How to Build a Simple Contact List Android App using MVVM and Room Database?

@Entity(tableName = "Contacts")
class Contacts (
    @PrimaryKey(autoGenerate = true)
    var id : Int?=null,
    var name : String,
    var number : String
)

@Dao
interface ContactDao  {
    @Query("Select * from Contacts")
    fun getAllContacts() : LiveData<List<Contacts>>

    @Insert(onConflict = OnConflictStrategy.REPLACE )
    fun insertContact(contact : Contacts)

    @Delete
    fun delete(contact: Contacts)
}

@Database(entities = [Contacts::class], version = 1, exportSchema = false)
abstract class ContactDatabase : RoomDatabase() {
    // Dao interface for the database
    abstract fun contactsDao() : ContactDao
    companion object {
        @Volatile
        var INSTANCE : ContactDatabase?=null

        // Singleton instance of the database
        fun getDatabaseInstance(context : Context) : ContactDatabase{
            val tempInstance = INSTANCE
            if(tempInstance!=null){
                return tempInstance
            }
            // Synchronized block to make sure that
            // only one instance of the database is created
            synchronized(this){
                val roomDatabaseInstance = Room.databaseBuilder(context,ContactDatabase::class.java,"Contacts").allowMainThreadQueries().build()
                INSTANCE = roomDatabaseInstance
                return roomDatabaseInstance
            }
        }

    }
}

class ContactRepository(val dao : ContactDao)
{
    // function to get all contacts from the database
    fun getAllContacts() : LiveData<List<Contacts>>{
        return dao.getAllContacts()
    }
    
    // function to insert a contact in the database
    fun insertContact(contact : Contacts) {dao.insertContact(contact)}

    // function to delete a contact from the database
    fun deleteContact(contact: Contacts) {
        dao.delete(contact)
    }
}

class ContactsAdapter(val context : Context, val list : List<Contacts>) : RecyclerView.Adapter<ContactsAdapter.ViewHolder>() {
    // Inner ViewHolder class
    class ViewHolder(val binding : ContactsLayoutBinding) : RecyclerView.ViewHolder(binding.root){}
    
    // DAO instance to interact with the database
    private val dao = ContactDatabase.getDatabaseInstance(context).contactsDao()
    
    // function to inflate the layout for each contact and create a new ViewHolder instance
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        return ViewHolder(
            ContactsLayoutBinding.inflate(LayoutInflater.from(parent.context),parent,false)
        )
    }

    // function to bind the data to the view elements of the ViewHolder
    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        holder.binding.contactName.text = list[position].name
        holder.binding.contactNumber.text = list[position].number
        // delete button onClickListener to delete the 
        // contact from the database and notify the
        // adapter of the change
        holder.binding.deleteButton.setOnClickListener{
            dao.delete(list[position])
            notifyItemRemoved(position)
        }
        
        // itemView onClickListener to make a phone call
        // to the number of the contact
        holder.itemView.setOnClickListener{
            val intent = Intent(Intent.ACTION_CALL, Uri.parse("" + list[position].number))
            context.startActivity(intent)
        }
    }

    // function returns the number of items in the list
    override fun getItemCount(): Int {
       return list.size
    }
}

class ContactViewModel(application: Application) : AndroidViewModel(application) {
    val repository : ContactRepository
    init {
        val dao = ContactDatabase.getDatabaseInstance(application).contactsDao()
        repository = ContactRepository(dao)
    }
    fun addContacts(contact : Contacts){
        repository.insertContact(contact)
    }
    fun getAllContacts() : LiveData<List<Contacts>> = repository.getAllContacts()
}

class CreateContact : AppCompatActivity() {
    // private variable to inflate the layout for the activity
    private lateinit var binding : ActivityCreateContactBinding
    
    // variable to access the ViewModel class
    val viewModel : ContactViewModel by viewModels()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        // inflate the layout
        binding = ActivityCreateContactBinding.inflate(layoutInflater)
        setContentView(binding.root)
        
        // set onClickListener for save button
        binding.save.setOnClickListener{
            createContact(it)
        }
    }

    // function to create new contact and call 
    // the addContacts function from the ViewModel class
    private fun createContact(it: View?) {
        // read name and number from EditTexts
        val name = binding.etName.text.toString()
        val number = binding.etNumber.text.toString()
        
        // create new contact object
        val data = Contacts(null,name = name , number = number)
        
        // call addContacts function from the ViewModel class
        viewModel.addContacts(data)
        
        // display a Toast message to confirm the save
        Toast.makeText(this@CreateContact, "Saved", Toast.LENGTH_SHORT).show()
        
        // start MainActivity
        startActivity(Intent(this@CreateContact,MainActivity::class.java))
    }
}

class MainActivity : AppCompatActivity() {
    // private variable to inflate the layout for the activity
    private lateinit var binding: ActivityMainBinding
    
    // variable to access the ViewModel class
    val viewModel : ContactViewModel  by viewModels()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        
        // inflate the layout
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)
        
        // set onClickListener for the floating action button
        binding.floatingActionButton.setOnClickListener{
            val intent = Intent(this , CreateContact::class.java)
            startActivity(intent)
        }
        
        // Observe the LiveData returned by the getAllContacts method
        viewModel.getAllContacts().observe(this , Observer {  list->
            // set the layout manager and the adapter for the recycler view
            binding.recyclerView.layoutManager = LinearLayoutManager(applicationContext)
            binding.recyclerView.adapter = ContactsAdapter(this,list)
        })
    }
}