private void DeleteAssignment(object assignmentInstance)
        {
            AssignmentViewModel assignmentViewModel = assignmentInstance as AssignmentViewModel;

            if (assignmentViewModel.Assignment != null && assignmentViewModel.Assignment.AssignmentId != 0)
            {
                using (AppDbContext db = App.GetContext())
                {
                    db.Assignments.Remove(assignmentViewModel.Assignment);
                    db.SaveChanges();
                }
            }
            Back();
        }
        private void SaveAssignment(object assignmentInstance)
        {
            AssignmentViewModel assignmentViewModel = assignmentInstance as AssignmentViewModel;

            if (assignmentViewModel.Assignment != null)
            {
                if (!assignmentViewModel.IsValid)
                {
                    Application.Current.MainPage.DisplayAlert("Предупреждение", "Задайте название задания, выберите рецепт и введите норму вылива", "Ok");
                    return;
                }

                if (assignmentViewModel.Size != null)
                {
                    if (String.IsNullOrEmpty(assignmentViewModel.Unit))
                    {
                        Application.Current.MainPage.DisplayAlert("Предупреждение", "Задайте ед. изм. размера задания", "Ok");
                        return;
                    }
                }

                using (AppDbContext db = App.GetContext())
                {
                    if (assignmentViewModel.Assignment.AssignmentId == 0)
                    {
                        db.Entry(assignmentViewModel.Assignment).State = EntityState.Added;
                    }
                    else
                    {
                        db.Assignments.Update(assignmentViewModel.Assignment);
                    }
                    db.SaveChanges();
                }
            }
            Back();
        }