Xamarin 绑定在 Behavior<> 中不起作用

如何解决Xamarin 绑定在 Behavior<> 中不起作用

我制作了将图像传输到 ViewModel 属性的行为。 当用户单击该元素时，图库将打开。当用户从图库中选择一些图像时，我的 ImageBytes 将包含该图像的字节数。但是在我为该属性分配了一个新值后，它并没有传递给我的 VM。

我的视图模型不响应行为的变化。

    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,(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) { }
    }

解决方法

    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;
        }
    }