gistfile1.cs

public class CreateEditPageViewModel : ViewModelBase
    {
        public RelayCommand CancelCommand { get; set; }
        public RelayCommand<TaskItem> SaveCommand { get; set; }

        public TaskItem TaskItem { get; set; }

        private IRepository<TaskItem> _repository;
        private IPageNavigationService _navigationService;
        private IValidationService<TaskItem> _validationService; 

        public CreateEditPageViewModel(IRepository<TaskItem> repository, IPageNavigationService navigationService, IValidationService<TaskItem> validationService)
        {
            _repository = repository;
            _navigationService = navigationService;
            _validationService = validationService;
            ReceiveMessage();
            InitCommands();
        }

        private void ReceiveMessage()
        {
            MessengerInstance.Register<TaskItem>(this, RegisterTaskItem);
        }

        private void RegisterTaskItem(TaskItem item)
        {
            TaskItem = item;
        }

        private void InitCommands()
        {
            CancelCommand = new RelayCommand(() =>
            {
                _navigationService.ToTasksPage();
            });

            SaveCommand = new RelayCommand<TaskItem>(item =>
            {
                if (_validationService.Validate(item))
                {
                    TaskItem = item;
                    _navigationService.ToTasksPage();
                    _repository.AddOrUpdate(TaskItem);
                }
            });
        }