Strengtening Caliburn.Micro conversion capabilities with Scal

If one looks into how Caliburn.Micro performs binding between models and views, a number of methods must be considered that play a role.

  • Conventions play a role how bindings are derived from naming and involved elements
  • VM getter / setter situation plays a role about the supported Binding directions
  • Does the VM implement anything Validation-specific?
  • Is there a type mismatch between the binding source and target? (classic example: Boolean -> Visibility)

Especially for the last one Scal introduces a configuration hook to provide your own Value Converters. Check out the usage in the sample application:

Converters.ApplyDefaults();
...
public static ConverterConfiguration ApplyDefaults(this ConverterConfiguration config)
{
    config
        .Add<bool, Visibility, BooleanToVisibilityConverter>()
        .Add<string, ImageSource, PathToImageSourceConverter>();
    return config;
}

This is supported by replacing a certain method deep in the bowels of Scal:

public class ValueConverterManagement
{
    private readonly Dictionary<Tuple<Type, Type>, IValueConverter> _converters;

    public ValueConverterManagement(AppModel model)
    {
        ConventionManager.ApplyValueConverter = ApplyConverter;
        _converters =  model.Converters.ToDictionary(k => Tuple.Create(k.Item1, k.Item2), v => v.Item3);
    }

    private void ApplyConverter(Binding binding, DependencyProperty dProp, PropertyInfo vmProp)
    {
        var t = Tuple.Create(vmProp.PropertyType, dProp.PropertyType);
        _converters.Get(t).Do(v => binding.Converter = v);
    }
}

The ConventionManager is a Caliburn.Micro class that (through the CM-typical override mechanism of replacing static Func-fields) provides a hook into the binding operations. The key to a value converter is the pair of bound ViewModel-Type to the target UIElement type.

An additional way to leverage Caliburn.Micro's Binding routine is through the ScalBinding markup extension:

<TextBlock x:Name="Hello" Visibility="{svc:ScalBinding IsVisible}"></TextBlock>

This markup extension will call into the relevant CM Binding-related methods beyond those that deal with the Convention-based binding. That way, the binding shown above will use Scal's knowledge of known value converters to map the Visibility property to the IsVisible bool.

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