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c#androidmvvmcross

Bind Alpha Android

I try bind to a ImageView Alpha property, I created a converter to set this for a boolean value. But I don't view setted value.

This is my converter

public class BooleanToOpacity : MvxValueConverter<bool,int>
{
    protected override int Convert(bool value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        var boolean = value as bool?;

        if (boolean.Value == true)
        {
            return 1;
        }
        else
        {
            return 127;
        }                
    }

    protected override bool ConvertBack(int value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (value > 127)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
}

I use a Integer value because the Alpha value is 0 to 255.

This my line to binding value. 

   local:MvxBind="alpha TwitterPost, Converter=BooleanToOpacity" />

And the Mvx trace me this

   MvxBind:Warning: 11.56 Failed to create target binding for binding alpha for TwitterPost
   [0:] MvxBind:Warning: 11.56 Failed to create target binding for binding alpha for TwitterPost
   10-21 15:54:22.280 I/mono-stdout(12096): MvxBind:Warning: 11.56 Failed to create target binding for binding alpha for TwitterPost

Any idea?

Thanks in advance.

Solution

  • There is no C# property available for "Alpha" in Xamarin.Android, so the Mvx framework doesn't know how to bind to it.

    You could create a custom binding for this if you wanted to - something like:

        public class ImageViewAlphaTargetBinding : MvxAndroidTargetBinding
    {
        public ImageViewAlphaTargetBinding (ImageView target) : base(target)
        {
        }

        protected override void SetValueImpl(object target, object value)
        {
            var imageView = (ImageView)target;
            imageView.SetMyProperty((int)value);
        }

        public override Type TargetType
        {
            get { return typeof(int); }
        }
    }

    registered in Setup.cs with:

    protected override void FillTargetFactories(Cirrious.MvvmCross.Binding.Bindings.Target.Construction.IMvxTargetBindingFactoryRegistry registry)
{
    registry.RegisterCustomBindingFactory<ImageView>(
                    "Alpha",
                    v=> new ImageViewAlphaTargetBinding (v) );
    base.FillTargetFactories(registry);
}

    Alternatively, you could inherit from ImageView and provide a custom view that exposes an Alpha property.

    For more on both these options, see: