AndroidViewModel has no zero argument constructor

Question

I am trying to create an instance of my AndroidViewModel in MainActivity. When I do this I get the following error has no zero argument constructor

Here is my RecipeViewModel

package com.example.kookrecepten;

import android.app.Application;

import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;

import java.util.List;

public class RecipeViewModel extends AndroidViewModel {
    private RecipeRepository recipeRepository;
    private LiveData<List<Recipe>> allRecipes;

    public RecipeViewModel(Application application) {
        super(application);
        recipeRepository = new RecipeRepository(application);
        allRecipes = recipeRepository.getAllRecipes();
    }

    public void insert(Recipe recipe) {
        recipeRepository.insert(recipe);
    }

    public void update(Recipe recipe) {
        recipeRepository.update(recipe);
    }

    public void delete(Recipe recipe) {
        recipeRepository.delete(recipe);
    }

    public void deleteAll() {
        recipeRepository.deleteAllRecipes();
    }

    public LiveData<List<Recipe>> getAllRecipes() {
        return allRecipes;
    }
}

Now correct me if I'm wrong but AndroidViewModel needs the Application context in the constructor and ViewModel doesn't. So I have no idea why android asks for a zero arguments constructor.

Here is my Main activity where I ask for an instance.

package com.example.kookrecepten;

import android.os.Bundle;
import android.view.View;
import android.widget.Toast;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;

import com.example.kookrecepten.databinding.ActivityMainBinding;

import java.util.List;


public class MainActivity extends AppCompatActivity {
    private ActivityMainBinding binding;
    private RecipeViewModel recipeViewModel;

    protected void onCreate (Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        View view = binding.getRoot();
        setContentView(view);

        //Get an instance of the RecipeViewModel.
        recipeViewModel = new ViewModelProvider(this).get(RecipeViewModel.class);
        recipeViewModel.getAllRecipes().observe(this, new Observer<List<Recipe>>() {
            @Override
            public void onChanged(List<Recipe> recipes) {
                //Update recycle view

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

And this are my implementations.

def lifecycle_version = "2.2.0"
def room_version = "2.2.5"

//LifeCycle Components
    // ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
    // LiveData
    implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
    // Annotation processor
    implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"

    //Room Components
    implementation "androidx.room:room-runtime:$room_version"
    annotationProcessor "androidx.room:room-compiler:$room_version"

Answer 1

if you're using hilt, you probably forgot to annotate your activity with @AndroidEntryPoint

Answer 2

apparently if I change

recipeViewModel = new ViewModelProvider(this).get(RecipeViewModel.class);

to this

recipeViewModel = new ViewModelProvider(this, ViewModelProvider.AndroidViewModelFactory.getInstance(this.getApplication())).get(RecipeViewModel.class);

It works. I don't know why this solution works could someone explain?

Answer 3

I had a look to Android Source code. If your activity was extending the class Activity, instead of AppCompatActivity you wouldn't have seen this issue. The class Activity provides an implementation for the interface HasDefaultViewModelProviderFactory which provides a correct factory for classes extending AndroidViewModel. Unfortunately, AppCompatActivity still doesn't implement that interface, and then, no correct factory is available. So, either you provide the factory as you did, or you might implement by yourself the interface HasDefaultViewModelProviderFactory.

Answer 4

Can you try with the old, deprecated

recipeViewModel = ViewModelProviders.of(this).get(RecipeViewModel.class);

This definitely used to work in our applications. If you're the adventureous type, you can also use the debugger and step into the get() method of the ViewModelProvider to check how it attempts to get an instance of the viewmodel.