Wpf – Binding off Visibility of another element – and inverting

bindingsilverlightwpf

I'd like to make Control A visibile if Control B is hidden, and vice versa. Right now I have this converter:

public class InvertVisibilityConverter : IValueConverter {

    public Object Convert(Object value, Type targetType, Object parameter, CultureInfo culture) {
        if (targetType == typeof(Visibility)) {
            Visibility vis = (Visibility)value;
            return vis == Visibility.Collapsed ? Visibility.Visible : Visibility.Collapsed;
        }
        throw new InvalidOperationException("Converter can only convert to value of type Visibility.");
    }

    public Object ConvertBack(Object value, Type targetType, Object parameter, CultureInfo culture) {
        throw new Exception("Invalid call - one way only");
    }
}

And this XAML:

<Button Visibility="{Binding ElementName=btn1, Path=Visibility, Converter={StaticResource InvertVisibilityConverter}}">Btn2</Button>

Which works. I'm just wondering if there's a more direct way in WPF / Silverlight to accomplish this? I don't mind having a converter, I just want to make sure there's no better way I'm not aware of.

Best Answer

In this case you could do it with triggers. Example with two Buttons

<Button Name="button1">
    <Button.Style>
        <Style>
            <Style.Triggers>
                <DataTrigger Binding="{Binding ElementName=button2, Path=Visibility}" Value="Visible">
                    <Setter Property="Button.Visibility" Value="Hidden"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Button.Style>
</Button>
<Button Name="button2"/>

Related Solutions

R – Silverlight Databinding – Binding a ValueConverter to a property on a view-model

Since you're using a ViewModel to sit between your actual model and your view, I wonder if it's easier just to implement the IValueConverter logic directly in there. Sort of like:

public class ViewModel
{
    public ObservableCollection Root { get; set: }

    public ObservableCollection Children
    {
        get { /* return children items */ }
    }
}

Then you can simply bind directly to your second property:

<control:HierarchicalDataTemplate ItemsSource="{Binding Children}">

I think the main purpose of a ViewModel object is to limit the number of "tricks" (such as IValueConverters) you need to pull to get the data you need from the original model. Since you have one, you might as well make use of it.

Edit 1

... and of course now that I re-read your post I see that there's more to it. You're getting the children for each item in your "Root" collection.

How about implementing the IValueConverter as a static instance in your ViewModel itself?

public class ViewModel : IValueConverter
{ 
    public static readonly IValueConverter ChildrenConverter
        = new LoadChildrenValueConverter();
}

Now you should be able to say:

<control:HierarchicalDataTemplate 
    ItemsSource="{Binding Converter={x:Static local:ViewModel.ChildrenConverter}}">

Edit 2

Ok, you're using Silverlight so {x:Static} isn't available to you.

The only other option I can think of that will let you reuse the one static resource instead of having to declare two is to implement the IValueConverter directly in your ViewModel. This is no good if you will need to do more than one type of conversion, but if your ViewModel is very narrow-focused then it could do the job. So:

public class ViewModel : IValueConverter
{
    // move your Convert and ConvertBack methods into here
}

Now you can do this:

<control:HierarchicalDataTemplate
    ItemsSource="{Binding Converter={StaticResource ViewModel}}">

.net – ImageSourceConverter error for Source=null

@AresAvatar is right in suggesting you use a ValueConverter, but that implementation does not help the situation. This does:

public class NullImageConverter :IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null)
            return DependencyProperty.UnsetValue;
        return value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        // According to https://msdn.microsoft.com/en-us/library/system.windows.data.ivalueconverter.convertback(v=vs.110).aspx#Anchor_1
        // (kudos Scott Chamberlain), if you do not support a conversion 
        // back you should return a Binding.DoNothing or a 
        // DependencyProperty.UnsetValue
        return Binding.DoNothing;
        // Original code:
        // throw new NotImplementedException();
    }
}

Returning DependencyProperty.UnsetValue also addresses the performance issues from throwing (and ignoring) all those exceptions. Returning a new BitmapSource(uri) would also get rid of the exceptions, but there is still a performance hit (and it isn't necessary).

Of course, you'll also need the plumbing:

In resources:

<local:NullImageConverter x:Key="nullImageConverter"/>

Your image:

<Image Source="{Binding Path=ImagePath, Converter={StaticResource nullImageConverter}}"/>

Best Answer

Related Solutions

R – Silverlight Databinding – Binding a ValueConverter to a property on a view-model

.net – ImageSourceConverter error for Source=null

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