Behavior<>中Xamarin结合不起作用

Question

我做出了将图像传输到ViewModel属性的行为。当用户с舔到元素时，库将打开。当用户从图片库中选择某个图像时，我的ImageBytes将有该图像的字节。但是，在我为该属性分配了一个新值之后，它不会传递给我的VM。

我的视图模型不响应行为中的更改。

    public class FolderDialogBehavior : Behavior<View>
    {
        public byte[] ImageBytes
        {
            get { return (byte[])GetValue(ImageBytesProperty); }
            private set 
            { 
                SetValue(ImageBytesProperty, value);
            }
        }

        public readonly static BindableProperty ImageBytesProperty = BindableProperty.Create(nameof(ImageBytes), typeof(byte[]),
            typeof(FolderDialogBehavior), null, BindingMode.TwoWay);


        private TapGestureRecognizer tapGestureRecognizer = new TapGestureRecognizer()
        {
            NumberOfTapsRequired = 1
        };

        protected override void OnAttachedTo(View view)
        {
            base.OnAttachedTo(view);

            tapGestureRecognizer.Tapped += OnTapGestureRecognizerTapped;
            view.GestureRecognizers.Add(tapGestureRecognizer);
        }

        protected override void OnDetachingFrom(View view)
        {
            base.OnDetachingFrom(view);

            tapGestureRecognizer.Tapped -= OnTapGestureRecognizerTapped;
            view.GestureRecognizers.Remove(tapGestureRecognizer);
        }

        private void OnTapGestureRecognizerTapped(object sender, EventArgs e)
        {
            GetPhotoAsync();
        }

        private async void GetPhotoAsync()
        {
            try
            {
                var photo = await MediaPicker.PickPhotoAsync();

                byte[] bytes;

                using (Stream sourceStream = await photo.OpenReadAsync())
                {
                    bytes = new byte[sourceStream.Length];
                    await sourceStream.ReadAsync(bytes, 0, (int)sourceStream.Length);
                }

                ImageBytes = bytes;
            }
            catch (Exception ex)
            {
                //await DisplayAlert("Сообщение об ошибке", ex.Message, "OK");
            }
        }
    }

     <Frame>
         <Frame.Behaviors>
             <local:FolderDialogBehavior ImageBytes="{Binding AddEmployee.UserImage, Mode=TwoWay}"/>
         </Frame.Behaviors>
     </Frame>

     public class EmployeeViewModel : OnPropertyChangedClass
     {
         private byte[] _userImage;

         public byte[] UserImage 
        {
            get => _userImage; 

            // *** I don't get here with debugging.***

            set => SetProperty(ref _userImage, value); 
        }
     }

    public abstract class OnPropertyChangedClass : INotifyPropertyChanged
    {
        /// <inheritdoc cref="INotifyPropertyChanged"/>
        public event PropertyChangedEventHandler PropertyChanged;

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

        protected void SetProperty<T>(ref T propertyFiled, T newValue, [CallerMemberName] string propertyName = null)
        {
            if (!object.Equals(propertyFiled, newValue))
            {
                T oldValue = propertyFiled;
                propertyFiled = newValue;
                RaisePropertyChanged(propertyName);

                OnPropertyChanged(propertyName, oldValue, newValue);
            }
        }

        protected virtual void OnPropertyChanged(string propertyName, object oldValue, object newValue) { }
    }

Answer 1

    protected override void OnAttachedTo(View bindable)
    {

        BindingContext = bindable.BindingContext;
        bindable.BindingContextChanged += Bindable_BindingContextChanged;
        /* Some Code */
        base.OnAttachedTo(bindable);
    }

    protected override void OnDetachingFrom(View bindable)
    {
        bindable.BindingContextChanged -= Bindable_BindingContextChanged;
        /* Some Code */
        base.OnDetachingFrom(bindable);
    }

    private void Bindable_BindingContextChanged(object sender, EventArgs e)
    {
        if (sender is View view)
        {
            BindingContext = (sender as View).BindingContext;
        }
    }