android - android fragment 之间的通信

Question

我有两个 fragment 。我正在使用界面将一个 fragment 与另一个 fragment 进行通信:

public class FragmentLanguages extends ListFragment {
LanguageArrayAdapter adapter;
OnLanguageChangedListener mCallback;

public interface OnLanguageChangedListener {
    public void onLanguageSelected(int position);
}

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

    View view = inflater.inflate(R.layout.languages, null);

    return view;
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    Language languages[] = new Language[] {
            new Language(R.drawable.ic_flag_german, "German"),
            new Language(R.drawable.ic_flag_spanish, "Spanish"),
            new Language(R.drawable.ic_flag_russian, "Russian"),
            new Language(R.drawable.ic_flag_ukrainian, "Ukrainian") };

    adapter = new LanguageArrayAdapter(getActivity(), R.layout.list_row,
            languages);
    getListView().setOnItemClickListener(new OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> parent, View view,
                int position, long id) {
            adapter.setSelectedItem(position);
            mCallback.onLanguageSelected(position + 1);
        }
    });
    setListAdapter(adapter);
}

public static class Language {
    public int image;
    public String text;

    public Language() {
        super();
    }

    public Language(int image, String text) {
        super();
        this.image = image;
        this.text = text;
    }
}

public void setOnLanguageChangedListener(OnLanguageChangedListener mCallback) {
    this.mCallback = mCallback;
}

还有一个

public class FragmentCircles extends Fragment implements OnClickListener,
    OnLanguageChangedListener { ...
.....
    @Override
public void onLanguageSelected(int position) {
    currentLang = position;
    translationCircle.removeAllViews();
    String strSelect = "SELECT Translation FROM Translation WHERE Lang_Id="
            + currentLang + " ORDER BY Word_Id ASC";
    translationList = DataBaseHelper.getInstance(getActivity())
            .SelectTranslationList(strSelect);
    Log.i("Lang ", "changed " + currentLang);


    calculateCentreTranslation(lLabelRadius, width, height);
}

但是在屏幕旋转之后，当我在 ListFragment 中选择一些项目时，它给我一个错误

12-07 19:06:15.163: E/AndroidRuntime(436): java.lang.NullPointerException

在线

                mCallback.onLanguageSelected(position + 1);

mCallback 似乎为空。但是我该如何解决呢？

Answer 1

您的FragmentLanguages 必须通知您的FragmentActivity，后者又通知其他 fragment 。这确保没有 fragment 链接在一起，如果一个 fragment 在另一个通知它时未实例化，则可能触发 NullPointerExceptions。

因此您的界面 OnLanguageChangedListener 保留在您的 LanguagesFragment

您的FragmentActivity必须实现setOnLanguageChangedListener

在 FragmentLanguages 中:

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

由于 onAttach() 甚至在 onCreate() 之前被调用，您可以随时在 fragment 中使用:

onLanguageChangedListener.onLanguageChanged(...);

在实现 OnLanguageChangedListener 的 FragmentActivity 中，您需要重写它:

@Override
public void onLanguageChanged(...) {
    // here you implement the logic to notify other fragments
}