每次我运行我的应用程序时,标题中都会显示此错误 我已经搜索了一些问题,有人说使ViewModel构造函数公开,而我的公开 对方说:
从HomeViewModel中删除Context上下文和LifecycleOwner lifecycleOwner构造函数参数,或者
创建一个可以构建HomeViewModel实例的ViewModelProvider.Factory,并将其与ViewModelProviders.of()一起使用
我已经做出了两种解决方案,但仍然出现相同的错误
MainActivity
package com.example.architectureexample;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;
import androidx.lifecycle.ViewModelProviders;
import android.os.Bundle;
import android.widget.Toast;
import java.util.List;
public class MainActivity extends AppCompatActivity {
// 5th video
private NoteViewModel noteViewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
noteViewModel = ViewModelProviders.of(this).get(NoteViewModel.class);
noteViewModel.getAllNotes().observe(this, new Observer<List<Note>>() {
@Override
public void onChanged(List<Note> notes) {
// update recycleView
Toast.makeText(MainActivity.this, "onChanged", Toast.LENGTH_SHORT).show();
}
});
}
}
ViewModel类
package com.example.architectureexample;
// 5th video
import android.app.Application;
import androidx.annotation.NonNull;
import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;
import java.util.List;
public class NoteViewModel extends AndroidViewModel {
private NoteRep rep;
private LiveData<List<Note>> allNotes;
public NoteViewModel(@NonNull Application application) {
super(application);
rep = new NoteRep(application);
allNotes = rep.getAllNotes();
}
public void insert(Note note){
rep.insert(note);
}
public void update(Note note){
rep.update(note);
}
public void delete(Note note){
rep.delete(note);
}
public void deleteAll(){
rep.deleteAll();
}
public LiveData<List<Note>> getAllNotes() {
return allNotes;
}
}
存储库类
package com.example.architectureexample;
// 4th video
import android.app.Application;
import android.os.AsyncTask;
import androidx.lifecycle.LiveData;
import java.util.List;
public class NoteRep {
private NoteDao noteDao;
private LiveData<List<Note>> allNotes;
public NoteRep(Application application){
NoteDatabase database = NoteDatabase.getInstance(application);
noteDao = database.noteDao();
allNotes = noteDao.getAllNotes();
}
// these methods works as API the the Repository Exports to the out side
public void insert(Note note){
new InsertNoteAsyncTask(noteDao).execute(note);
}
public void update(Note note){
new UpdateNoteAsyncTask(noteDao).execute(note);
}
public void delete(Note note){
new DeleteNoteAsyncTask(noteDao).execute(note);
}
public void deleteAll(){
new DeleteAllNotesAsyncTask(noteDao).execute();
}
public LiveData<List<Note>> getAllNotes() {
return allNotes;
}
private static class InsertNoteAsyncTask extends AsyncTask<Note ,Void,Void>{
// we need it to do database operations
private NoteDao noteDao;
// because the class is static we can't access the NoteDao directly so we have to pass it throw a constructor
public InsertNoteAsyncTask(NoteDao noteDao) {
this.noteDao = noteDao;
}
@Override
protected Void doInBackground(Note... notes) {
noteDao.insert(notes[0]);
return null;
}
}
private static class UpdateNoteAsyncTask extends AsyncTask<Note ,Void,Void>{
// we need it to do database operations
private NoteDao noteDao;
// because the class is static we can't access the NoteDao directly so we have to pass it throw a constructor
public UpdateNoteAsyncTask(NoteDao noteDao) {
this.noteDao = noteDao;
}
@Override
protected Void doInBackground(Note... notes) {
noteDao.update(notes[0]);
return null;
}
}
private static class DeleteNoteAsyncTask extends AsyncTask<Note ,Void,Void>{
// we need it to do database operations
private NoteDao noteDao;
// because the class is static we can't access the NoteDao directly so we have to pass it throw a constructor
public DeleteNoteAsyncTask(NoteDao noteDao) {
this.noteDao = noteDao;
}
@Override
protected Void doInBackground(Note... notes) {
noteDao.delete(notes[0]);
return null;
}
}
private static class DeleteAllNotesAsyncTask extends AsyncTask<Void ,Void,Void>{
// we need it to do database operations
private NoteDao noteDao;
// because the class is static we can't access the NoteDao directly so we have to pass it throw a constructor
public DeleteAllNotesAsyncTask(NoteDao noteDao) {
this.noteDao = noteDao;
}
@Override
protected Void doInBackground(Void... voids) {
noteDao.deleteAllNotes();
return null;
}
}
}
数据库类
package com.example.architectureexample;
// 3rd video
// this is a class where we connect the database table with the dao to get the to communicate with each other
import android.content.Context;
import android.os.AsyncTask;
import androidx.annotation.NonNull;
import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
import androidx.sqlite.db.SupportSQLiteDatabase;
@Database(entities = {Note.class},version = 1)
public abstract class NoteDatabase extends RoomDatabase {
private static NoteDatabase instance;
// this will access our Dao
public abstract NoteDao noteDao();
public static synchronized NoteDatabase getInstance(Context context){
if(instance!=null){
instance = Room.databaseBuilder(context.getApplicationContext(),
NoteDatabase.class,"note_database")
.fallbackToDestructiveMigration()
// added call back 4th video
.addCallback(roomCallback)
.build();
}
return instance;
}
// 4th video
private static RoomDatabase.Callback roomCallback = new RoomDatabase.Callback(){
@Override
public void onCreate(@NonNull SupportSQLiteDatabase db) {
super.onCreate(db);
new PopulateDbAsyncTask(instance).execute();
}
};
private static class PopulateDbAsyncTask extends AsyncTask<Void,Void,Void>{
private NoteDao noteDao;
private PopulateDbAsyncTask(NoteDatabase database){
noteDao = database.noteDao();
}
@Override
protected Void doInBackground(Void... voids) {
noteDao.insert(new Note("Title1","des1",1));
noteDao.insert(new Note("Title2","des2",2));
noteDao.insert(new Note("Title3","des3",3));
return null;
}
}
}
表类
package com.example.architectureexample;
// 2nd video
// this is the database table
import androidx.room.Entity;
import androidx.room.PrimaryKey;
@Entity(tableName = "note_table")
public class Note {
@PrimaryKey(autoGenerate = true)
private int id;
private String title ;
private String description;
private int priorty;
public Note(String title, String description, int priorty) {
this.title = title;
this.description = description;
this.priorty = priorty;
}
public void setId(int id) {
this.id = id;
}
public int getId() {
return id;
}
public String getTitle() {
return title;
}
public String getDescription() {
return description;
}
public int getPriorty() {
return priorty;
}
}
DAO界面
package com.example.architectureexample;
import androidx.lifecycle.LiveData;
import androidx.room.Dao;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;
import androidx.room.Update;
// 3rd video
// this is the dao that will interact with the databse to get the data
import java.util.List;
@Dao
public interface NoteDao {
@Insert
void insert(Note note);
@Update
void update(Note note);
@Delete
void delete(Note note);
// sql code delete from (table name)
@Query("DELETE FROM note_table")
void deleteAllNotes();
@Query("SELECT * FROM note_table ORDER BY priorty DESC")
LiveData<List<Note>> getAllNotes();
}
logcat错误
2019-09-18 17:38:07.042 15476-15476/com.example.architectureexample E/Minikin: Could not get cmap table size!
2019-09-18 17:38:07.088 15476-15497/com.example.architectureexample E/MemoryLeakMonitorManager: MemoryLeakMonitor.jar is not exist!
2019-09-18 17:38:07.373 15476-15476/com.example.architectureexample E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.architectureexample, PID: 15476
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.architectureexample/com.example.architectureexample.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.architectureexample.NoteViewModel
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3303)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3411)
at android.app.ActivityThread.-wrap12(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1994)
at android.os.Handler.dispatchMessage(Handler.java:108)
at android.os.Looper.loop(Looper.java:166)
at android.app.ActivityThread.main(ActivityThread.java:7529)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)
Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.architectureexample.NoteViewModel
at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:238)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:164)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:130)
at com.example.architectureexample.MainActivity.onCreate(MainActivity.java:22)
at android.app.Activity.performCreate(Activity.java:7383)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1218)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3256)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3411)
at android.app.ActivityThread.-wrap12(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1994)
at android.os.Handler.dispatchMessage(Handler.java:108)
at android.os.Looper.loop(Looper.java:166)
at android.app.ActivityThread.main(ActivityThread.java:7529)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Constructor.newInstance0(Native Method)
at java.lang.reflect.Constructor.newInstance(Constructor.java:334)
at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:230)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:164)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:130)
at com.example.architectureexample.MainActivity.onCreate(MainActivity.java:22)
at android.app.Activity.performCreate(Activity.java:7383)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1218)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3256)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3411)
at android.app.ActivityThread.-wrap12(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1994)
at android.os.Handler.dispatchMessage(Handler.java:108)
at android.os.Looper.loop(Looper.java:166)
at android.app.ActivityThread.main(ActivityThread.java:7529)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.example.architectureexample.NoteDao com.example.architectureexample.NoteDatabase.noteDao()' on a null object reference
at com.example.architectureexample.NoteRep.<init>(NoteRep.java:19)
at com.example.architectureexample.NoteViewModel.<init>(NoteViewModel.java:18)
at java.lang.reflect.Constructor.newInstance0(Native Method)
at java.lang.reflect.Constructor.newInstance(Constructor.java:334)
at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:230)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:164)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:130)
at com.example.architectureexample.MainActivity.onCreate(MainActivity.java:22)
at android.app.Activity.performCreate(Activity.java:7383)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1218)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3256)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3411)
at android.app.ActivityThread.-wrap12(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1994)
at android.os.Handler.dispatchMessage(Handler.java:108)
at android.os.Looper.loop(Looper.java:166)
at android.app.ActivityThread.main(ActivityThread.java:7529)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:245)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:921)
答案 0 :(得分:0)
您的数据库从未初始化。
instance!=null
应该是instance==null
。
答案 1 :(得分:0)
我也遇到了同样的问题,因为不推荐使用ViewModelProviders。 所以我用过:
NoteViewModel viewModel = new ViewModelProvider.AndroidViewModelFactory(getApplication()).create(NoteViewModel.class);