public MainPageViewModel(INavigationService navigationService, IEventAggregator eventAggregator,
                                 IPageDialogService dialogService, IPicture picture
                                 )
        {
            //IUnityContainer myContainer = (App.Current as PrismApplication).Container;
            //_picture= myContainer.Resolve<IPicture>();
            // 相依性服務注入的物件
            _picture           = picture;
            _dialogService     = dialogService;
            _eventAggregator   = eventAggregator;
            _navigationService = navigationService;

            // 頁面中綁定的命令
            TakePictureCommand = new DelegateCommand(async() =>
            {
                var fooAction = await _dialogService.DisplayActionSheetAsync("請選擇圖片來源", "取消", null, "相簿", "拍照");
                if (fooAction == "相簿")
                {
                    App.CroppedImage = null;
                    await SelectPicture();
                }
                else if (fooAction == "拍照")
                {
                    App.CroppedImage = null;
                    await TakePicture();
                }
                else if (fooAction == "取消")
                {
                }
            });

            SavePictureCommand = new DelegateCommand(() =>
            {
                if (App.CroppedImage != null)
                {
                    _picture.SavePictureToDisk("MyCrop", App.CroppedImage);
                }
            });

            LoadPictureCommand = new DelegateCommand(() =>
            {
                App.CroppedImage   = _picture.LoadPictureFromDisk("MyCrop");
                Stream stream      = new MemoryStream(App.CroppedImage);
                fooLoadImageSource = ImageSource.FromStream(() => stream);
            });
        }