Android Studio中底部导航关联Fragment间Bundle传递数据失败

问题根因 #

你在GoalsFragment的SET GOAL点击事件中创建的ProgressFragment是临时的全新实例，和底部导航管理器中已经初始化、实际显示的ProgressFragment不是同一个对象。你给临时实例设置的Arguments不会作用到实际展示的页面上，因此实际运行时ProgressFragment拿不到传递的参数，int类型默认返回0。

推荐解决方案 #

方案1：使用共享ViewModel（Jetpack官方推荐同Activity下Fragment传值方案） #

该方案无需Fragment互相持有引用，代码耦合度低，支持数据变化实时响应。

1. 新建共享ViewModel类：

import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

public class SharedGoalViewModel extends ViewModel {
    private final MutableLiveData<Integer> selectedGoal = new MutableLiveData<>(0);

    public void setSelectedGoal(int goal) {
        selectedGoal.setValue(goal);
    }

    public LiveData<Integer> getSelectedGoal() {
        return selectedGoal;
    }
}

2. 修改GoalsFragment逻辑，点击按钮时给ViewModel存值：

// 先在onCreate中初始化共享ViewModel，注意要依附Activity才能跨Fragment共享
private SharedGoalViewModel sharedViewModel;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    sharedViewModel = new ViewModelProvider(requireActivity()).get(SharedGoalViewModel.class);
}

// 修改SET GOAL按钮的点击逻辑，删掉原来创建ProgressFragment实例的无用代码
case R.id.fg_goals_setgoal:
    if(check == 0) {
        Toast.makeText(getActivity(), "Please select one of the 3 Goals!", Toast.LENGTH_LONG).show();
        break;
    }
    // 存值到共享ViewModel
    sharedViewModel.setSelectedGoal(check);
    String toastText;
    if (check == 1) {
        toastText = "Current Goal: Socialize More";
    } else if (check == 2) {
        toastText = "Current Goal: Enhance Study Motives";
    } else {
        toastText = "Current Goal: Physical Exercise";
    }
    Toast.makeText(getActivity(), toastText, Toast.LENGTH_LONG).show();
    // 如果需要点击后自动切到Progress页面，添加如下代码调用底部导航切换即可
    // ((你的主Activity类名) requireActivity()).底部导航控件id.setSelectedItemId(R.id.nav_progress);
    break;

3. 修改ProgressFragment逻辑，观察ViewModel的值更新UI：

private SharedGoalViewModel sharedViewModel;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    sharedViewModel = new ViewModelProvider(requireActivity()).get(SharedGoalViewModel.class);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_progress, container, false);
    TextView fg_progress_header2 = rootView.findViewById(R.id.fg_progress_header2);

    // 观察目标值变化，自动更新UI
    sharedViewModel.getSelectedGoal().observe(getViewLifecycleOwner(), goal -> {
        System.out.println(goal);
        if (goal == 0) {
            fg_progress_header2.setText("请先选择目标");
            return;
        }
        String headerText;
        if (goal == 1) {
            headerText = "Socialize More";
        } else if (goal == 2) {
            headerText = "Enhance Study Motives";
        } else {
            headerText = "Physical Exercise";
        }
        fg_progress_header2.setText(headerText);
    });
    return rootView;
}

方案2：使用SharedPreferences存储（适合需要持久化目标选择的场景） #

如果需要退出APP再打开仍然保留上次选择的目标，可以用本地存储实现：

1. 修改GoalsFragment的SET GOAL点击逻辑：

case R.id.fg_goals_setgoal:
    if(check == 0) {
        Toast.makeText(getActivity(), "Please select one of the 3 Goals!", Toast.LENGTH_LONG).show();
        break;
    }
    // 存储到本地SharedPreferences
    SharedPreferences sp = requireContext().getSharedPreferences("app_config", Context.MODE_PRIVATE);
    sp.edit().putInt("selected_goal", check).apply();
    // 吐司逻辑同上
    break;

2. 修改ProgressFragment逻辑，页面显示时读取本地值：

@Override
public void onResume() {
    super.onResume();
    TextView fg_progress_header2 = getView().findViewById(R.id.fg_progress_header2);
    SharedPreferences sp = requireContext().getSharedPreferences("app_config", Context.MODE_PRIVATE);
    int goal = sp.getInt("selected_goal", 0);
    // 更新UI逻辑同上
}

注意事项 #

你原来的代码中直接调用b.getInt("goal")如果Bundle为空会触发空指针异常，建议改用getInt("goal", 0)的重载方法指定默认值，避免崩溃。

内容的提问来源于stack exchange，提问作者Loizos Aristeidis