Android Fragment监听器空对象引用问题求助

这个空指针异常的原因很明确——你Fragment里的Listener变量始终是null！虽然你在MainActivity里实现了MessageFragment.OnItemSelectedListener接口，但你从来没把Activity的实例赋值给Fragment里的这个Listener对象，所以当点击ListView触发回调时，调用Listener.onItemSelectedMessageFragment(position)就会抛出空指针。

修复步骤 #

你需要在Fragment和Activity建立关联的时候，完成Listener的赋值，具体可以通过重写Fragment的onAttach方法来实现：

1. 重写onAttach方法，将依附的Activity实例赋值给Listener

@Override
public void onAttach(@NonNull Context context) {
    super.onAttach(context);
    try {
        // 把上下文强制转换为我们定义的监听接口
        Listener = (OnItemSelectedListener) context;
    } catch (ClassCastException e) {
        // 如果Activity没实现接口，抛出明确的异常信息，方便排查
        throw new ClassCastException(context.toString() + " must implement OnItemSelectedListener");
    }
}

2. 重写onDetach方法，清空Listener避免内存泄漏

@Override
public void onDetach() {
    super.onDetach();
    Listener = null;
}

修改后的完整Fragment代码 #

public class MessageFragment extends Fragment { 
    OnItemSelectedListener Listener; 

    @Nullable 
    @Override 
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { 
        View view = inflater.inflate(R.layout.fragment_message, container, false); 
        List<String> dummyList = new ArrayList<>(); 
        for (int i = 0; i < DummyDataProvider.getDummyData().size(); i++) { 
            dummyList.add(DummyDataProvider.getDummyData().get(i).getDummyData() +" "+ i); 
        } 
        ListView listView = (ListView) view.findViewById(R.id.messageFragmentListView); 
        ArrayAdapter<String> listAdapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1,dummyList); 
        listView.setAdapter(listAdapter); 
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
            @Override 
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
                Listener.onItemSelectedMessageFragment(position); 
            } 
        }); 
        return view; 
    } 

    @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        try {
            Listener = (OnItemSelectedListener) context;
        } catch (ClassCastException e) {
            throw new ClassCastException(context.toString() + " must implement OnItemSelectedListener");
        }
    }

    @Override
    public void onDetach() {
        super.onDetach();
        Listener = null;
    }

    public interface OnItemSelectedListener{ 
        void onItemSelectedMessageFragment(int position); 
    } 
}

这样修改后，当Fragment依附到MainActivity时，Listener就会被正确赋值为MainActivity的实例，点击ListView时就能正常触发Activity里的Toast提示了。

内容的提问来源于stack exchange，提问作者Marko Dunovic