如何修复“无法创建ViewModel类的实例”？

Question

我尝试了MVVM架构，实现了所有需要的类和方法。在MainActivity中创建ViewModel类的对象时，我得到以下错误java.lang.RuntimeException：无法创建类com.prathameshmore.getNotes.ViewModel.NoteViewModel的实例。

我试了一个YouTube教程中的例子。我把所有的实现都做对了。我尝试过将ViewModel类和构造函数公开，但应用程序仍然在运行时崩溃。

mainactivity.java

public class MainActivity extends AppCompatActivity {

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) {
            Toast.makeText(MainActivity.this, "Updated", Toast.LENGTH_SHORT).show();
        }
    });

}
}

NoteViewModel.java

public class NoteViewModel extends AndroidViewModel {

private NoteRepository noteRepository;
private LiveData<List<Note>> allNotes;

public NoteViewModel(@NonNull Application application) {
    super(application);
    noteRepository = new NoteRepository(application);
    allNotes = noteRepository.getAllNotes();
}

public void insert(Note note) {
    noteRepository.insert(note);
}

public void update(Note note) {
    noteRepository.update(note);
}

public void delete(Note note) {
    noteRepository.delete(note);

}

public void deleteAllNotes() {
    noteRepository.deleteAllNotes();
}

public LiveData<List<Note>> getAllNotes() {
    return allNotes;
}

}

noteRepository.java

public class NoteRepository {

private NoteDao noteDao;
private LiveData<List<Note>> allNotes;

public NoteRepository(Application application) {
    NoteDatabase database = NoteDatabase.getInstance(application);
    noteDao = database.noteDao();
    allNotes = noteDao.getAllNotes();
}

public void insert(Note note){
    new InsertNoteAsyncTask(noteDao).execute(note);
}

public void delete(Note note) {
    new DeleteNoteAsyncTask(noteDao).execute(note);
}

public void update(Note note) {
    new UpdateNoteAsyncTask(noteDao).execute(note);
}

public void deleteAllNotes() {
    new DeleteAllNotesAsyncTask(noteDao).execute();
}

public LiveData<List<Note>> getAllNotes() {
    return allNotes;
}

private static class InsertNoteAsyncTask extends AsyncTask<Note, Void, Void> {

    private NoteDao noteDao;

    private 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> {

    private NoteDao noteDao;

    private 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> {

    private NoteDao noteDao;

    private 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> {

    private NoteDao noteDao;

    private DeleteAllNotesAsyncTask(NoteDao noteDao) {
        this.noteDao = noteDao;
    }

    @Override
    protected Void doInBackground(Void...voids) {
        noteDao.deleteAllNotes();
        return null;
    }
}


}

日志

Answer 1

您需要为ViewModel提供一个工厂类。

public class MyViewModelFactory implements ViewModelProvider.Factory {
    private Application mApplication;
    private String mParam;


    public MyViewModelFactory(Application application, String param) {
        mApplication = application;
        mParam = param;
    }


    @Override
    public <T extends ViewModel> T create(Class<T> modelClass) {
        return (T) new MyViewModel(mApplication, mParam);
    }
}

并且在实例化视图模型时,您可以这样做：

MyViewModel myViewModel = ViewModelProviders.of(this, new MyViewModelFactory(this.getApplication(), "my awesome param")).get(MyViewModel.class);