onlyrepository.cs

    public class TipCalculatorViewModel : INotifyPropertyChanged
    {
        private readonly TipCalculation _model;
        private readonly ITipCalculatorRepository _repository;

        public event PropertyChangedEventHandler PropertyChanged;

        public TipCalculatorViewModel(ITipCalculatorRepository repository)
        {
            _model = new TipCalculation();
            _repository = repository;
            SaveCalculationCommand = new RelayCommand<TipCalculation>(SaveCalculation);
        }

        public decimal BillAmount
        {
            get { return _model.BillAmount; }
            set
            {
                _model.BillAmount = value;
                OnPropertyChanged(nameof(BillAmount));
            }
        }

        public decimal TipRate
        {
            get { return _model.TipRate; }
            set
            {
                _model.TipRate = value;
                OnPropertyChanged(nameof(TipRate));
            }
        }

        public decimal Tip => Math.Round(BillAmount * (TipRate / 100), 2);
        public decimal Total => BillAmount + TipRate;

        public ICommand SaveCalculationCommand { get; private set; }

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        private void SaveCalculation(TipCalculation model)
        {
            model.Tip = Tip;
            model.Total = Total;

            //Save it
            _repository.Save(model);
        }
    }

repositoryandmodel.cs

    public class TipCalculatorViewModel : INotifyPropertyChanged
    {
        private readonly TipCalculation _model;
        private readonly ITipCalculatorRepository _repository;

        public event PropertyChangedEventHandler PropertyChanged;

        public TipCalculatorViewModel(TipCalculation model, ITipCalculatorRepository repository)
        {
            _model = model;
            _repository = repository;
            SaveCalculationCommand = new RelayCommand<TipCalculation>(SaveCalculation);
        }

        public decimal BillAmount
        {
            get { return _model.BillAmount; }
            set
            {
                _model.BillAmount = value;
                OnPropertyChanged(nameof(BillAmount));
            }
        }

        public decimal TipRate
        {
            get { return _model.TipRate; }
            set
            {
                _model.TipRate = value;
                OnPropertyChanged(nameof(TipRate));
            }
        }

        public decimal Tip => Math.Round(BillAmount * (TipRate / 100), 2);
        public decimal Total => BillAmount + TipRate;

        public ICommand SaveCalculationCommand { get; private set; }

        [NotifyPropertyChangedInvocator]
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        private void SaveCalculation(TipCalculation model)
        {
            model.Tip = Tip;
            model.Total = Total;

            //Save it
            _repository.Save(model);
        }
    }