我相信我会遵循为我提供的用于建立ViewModelProvider.Factory的代码示例。这发生在OnAttach方法中CheckInRecentList代码中的此语句上。您可以提供的任何帮助将不胜感激。
mViewModel = new ViewModelProvider（this，viewModelFactory）.get（MainViewModel.class）;
获取以下空指针异常，如下所示
02-01 06:08:08.319 22068-22068/com.example.checkingin I/Constraints: onAttach: OnAttach
02-01 06:17:22.773 22068-22068/com.example.checkingin D/AndroidRuntime: Shutting down VM
02-01 06:17:22.773 22068-22068/com.example.checkingin E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.checkingin, PID: 22068
java.lang.NullPointerException: Attempt to invoke interface method 'androidx.lifecycle.ViewModel androidx.lifecycle.ViewModelProvider$Factory.create(java.lang.Class)' on a null object reference
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
at com.example.checkingin.CheckInRecentList.onAttach(CheckInRecentList.java:117)
at androidx.fragment.app.Fragment.performAttach(Fragment.java:2669)
at androidx.fragment.app.FragmentStateManager.attach(FragmentStateManager.java:229)
at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1215)
at androidx.fragment.app.FragmentTransition.addToFirstInLastOut(FragmentTransition.java:1240)
at androidx.fragment.app.FragmentTransition.calculateFragments(FragmentTransition.java:1123)
at androidx.fragment.app.FragmentTransition.startTransitions(FragmentTransition.java:136)
at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:2117)
at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:2075)
at androidx.fragment.app.FragmentManager.execSingleAction(FragmentManager.java:1946)
at androidx.fragment.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:303)
at androidx.fragment.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:238)
at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1244)
at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1092)
at androidx.viewpager.widget.ViewPager.onMeasure(ViewPager.java:1622)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at androidx.coordinatorlayout.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:733)
at com.google.android.material.appbar.HeaderScrollingViewBehavior.onMeasureChild(HeaderScrollingViewBehavior.java:95)
at com.google.android.material.appbar.AppBarLayout$ScrollingViewBehavior.onMeasureChild(AppBarLayout.java:1556)
at androidx.coordinatorlayout.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:803)
at android.view.View.measure(View.java:18788)
at androidx.drawerlayout.widget.DrawerLayout.onMeasure(DrawerLayout.java:1119)
at android.view.View.measure(View.java:18788)
at androidx.drawerlayout.widget.DrawerLayout.onMeasure(DrawerLayout.java:1119)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at androidx.appcompat.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:143)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1465)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:748)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:630)
at android.view.View.measure(View.java:18788)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5951)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2643)
at android.view.View.measure(View.java:18788)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2100)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1216)
a
CheckInRecentList.java
package com.example.checkingin;
import android.content.Context;
import android.net.Uri;
import android.os.Bundle;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;
import androidx.lifecycle.ViewModelProvider.Factory;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import java.util.List;
import static androidx.constraintlayout.widget.Constraints.TAG;
/**
* A simple {@link Fragment} subclass.
* Activities that contain this fragment must implement the
* {@link CheckInRecentList.OnFragmentInteractionListener} interface
* to handle interaction events.
* Use the {@link CheckInRecentList#newInstance} factory method to
* create an instance of this fragment.
*/
public class CheckInRecentList extends Fragment {
// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private static final String ARG_PARAM1 = "param1";
private static final String ARG_PARAM2 = "param2";
private RecyclerView recyclerView;
private RecyclerView.Adapter checkInListAdapter;
//private RecyclerView.LayoutManager layoutManager;
// TODO: Rename and change types of parameters
private String mParam1;
private String mParam2;
private MainViewModel mViewModel;
private CheckInListAdapter adapter;
private MainViewModelProviderFactory viewModelFactory;
private TextView checkInLastDateTime;
private TextView checkInTitle;
private TextView checkInDestinationName;
private TextView checkInComments;
private OnFragmentInteractionListener mListener;
public CheckInRecentList() {
// Required empty public constructor
}
/**
* Use this factory method to create a new instance of
* this fragment using the provided parameters.
*
* @param param1 Parameter 1.
* @param param2 Parameter 2.
* @return A new instance of fragment CheckInRecentList.
*/
// TODO: Rename and change types and number of parameters
public static CheckInRecentList newInstance(String param1, String param2) {
CheckInRecentList fragment = new CheckInRecentList();
Bundle args = new Bundle();
args.putString(ARG_PARAM1, param1);
args.putString(ARG_PARAM2, param2);
fragment.setArguments(args);
return fragment;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
mParam1 = getArguments().getString(ARG_PARAM1);
mParam2 = getArguments().getString(ARG_PARAM2);
}
// These were originally set up from the recycler view add to the fragment
// recyclerView = findViewById(R.id.check_in_recent_recycler_view);
// use this setting to improve performance if you know that changes
// in content do not change the layout size of the RecyclerView
//recyclerView.setHasFixedSize(true);
/*
// use a linear layout manager
layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);
*/
// specify an adapter (see also next example)
//checkInListAdapter = new CheckInListAdapter();
// recyclerView.setAdapter(checkInListAdapter);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_check_in_recent_list, container, false);
}
// TODO: Rename method, update argument and hook method into UI event
public void onButtonPressed(Uri uri) {
if (mListener != null) {
mListener.onFragmentInteraction(uri);
}
}
@Override
public void onAttach(Context context) {
super.onAttach(context);
Log.i(TAG, "onAttach: OnAttach");
mViewModel = new ViewModelProvider(this, viewModelFactory).get(MainViewModel.class);
if (context instanceof OnFragmentInteractionListener) {
mListener = (OnFragmentInteractionListener) context;
} else {
throw new RuntimeException(context.toString()
+ " must implement OnFragmentInteractionListener");
}
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
Log.i(TAG, "onActivityCreated: OnCreateView");
//mViewModel = new ViewModelProvider(this, viewModelFactory).get(MainViewModel.class);
checkInLastDateTime = getView().findViewById(R.id.checkInLastDateTime);
checkInTitle = getView().findViewById(R.id.checkInTitle);
checkInDestinationName = getView().findViewById(R.id.checkInDestinationName);
checkInComments = getView().findViewById(R.id.checkInComments);
listenerSetup();
observerSetup();
recyclerSetup();
}
@Override
public void onDetach() {
super.onDetach();
mListener = null;
}
/**
* This interface must be implemented by activities that contain this
* fragment to allow an interaction in this fragment to be communicated
* to the activity and potentially other fragments contained in that
* activity.
* <p>
* See the Android Training lesson <a href=
* "http://developer.android.com/training/basics/fragments/communicating.html"
* >Communicating with Other Fragments</a> for more information.
*/
public interface OnFragmentInteractionListener {
// TODO: Update argument type and name
void onFragmentInteraction(Uri uri);
}
private void clearFields() {
checkInLastDateTime.setText("");
checkInDestinationName.setText("");
checkInTitle.setText("");
checkInComments.setText("");
}
private void listenerSetup() {
Button editCheckInButton = getView().findViewById(R.id.checkInEditButton);
Button resendCheckInButton = getView().findViewById(R.id.checkInResendButton);
editCheckInButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
//put in edit check in logic
}
});
resendCheckInButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
//put in resend logic
}
});
}
private void observerSetup() {
mViewModel.getAllCheckIn().observe(getViewLifecycleOwner(), new Observer<List<CheckInTable>> ()
{
@Override
public void onChanged(@Nullable final List<CheckInTable> checkIn) {
adapter.setCheckInList(checkIn);
}
});
mViewModel.getSearchCheckInResults().observe(getViewLifecycleOwner(), new Observer<List<CheckInTable>>() {
@Override
public void onChanged(@Nullable final List<CheckInTable> checkIn) {
if (checkIn.size() > 0) {
checkInLastDateTime.setText(checkIn.get(0).getCheckInLastDateTime());
checkInDestinationName.setText(checkIn.get(0).getCheckInDestinationName());
checkInTitle.setText(checkIn.get(0).getCheckInTitle());
checkInComments.setText(checkIn.get(0).getCheckInComments());
} else {
checkInLastDateTime.setText("None Found");
}
}
});
}
private void recyclerSetup() {
RecyclerView recyclerView;
adapter = new CheckInListAdapter(R.layout.recycler_view_item);
recyclerView = getView().findViewById(R.id.check_in_recent_recycler_view);
recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));
recyclerView.setAdapter(adapter);
}
}
MainViewModelProviderFactory
package com.example.checkingin;
import android.app.Application;
import android.util.Log;
import androidx.annotation.NonNull;
import androidx.lifecycle.ViewModel;
import androidx.lifecycle.ViewModelProvider;
import static androidx.constraintlayout.widget.Constraints.TAG;
public class MainViewModelProviderFactory implements ViewModelProvider.Factory {
private final Application mDataSource;
public MainViewModelProviderFactory(Application dataSource) {
mDataSource = dataSource;
}
@Override
@NonNull
public <T extends ViewModel>
T create(@NonNull Class<T> modelClass) {
Log.i(TAG, "MainViewModelProviderFactory: ");
if (modelClass.isAssignableFrom(MainViewModel.class)) {
Log.i(TAG, "create: is assignable");
return (T) new MainViewModel(mDataSource);
}
//noinspection unchecked
throw new IllegalArgumentException("Unknown ViewModel class");
}}
MainViewModel.java
package com.example.checkingin;
import android.app.Application;
import android.util.Log;
import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import java.util.List;
import static androidx.constraintlayout.widget.Constraints.TAG;
public class MainViewModel extends AndroidViewModel {
private CheckInRecipientsTableRepository checkInRecipientsTableRepository;
private LiveData<List<CheckInRecipientsTable>> allRecipients;
private MutableLiveData<List<CheckInRecipientsTable>> searchRecipientResults;
private ContactGroupsTableRepository contactGroupsTableRepository;
private LiveData<List<ContactGroupsTable>> allContactGroups;
private MutableLiveData<List<ContactGroupsTable>> searchContactGroupsResults;
private CheckInTableRepository checkInTableRepository;
private LiveData<List<CheckInTable>> allCheckIn;
private MutableLiveData<List<CheckInTable>> searchCheckInResults;
public MainViewModel (Application application) {
super(application);
Log.i(TAG, "MainViewModel: In the Main View Model");
checkInRecipientsTableRepository = new CheckInRecipientsTableRepository(application);
allRecipients = checkInRecipientsTableRepository.getAllCheckInRecipients();
searchRecipientResults = checkInRecipientsTableRepository.getSearchRecipientResults();
checkInTableRepository = new CheckInTableRepository(application);
allCheckIn = checkInTableRepository.getAllCheckIn();
searchCheckInResults = checkInTableRepository.getSearchCheckInResults();
contactGroupsTableRepository = new ContactGroupsTableRepository(application);
allContactGroups = contactGroupsTableRepository.getAllContactGroups();
searchContactGroupsResults = contactGroupsTableRepository.getSearchContactGroupsResults();
}
MutableLiveData<List<CheckInRecipientsTable>> getSearchRecipientResults() {
return getSearchRecipientResults();
}
LiveData<List<CheckInRecipientsTable>> getAllCheckInRecipients() {
return getAllCheckInRecipients();
}
public void insertCheckInRecipientsTable(CheckInRecipientsTable checkInRecipientsTable) {
checkInRecipientsTableRepository.insertCheckInRecipientsTable(checkInRecipientsTable);
}
public void deleteCheckInRecipient(int checkInPrimaryKey) {
checkInRecipientsTableRepository.deleteCheckInRecipient(checkInPrimaryKey);
}
public void findCheckInRecipient(int checkInPrimaryKey) {
checkInRecipientsTableRepository.findCheckInRecipient(checkInPrimaryKey);
}
MutableLiveData<List<ContactGroupsTable>> getSearchContactGroupsResults() {
return getSearchContactGroupsResults();
}
LiveData<List<ContactGroupsTable>> getAllContactGroups() {
return getAllContactGroups();
}
public void insertContactGroupsTable(ContactGroupsTable contactGroupsTable) {
contactGroupsTableRepository.insertContactGroups(contactGroupsTable);
}
public void deleteContactGroups(int contactGroupsTablePrimaryKey) {
contactGroupsTableRepository.deleteContactGroups(contactGroupsTablePrimaryKey);
}
public void findContactGroups(int contactGroupsTablePrimaryKey) {
contactGroupsTableRepository.findContactGroups(contactGroupsTablePrimaryKey);
}
MutableLiveData<List<CheckInTable>> getSearchCheckInResults() {
return getSearchCheckInResults();
}
LiveData<List<CheckInTable>> getAllCheckIn() {
return getAllCheckIn();
}
public void insertCheckInTable(CheckInTable checkInTable) {
checkInTableRepository.insertCheckIn(checkInTable);
}
public void deleteCheckIn(int checkInTablePrimaryKey) {
checkInTableRepository.deleteCheckIn(checkInTablePrimaryKey);
}
public void findCheckIn(int checkInTablePrimaryKey) {
checkInTableRepository.findCheckIn(checkInTablePrimaryKey);
}
}
最佳答案
您应该将ViewModelProvider.Factory接口的初始化对象传递给ViewModelProvider构造函数。
private MainViewModelProviderFactory viewModelFactory;
// Note that it is better to use onCreate or onCreateView for initialization.
// Since your ViewModel usually should be alive at a time, when `onAttach` is called.
@Override
public void onAttach(Context context) {
super.onAttach(context);
viewModelFactory = new MainViewModelProviderFactory(context.getApplicationContext());
mViewModel = new ViewModelProvider(this, viewModelFactory).get(MainViewModel.class);
}
将应用程序类作为数据源传递，而是使用上下文是不寻常的做法。因此，更改您的
MainViewModelProviderFactory类：private final Context mDataSource;
public MainViewModelProviderFactory(Context appContext) {
mDataSource = appContext;
}
或者，如果您不想这样做，请在
onAttach中更改行viewModelFactory = new MainViewModelProviderFactory((Application)context.getApplicationContext());