我如何将共享首选项中的数据检索到片段中

Question

我有 SharedPrefManger 类，它保存用户信息和检索数据，但是当我将它用于片段时返回空指针异常

package com.example.e_learning.storage;

import android.content.Context;
import android.content.SharedPreferences;

import com.example.e_learning.jsondata.StudentData;
import com.example.e_learning.pojo.StudentModel;

public class SharedPrefManger {
    private static SharedPrefManger INSTANCE;
    private Context context;
    private static final String SHARED_PREF_NAME = "MySharedPref";

    public SharedPrefManger(Context context) {
        this.context = context;
    }

    public static SharedPrefManger getINSTANCE(Context context) {
        if (INSTANCE == null)
            INSTANCE = new SharedPrefManger(context);
        return INSTANCE;

    }

    public void saveUser(StudentData data) {
        SharedPreferences preferences = context.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE);
        SharedPreferences.Editor editor = preferences.edit();

        editor.putInt("id", data.getId());
        editor.putString("email", data.getEmail());
        editor.putString("name", data.getName());
        editor.putString("level", data.getLevel());

        editor.apply();
    }

    public boolean isLoggedIn() {
        SharedPreferences preferences = context.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE);
        return preferences.getInt("id", -1) != -1;
    }

    public StudentData getUser() {
        SharedPreferences preferences = context.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE);

        return new StudentData(
                preferences.getInt("id", -1),
                preferences.getString("name", null),
                preferences.getString("email", null),
                preferences.getString("level", null)
        );

    }
    public void clear(){
        SharedPreferences preferences = context.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE);
        SharedPreferences.Editor editor = preferences.edit();
        editor.clear();
        editor.apply();

    }
}

我有一个片段，我想将用户检索到其中 但是当我调用 getUser Id 时返回 null

这是我的片段

package com.example.e_learning.ui.fragments.announcement;

import android.os.Bundle;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.databinding.DataBindingUtil;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.ViewModelProvider;
import androidx.recyclerview.widget.LinearLayoutManager;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;

import com.example.e_learning.R;
import com.example.e_learning.databinding.FragmentAnnouncementBinding;
import com.example.e_learning.jsondata.AnnouncementsData;
import com.example.e_learning.storage.SharedPrefManger;

import java.util.ArrayList;

/**
 * A simple {@link Fragment} subclass.
 */
public class AnnouncementFragment extends Fragment {


    private FragmentAnnouncementBinding binding;
    private AnnouncementsViewModel announcementsViewModel;

    public AnnouncementFragment() {
    }



    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        binding = DataBindingUtil.inflate(inflater, R.layout.fragment_announcement, container, false);
        View view = binding.getRoot();
        return view;


    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        AnnouncementAdapter announcementAdapter = new AnnouncementAdapter();
        binding.announcementRecycler.setLayoutManager(new LinearLayoutManager(getContext()));
        binding.announcementRecycler.setAdapter(announcementAdapter);
        announcementsViewModel.getStudentId().setValue(SharedPrefManger.getINSTANCE(this.getActivity()).getUser().getId());

        announcementsViewModel = new ViewModelProvider(requireActivity()).get(AnnouncementsViewModel.class);

        announcementsViewModel.getAnnouncements();
        announcementsViewModel.announcementsModelMutableLiveData.observe(getViewLifecycleOwner(), announcementsModel -> {
            if (announcementsModel.getStatus()) {
                announcementAdapter.setList((ArrayList<AnnouncementsData>) announcementsModel.getData());

            } else if (!announcementsModel.getStatus()) {
                Toast.makeText(getContext(), announcementsModel.getError(), Toast.LENGTH_SHORT).show();
            }
        });


    }


}

此行返回空指针异常

        announcementsViewModel.getStudentId().setValue(SharedPrefManger.getINSTANCE(this.getActivity()).getUser().getId());

如何将用户数据检索到此片段中？ 这门课与其他活动配合得很好

Answer 1

announcementsViewModel在使用前必须先初始化，所以需要移动这一行

announcementsViewModel = new ViewModelProvider(requireActivity()).get(AnnouncementsViewModel.class);

在此之前

announcementsViewModel.getStudentId().setValue(SharedPrefManger.getINSTANCE(this.getActivity()).getUser().getId());

Answer 2

创建类 Shared Preference 的对象，具有构造函数。 以及您想在哪里获取它，只需使用 new 关键字和该首选项名称方法调用构造函数。 喜欢： 新的 PrefManager(getActivity).getData();