java.lang.RuntimeException：无法创建 ViewModel class 的实例

Question

I am trying to instantiate UserViewModel in my activity however it keeps giving me a java.lang.RuntimeException: Cannot create an instance of viewmodel class kindly assist.

This is how my ViewModel looks like

public class UserViewModel extends AndroidViewModel {

    private NodeAuthService api;
    private SharedPreferences pref;
    private static MutableLiveData<List<User>> userDetails = new MutableLiveData<>();

    public UserViewModel(@NonNull Application application) {
        super(application);
        api = AuthRetrofitClient.getInstance().create(NodeAuthService.class);
    }


    private String email = pref.getString("email", "");

    public void loadUser(){
        Call<List<User>> call;
        call = api.getUser(email);
        call.enqueue(new Callback<List<User>>() {
            @Override
            public void onResponse(Call<List<User>> call, Response<List<User>> response) {
                userDetails.postValue(response.body());

            }

            @Override
            public void onFailure(Call<List<User>> call, Throwable t) {
                Log.d("USER",t.getMessage());
            }
        });

    }

   public MutableLiveData<List<User>>getUserDetails(){
        return userDetails;
   }
}

This is how my activity is setup

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.navigation_drawer);
    String nameVm;

    userViewModel = ViewModelProviders.of(this).get(UserViewModel.class);

    userViewModel.loadUser();

    userViewModel.getUserDetails().observe(this, new Observer<List<User>>() {
        @Override
        public void onChanged(List<User> users) {
            if (users != null){
                for (int i = 0; i<users.size(); i++){
                    nameVm = String.valueOf(users.get(0));
                }
            }
        }
    });

}

Answer 1

Create ViewModelFactory class

public class MyViewModelFactory implements ViewModelProvider.Factory {
    private Application mApplication;
    public MyViewModelFactory(Application application) {
        mApplication = application;           
    }

    @Override
    public <T extends ViewModel> T create(Class<T> modelClass) {
        // Replace UserViewModel →  with whatever or however you create your ViewModel
        return (T) new UserViewModel(mApplication);
    }
}

and init ViewModel like

UserViewModel myViewModel = ViewModelProviders.of(this, new MyViewModelFactory(this.getApplication())).get(UserViewModel.class);