Converting Binding Values
Often you will need to negate a value that you're binding to. A frequent use for this is to show/hide a control or to enable/disable it. You can negate a binding value by prepending a "bang" operator:
!.For example you might want to show one control when another control is disabled.
<StackPanel>
<TextBox Name="input" IsEnabled="{Binding AllowInput}"/>
<TextBlock IsVisible="{Binding !AllowInput}">Sorry, no can do!</TextBlock>
</StackPanel>
Negation also works when binding to non-boolean values. First of all, the value is converted to a boolean using
Convert.ToBoolean and the result from this is negated. Because the integer value 0 is considered false and all other integer values are considered true, you can use this to show a message when a collection is empty:<Panel>
<ListBox Items="{Binding Items}"/>
<TextBlock IsVisible="{Binding !Items.Count}">No results found</TextBlock>
</Panel>
A "double-bang" can be used to convert a non-boolean value to a boolean value. For example to hide a control when a collection is empty:
<Panel>
<ListBox Items="{Binding Items}" IsVisible="{Binding !!Items.Count}"/>
</Panel>
Note: TheIValueConverterinterface is not available in .NET standard 2.0 so we ship our own, in theAvalonia.Data.Convertersnamespace.
Usage is identical to other XAML frameworks:
<Window xmlns="https://github.com/avaloniaui"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:ExampleApp;assembly=ExampleApp">
<Window.Resources>
<local:MyConverter x:Key="myConverter"/>
</Window.Resources>
<TextBlock Text="{Binding Value, Converter={StaticResource myConverter}}"/>
</Window>
Avalonia supplies a number of built-in value converters for common scenarios:
Converter | Description |
|---|---|
StringConverters.IsNullOrEmpty | Returns true if the input string is null or empty |
StringConverters.IsNotNullOrEmpty | Returns false if the input string is null or empty |
ObjectConverters.IsNull | Returns true if the input is null |
ObjectConverters.IsNotNull | Returns false if the input is null |
BoolConverters.And | A multi-value converter that returns true if all inputs are true. |
BoolConverters.Or | A multi-value converter that returns true if any input is true. |
Hiding a
TextBlock if the bound text is null or empty:<TextBlock Text="{Binding MyText}"
IsVisible="{Binding MyText, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"/>
Hiding a
ContentControl if the bound content is null or empty:<ContentControl Content="{Binding MyContent}"
IsVisible="{Binding MyContent, Converter={x:Static ObjectConverters.IsNotNull}}"/>
from now on assume converters are imported as shown in the previous "Binding Converters" section
Convert text to specific case from a parameter
<TextBlock Text="{Binding TheContent,
Converter={StaticResource textCaseConverter},
ConverterParameter=lower}" />
public class TextCaseConverter : IValueConverter
{
public static readonly TextCaseConverter Instance = new();
public object? Convert( object? value, Type targetType, object? parameter, CultureInfo culture )
{
if (value is string sourceText && parameter is string targetCase
&& targetType.IsAssignableTo(typeof(string)))
{
switch (targetCase)
{
case "upper":
case "SQL":
return sourceText.ToUpper();
case "lower":
return sourceText.ToLower();
case "title": // Every First Letter Uppercase
var txtinfo = new System.Globalization.CultureInfo("en-US",false).TextInfo;
return txtinfo.ToTitleCase(sourceText);
default:
// invalid option, return the exception below
break;
}
}
// converter used for the wrong type
return new BindingNotification(new InvalidCastException(), BindingErrorType.Error);
}
public object ConvertBack( object? value, Type targetType, object? parameter, CultureInfo culture )
{
throw new NotSupportedException();
}
}
Converting a bound object to different target types contextually
<Image Width="42"
Source="{Binding Animal, Converter={StaticResource animalConverter}}"/>
<TextBlock
Text="{Binding Animal, Converter={StaticResource animalConverter}}" />
public class AnimalConverter : IValueConverter
{
public static readonly AnimalConverter Instance = new();
public object? Convert( object? value, Type targetType, object? parameter, CultureInfo culture )
{
if (value is Animal animal)
{
if (targetType.IsAssignableTo(typeof(IImage)))
{
img = @"icons/generic-animal-placeholder.png"
switch (animal)
{
case Dog d:
img = d.IsGoodBoy ? @"icons/dog-happy.png" : @"icons/dog.png";
break;
case Cat:
img = @"icons/cat.png";
break;
// etc. etc.
}
// see https://docs.avaloniaui.net/docs/controls/image
return BitmapAssetValueConverter.Instance
.Convert(img, typeof(Bitmap), parameter, culture);
}
else if (targetType.IsAssignableTo(typeof(string)))
{
return !string.IsNullOrEmpty(animal.NickName) ?
$"{animal.Name} \"{animal.NickName}\"" : animal.Name;
}
}
// converter used for the wrong type
return new BindingNotification(new InvalidCastException(), BindingErrorType.Error);
}
public object ConvertBack( object? value, Type targetType, object? parameter, CultureInfo culture )
{
throw new NotSupportedException();
}
}
Last modified 8mo ago