英文:
Cannot create instance of ViewModel in android
问题
以下是翻译好的内容：
你好，我是Android的新手，一直在尝试学习MVVM，但是我无法弄清楚为什么会出现错误，显示无法创建ViewModel类的实例：
以下是我收到的错误信息：
2020-06-29 12:30:26.293 15174-15174/com.carrot.roomdatabase E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.carrot.roomdatabase, PID: 15174
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.carrot.roomdatabase/com.carrot.roomdatabase.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.carrot.roomdatabase.ViewModel
...
Caused by: java.lang.RuntimeException: Cannot create an instance of class com.carrot.roomdatabase.ViewModel
...
Caused by: java.lang.InstantiationException: java.lang.Class<com.carrot.roomdatabase.ViewModel> has no zero argument constructor
...
以下是我的类：
MainActivity.java类：
public class MainActivity extends AppCompatActivity {
private RecyclerView recyclerView;
private recycleViewAdapter adapter;
private ViewModel viewModel;
private List<NotesEntity> notesEntityList = new ArrayList<>();
private FloatingActionButton btn;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
viewModel = new ViewModelProvider(this).get(ViewModel.class);
viewModel.getAllNotes().observe(this, new Observer<List<NotesEntity>>() {
@Override
public void onChanged(List<NotesEntity> notesEntities) {
Toast.makeText(MainActivity.this, "onChanged", Toast.LENGTH_SHORT).show();
adapter.notifyDataSetChanged();
}
});
btn = findViewById(R.id.floatingActionButton);
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(MainActivity.this , AddNote.class));
}
});
init();
}
private void init(){
adapter = new recycleViewAdapter(notesEntityList);
recyclerView.setAdapter(adapter);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
}
}
ViewModel.java类：
public class ViewModel extends AndroidViewModel {
private Repository repository;
private LiveData<List<NotesEntity>> allNotes;
public ViewModel(@NonNull Application application) {
super(application);
repository = new Repository(application);
allNotes = repository.getNotes();
}
public void insertThis(NotesEntity notesEntity){
repository.Insert(notesEntity);
}
public void deleteThis(NotesEntity notesEntity){
repository.Delete(notesEntity);
}
public LiveData<List<NotesEntity>> getAllNotes(){
return allNotes;
}
}
有人能帮助我找出我做错了什么吗？谢谢。
英文:
Hello I am new to android and have been trying to learn MVVM but I am not able to figure out why I am error saying cannot create instance of viewmodel class :
These are the errors that I am getting:
2020-06-29 12:30:26.293 15174-15174/com.carrot.roomdatabase E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.carrot.roomdatabase, PID: 15174
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.carrot.roomdatabase/com.carrot.roomdatabase.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.carrot.roomdatabase.ViewModel
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3388)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3527)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2123)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7710)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)
Caused by: java.lang.RuntimeException: Cannot create an instance of class com.carrot.roomdatabase.ViewModel
at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:221)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
at com.carrot.roomdatabase.MainActivity.onCreate(MainActivity.java:31)
at android.app.Activity.performCreate(Activity.java:7820)
at android.app.Activity.performCreate(Activity.java:7809)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1318)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3363)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3527) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2123) 
at android.os.Handler.dispatchMessage(Handler.java:107) 
at android.os.Looper.loop(Looper.java:214) 
at android.app.ActivityThread.main(ActivityThread.java:7710) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950) 
Caused by: java.lang.InstantiationException: java.lang.Class<com.carrot.roomdatabase.ViewModel> has no zero argument constructor
at java.lang.Class.newInstance(Native Method)
at androidx.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:219)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:187) 
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150) 
at com.carrot.roomdatabase.MainActivity.onCreate(MainActivity.java:31) 
at android.app.Activity.performCreate(Activity.java:7820) 
at android.app.Activity.performCreate(Activity.java:7809) 
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1318) 
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3363) 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3527) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2123) 
at android.os.Handler.dispatchMessage(Handler.java:107) 
at android.os.Looper.loop(Looper.java:214) 
at android.app.ActivityThread.main(ActivityThread.java:7710) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:516) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950) 
2020-06-29 12:30:26.304 15174-15174/? I/Process: Sending signal. PID: 15174 SIG: 9
Below are my classes
MainActivity.java class:
public class MainActivity extends AppCompatActivity {
private RecyclerView recyclerView;
private recycleViewAdapter adapter;
private ViewModel viewModel;
private List<NotesEntity> notesEntityList = new ArrayList<>();
private FloatingActionButton btn;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
viewModel = new ViewModelProvider(this).get(ViewModel.class);
viewModel.getAllNotes().observe(this, new Observer<List<NotesEntity>>() {
@Override
public void onChanged(List<NotesEntity> notesEntities) {
Toast.makeText(MainActivity.this, "onChanged", Toast.LENGTH_SHORT).show();
adapter.notifyDataSetChanged();
}
});
btn = findViewById(R.id.floatingActionButton);
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
startActivity(new Intent(MainActivity.this , AddNote.class));
}
});
init();
}
private void init(){
adapter = new recycleViewAdapter(notesEntityList);
recyclerView.setAdapter(adapter);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
}
}
ViewModel.java class:
public class ViewModel extends AndroidViewModel {
private Repository repository;
private LiveData<List<NotesEntity>> allNotes;
public ViewModel(@NonNull Application application) {
super(application);
repository = new Repository(application);
allNotes = repository.getNotes();
}
public void insertThis(NotesEntity notesEntity){
repository.Insert(notesEntity);
}
public void deleteThis(NotesEntity notesEntity){
repository.Delete(notesEntity);
}
public LiveData<List<NotesEntity>> getAllNotes(){
return allNotes;
}
}
can someone please help me in figuring out what is it that I am doing wrong.
Thank you.
答案1
得分: 3
初始化 ViewModel 的代码如下。您还需要在 ViewModelProvider 构造函数中传递 ViewModelFactory。
viewModel = new ViewModelProvider(this, new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(ViewModel.class);
希望这对您有所帮助......
英文:
Initialise ViewModel like this. You need to also pass ViewModelFactory with ViewModelProvider constructor.
viewModel = new ViewModelProvider(this, new ViewModelProvider.AndroidViewModelFactory(getApplication())).get(ViewModel.class);
Hope this helps.......
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