如何用ViewModel传递AlertDialog Fragment的结果？

我有一个自定义DialogFragment，原本通过Intent和父组件通信，但用到的onActivityResult、getTargetFragment等方法已经废弃。我知道有新方法可以替代，但只是要获取Yes/No的结果，犯不上大费周章！有没有更简单的实现方式？我尝试用ViewModel但没成功！

我尝试的ViewModel代码 #

AlertDialogViewModel.java #

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

public class AlertDialogViewModel extends ViewModel {
    private final MutableLiveData<Boolean> mAlertDialogResult = new MutableLiveData();
    
    public LiveData<Boolean> getAlertDialogResult() {
        return mAlertDialogResult;
    }
    
    public void setAlertDialogResult(boolean mResult) {
        mAlertDialogResult.setValue(mResult);
    }
}

MyAlertDialog.java #

public class MyAlertDialog extends DialogFragment {
    AlertDialogViewModel mViewModel;

    @NonNull
    @Override
    public Dialog onCreateDialog(@Nullable Bundle savedInstanceState) {
        return new MaterialAlertDialogBuilder(requireContext())
                .setTitle(Title)
                .setMessage(MESSAGE)
                .setPositiveButton("YES", (d, b) -> onResult(true))
                .setNegativeButton("NO", (d, b) -> onResult(false))
                .create();
    }

    private void onResult(boolean result) {
       mViewModel.setAlertDialogResult(result);
    }
    
    public static void showResult(Fragment fragment) {
       MyAlertDialog dialog = new MyAlertDialog();
       dialog.show(fragment.getParentFragmentManager(), null);
    }
}

调用代码 #

MyAlertDialog.showResult(this);
AlertDialogViewModel model = new ViewModelProvider(this).get(AlertDialogViewModel.class);
model.getAlertDialogResult().observe(this, result-> {
    Toast.makeText(this, ""+result, Toast.LENGTH_SHORT).show();
});

问题分析 #

你的ViewModel方案失效的原因有两个：

• ViewModel未初始化：DialogFragment里的mViewModel没有赋值，调用setAlertDialogResult时会直接空指针。
• ViewModel作用域不共享：如果在DialogFragment里单独初始化ViewModel，默认会创建属于自己的实例，和宿主Fragment/Activity的ViewModel不是同一个，导致宿主观察不到结果。

两种简易解决方案 #

方案一：修复ViewModel共享逻辑 #

修改MyAlertDialog，让它使用宿主的ViewModelStoreOwner：

public class MyAlertDialog extends DialogFragment {
    private AlertDialogViewModel mViewModel;

    @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        // 使用宿主的ViewModelStoreOwner，保证和宿主共享同一个ViewModel实例
        mViewModel = new ViewModelProvider(requireParentFragment()).get(AlertDialogViewModel.class);
        // 如果是和Activity共享，就用：
        // mViewModel = new ViewModelProvider(requireActivity()).get(AlertDialogViewModel.class);
    }

    @NonNull
    @Override
    public Dialog onCreateDialog(@Nullable Bundle savedInstanceState) {
        return new MaterialAlertDialogBuilder(requireContext())
                .setTitle("标题")
                .setMessage("提示内容")
                .setPositiveButton("YES", (d, b) -> {
                    mViewModel.setAlertDialogResult(true);
                    dismiss();
                })
                .setNegativeButton("NO", (d, b) -> {
                    mViewModel.setAlertDialogResult(false);
                    dismiss();
                })
                .create();
    }

    public static void showResult(Fragment fragment) {
        MyAlertDialog dialog = new MyAlertDialog();
        dialog.show(fragment.getParentFragmentManager(), "MyAlertDialog");
    }
}

调用逻辑不变，现在宿主就能正确观察到结果了。

方案二：更轻量的接口回调 #

如果只是需要Yes/No的简单结果，接口回调比ViewModel更直接，完全不需要额外依赖：

1. 定义回调接口：

public interface AlertDialogCallback {
    void onDialogResult(boolean isPositive);
}

2. 修改MyAlertDialog：

public class MyAlertDialog extends DialogFragment {
    private AlertDialogCallback mCallback;

    // 绑定回调，确保宿主实现了接口
    @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
        try {
            mCallback = (AlertDialogCallback) requireParentFragment();
            // 如果是和Activity绑定，就用：
            // mCallback = (AlertDialogCallback) requireActivity();
        } catch (ClassCastException e) {
            throw new ClassCastException("宿主必须实现AlertDialogCallback接口");
        }
    }

    @NonNull
    @Override
    public Dialog onCreateDialog(@Nullable Bundle savedInstanceState) {
        return new MaterialAlertDialogBuilder(requireContext())
                .setTitle("标题")
                .setMessage("提示内容")
                .setPositiveButton("YES", (d, b) -> {
                    mCallback.onDialogResult(true);
                    dismiss();
                })
                .setNegativeButton("NO", (d, b) -> {
                    mCallback.onDialogResult(false);
                    dismiss();
                })
                .create();
    }

    public static void showResult(Fragment fragment) {
        MyAlertDialog dialog = new MyAlertDialog();
        dialog.show(fragment.getParentFragmentManager(), "MyAlertDialog");
    }
}

3. 宿主实现接口并调用：

public class HostFragment extends Fragment implements AlertDialogCallback {
    // ... 其他代码

    private void showDialog() {
        MyAlertDialog.showResult(this);
    }

    @Override
    public void onDialogResult(boolean isPositive) {
        Toast.makeText(getContext(), isPositive ? "YES" : "NO", Toast.LENGTH_SHORT).show();
    }
}

这个方案代码更少，逻辑更直观，完全满足你“简单获取Yes/No结果”的需求。

内容的提问来源于stack exchange，提问作者user21193451