
Spinner newspaperEditionSelector;
TextView newspaperEditionViewer;
GridView thumbnailView;
ImageSwitcher nothingLoaded;

int newspaperEditionPosition = -1;
String newspaperDate = null;
protected Adapter initializedAdapter = null;

@Override
public void updateView() {
    newspaperEditionSelector = (Spinner) getActivity().findViewById(R.id.newspaperEditionSelector);
    newspaperEditionViewer = (TextView) getActivity().findViewById(R.id.newspaperEditionViewer);
    thumbnailView = (GridView) getActivity().findViewById(R.id.thumbnailFragment_thumbnailList);
    datePicker = (Button) getActivity().findViewById(R.id.choosenDate);
    nothingLoaded = (ImageSwitcher) getActivity().findViewById(R.id.nothingLoaded);

    if (newspaperEditionSelector == null || newspaperEditionViewer == null || thumbnailView == null || datePicker == null || nothingLoaded == null)
        return;

    if (model.getSubscriptions() == null || model.getSubscriptions().size() < 1) {
        Bundle args = new Bundle();
        args.putBoolean("promptForLoginData", true);
        MainActivity.showFragment(getActivity(), getFragmentManager(), SettingsFragment.class.getName(), args, true);
    } else {
        if (model.getSubscriptions().size() > 1) {
            // Use spinner for user to select desired edition
            ArrayAdapter<String> dataAdapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_spinner_item, model.getSubscriptions());
            dataAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
            newspaperEditionSelector.setAdapter(dataAdapter);
            newspaperEditionSelector.setSelection(newspaperEditionPosition);
            this.newspaperEditionSelector.setVisibility(View.VISIBLE);
            this.newspaperEditionViewer.setVisibility(View.GONE);
        } else {
            // because only one edition for user exists use textview instead
            // of spinner
            this.newspaperEditionViewer.setText(model.getSubscriptions().get(0));
            this.newspaperEditionViewer.setVisibility(View.VISIBLE);
            this.newspaperEditionSelector.setVisibility(View.GONE);
            preferences.setIntegerPreference("ChoosenThumbnailNewspaperEdition", 0);
        }

        // Create on item choose listener
        newspaperEditionSelector.setOnItemSelectedListener(new OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parent, View view, int pos, long arg3) {
                if (initializedAdapter != parent.getAdapter()) {
                    initializedAdapter = parent.getAdapter();
                    return;
                } else {
                    newspaperEditionPosition = (int) parent.getSelectedItemId();
                    preferences.setIntegerPreference("ChoosenThumbnailNewspaperEdition", newspaperEditionPosition);
                    controller.loadNewspaper(newspaperDate, model.getSubscriptions().get(newspaperEditionPosition));
                }
            }

            @Override
            public void onNothingSelected(AdapterView<?> arg0) {
            }
        });
        // Prepare date picker ui
        datePicker.setText(newspaperDate);
        datePicker.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                DialogFragment newFragment = new DatePickerFragment();
                newFragment.show(getFragmentManager(), "datePicker");
            }
        });

        datePicker.addTextChangedListener(new TextWatcher() {
            @Override
            public void afterTextChanged(Editable s) {
                //controller.loadNewspaper(newspaperDate, model.getSubscriptions().get(newspaperEditionPosition));
            }

            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
            }
        });

        String title = "";
        if (model.getThumbnailsAdapter() == null) {
            nothingLoaded.setVisibility(View.VISIBLE);
            title += "...";
        } else {
            nothingLoaded.setVisibility(View.GONE);
            title += "Zeitung - " + model.getSubscriptions().get(newspaperEditionPosition) + (newspaperDate != null ? " von " + newspaperDate : "");
        }
        getActivity().getActionBar().setTitle(title);
    }
}


@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    setRetainInstance(true);

        preferences = new PreferencesProvider(getActivity());
        this.newspaperEditionPosition = preferences.getIntegerPreference("ChoosenThumbnailNewspaperEdition");
        this.newspaperDate = (preferences.getStringPreference("ChoosenThumbnailNewspaperDate").equals("") ? "Aktuell" : preferences.getStringPreference("ChoosenThumbnailNewspaperDate"));

        model = new GlobalModel().getInstance();
        model.init(getActivity().getApplicationContext());
        model.registerView(new WeakReference<IView>(this));
        controller = new ThumbnailController(model, this.getActivity());
        controller.loadSubscriptions();

        if (model.getSubscriptions() != null)
            controller.loadNewspaper(newspaperDate, model.getSubscriptions().get(newspaperEditionPosition));


            thumbnailView.setAdapter(model.getThumbnailsAdapter());
            thumbnailView.setOnItemClickListener(new OnItemClickListener() {
                @Override
                public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                    // Change fragment to pageview
                    Bundle args = new Bundle();
                    args.putString("newspaperEdition", model.getSubscriptions().get(newspaperEditionPosition));
                    args.putString("newspaperDate", newspaperDate);
                    args.putInt("newspaperPagePosition", position + 1);
                    args.putInt("newspaperPagesCount", parent.getAdapter().getCount());
                    MainActivity.showFragment(getActivity(), getFragmentManager(), PageFragment.class.getName(), args, true);
                }
            });
}



@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    super.onCreateView(inflater, container, savedInstanceState);

    setHasOptionsMenu(true);
    return inflater.inflate(R.layout.fragment_thumbnail, container, false);
}


@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
    // Clear and replace menu in actionbar and hide listbox
    final ActionBar actionBar = getActivity().getActionBar();
    actionBar.setDisplayShowTitleEnabled(true);
    actionBar.setTitle(R.string.title_thumbnail_fragment);
    actionBar.setDisplayHomeAsUpEnabled(false);
    menu.clear();
    inflater.inflate(R.menu.thumbnail_fragment_menu, menu);
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case R.id.menu_goto_search:
            MainActivity.showFragment(getActivity(), getFragmentManager(), SearchFragment.class.getName(), null, true);
            break;
        case R.id.menu_export_pdf:
            NewspaperDownloader task = new NewspaperDownloader(getActivity(), model.getSubscriptions().get(newspaperEditionPosition), this.newspaperDate, -1);
            task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
            break;
        case R.id.menu_settings:
            MainActivity.showFragment(getActivity(), getFragmentManager(), SettingsFragment.class.getName(), null, true);
            break;
        case R.id.menu_exit:
            getActivity().finish();
            System.exit(0);
            break;
        default:
            return super.onOptionsItemSelected(item);
    }
    return false;
}


}