在对话框 window 第二次打开后，在 dialogfragment 中使用观察器会导致程序崩溃

Question

我最近问了这个问题，但它有点乱，所以我想我对所有信息的理解要好一些，然后再试一次。

我想在常规片段和对话框片段之间共享数据以实现此目的我使用共享视图模型。正如您从标题中看到的那样，我第二次打开对话框时软件崩溃了window。

这是具有单个按钮的主片段，单击该按钮会打开一个对话框片段。

public class MainFragment extends Fragment {

    private MainViewModel mViewModel;
    private View fragmentView;
    private SharedViewModel sharedViewModel;


    public static MainFragment newInstance() {
        return new MainFragment();
    }

    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {
        fragmentView = inflater.inflate(R.layout.main_fragment, container, false);
        return fragmentView;
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        mViewModel = ViewModelProviders.of(this).get(MainViewModel.class);

        sharedViewModel = new ViewModelProvider(requireActivity()).get(SharedViewModel.class);

        Button aButton = fragmentView.findViewById(R.id.button);
        aButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                sharedViewModel.setColor("Green");
                NavDirections action = MainFragmentDirections.actionMainFragmentToBlankFragment();
                Navigation.findNavController(getView()).navigate(action);

            }
        });

    }

}

这是我单击按钮时弹出的对话框片段的代码。

public class BlankFragment extends DialogFragment {
    private View dialogView;
    private SharedViewModel sharedViewModel;

    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        LayoutInflater layoutInflater = requireActivity().getLayoutInflater();
        dialogView = layoutInflater.inflate(R.layout.blank_fragment, null);
        AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
        builder.setView(dialogView);

        return builder.create();
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        sharedViewModel = new ViewModelProvider(requireActivity()).
                get(SharedViewModel.class); //gets the shared view model from the associsated fragment.
        MutableLiveData<String> tableColor = sharedViewModel.getColor();
        tableColor.observe(getParentFragment().getViewLifecycleOwner(), new Observer<String>() {
            @Override
            public void onChanged(String s) {
                Toast.makeText(getActivity(), "IF I GET HERE TWICE I DIE", Toast.LENGTH_SHORT).show();
            }

        });
    }
}

您可能已经看到我使用导航图在它们之间导航。

共享视图模型如下所示

public class SharedViewModel extends ViewModel {
    private MutableLiveData<String> color = new MutableLiveData<>();


    public void setColor(String color){
        this.color.setValue(color);
    }
    public MutableLiveData<String> getColor(){
        return color;
    }

}

错误信息如下：

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.dialogfragmentcrashing, PID: 32620 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference at android.widget.Toast.(Toast.java:121) at android.widget.Toast.makeText(Toast.java:286) at android.widget.Toast.makeText(Toast.java:276) at com.example.dialogfragmentcrashing.BlankFragment.onChanged(BlankFragment.java:47) at com.example.dialogfragmentcrashing.BlankFragment.onChanged(BlankFragment.java:44) at androidx.lifecycle.LiveData.considerNotify(LiveData.java:131) at androidx.lifecycle.LiveData.dispatchingValue(LiveData.java:149) at androidx.lifecycle.LiveData.setValue(LiveData.java:307) at androidx.lifecycle.MutableLiveData.setValue(MutableLiveData.java:50) at com.example.dialogfragmentcrashing.SharedViewModel.setColor(SharedViewModel.java:11) at com.example.dialogfragmentcrashing.MainFragment.onClick(MainFragment.java:49) at android.view.View.performClick(View.java:7155) at android.view.View.performClickInternal(View.java:7124) at android.view.View.access00(View.java:808) at android.view.View$PerformClick.run(View.java:27370) at android.os.Handler.handleCallback(Handler.java:883) at android.os.Handler.dispatchMessage(Handler.java:100) at android.os.Looper.loop(Looper.java:359) at android.app.ActivityThread.main(ActivityThread.java:7418) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:935)

Answer 1

getParentFragment().getViewLifecycleOwner() 总是错误的 LifecycleOwner 使用，因为当 DialogFragment 被销毁时它不会被销毁。将所有代码移至 onCreate() 并使用 this:

@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    sharedViewModel = new ViewModelProvider(requireActivity()).
            get(SharedViewModel.class); //gets the shared view model from the associsated fragment.
    MutableLiveData<String> tableColor = sharedViewModel.getColor();
    tableColor.observe(this, new Observer<String>() {
        @Override
        public void onChanged(String s) {
            Toast.makeText(getActivity(), "IF I GET HERE TWICE I DIE", Toast.LENGTH_SHORT).show();
        }

    });
}

Answer 2

如果您使用 viewBinding 和 kotlin，试试这个：

override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
    _binding = FragmentBinding.inflate(LayoutInflater.from(context))

    val builder = AlertDialog.Builder(requireActivity()).apply {
        setTitle(R.string.add_title)
        setPositiveButton(R.string.add_dialog_ok, null)
        setNegativeButton(R.string.add_dialog_cancel, null)
        setView(binding.root)
    }

    setupViewModel()

    val dialog = builder.create()
    return dialog
}

override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, 
                          savedInstanceState: Bundle?): View? = binding.root

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    setupObservers()
}

在对话框 window 第二次打开后，在 dialogfragment 中使用观察器会导致程序崩溃

Using observer inside dialogfragment causes program crash after the dialog window opens a second time

java

android

viewmodel