MarkupExtension that uses a DataBinding value

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Nevermind, I finally found out how the referenced code works and could come up with a solution. Here’s just a short explanation for the record.

<TextBlock Text="{t:Translate 'import files', {Binding FileCount}}"/>

This requires a class TranslateExtension, inherited from MarkupExtension, with a constructor accepting two parameters, one String and one Binding. Store both values in the instance. The classes’ ProvideValue method then uses the binding it gets, adds a custom converter instance to it and returns the result from binding.ProvideValue, which is a BindingExpression instance IIRC.

public class TranslateExtension : MarkupExtension
{
    public TranslateExtension(string key, Binding countBinding)
    {
        // Save arguments to properties
    }
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        countBinding.Converter = new TranslateConverter(key);
        return countBinding.ProvideValue(serviceProvider);
    }
}

The converter, say of class TranslateConverter, has a constructor that accepts one parameter, a String. This is my key argument from the TranslateExtension above. It remembers it for later.

Whenever the Count value changes (it comes through the binding), WPF will request its value anew. It seems to walk from the source of the binding, through the converter, to the surface where it’s displayed. By using a converter, we don’t have to worry about the binding at all, because the converter gets the binding’s current value as a method argument and is expected to return something else. Counting value (int) in, translated text (string) out. This is my code.

So it’s the converter’s task to adapt the number to a formulated text. It uses the stored text key for that. So what happens is basically a kinda backwards data flow. Instead of the text key being the main information and the count value being added to it, we need to treat the count value as the primary information and just use the text key as a side parameter to make it whole. This isn’t exactly straightforward, but the binding needs to be the primary trigger. Since the key won’t change, it can be stored for good in the instance of the converter. And every occurence of a translated text gets its own copy of the converter, each with an individual key programmed in.

This is what the converter could look like:

class TranslateConverter : IValueConverter
{
    private string key;
    public TranslateConverter(string key)
    {
        this.key = key;
    }
    public object Convert(object value, ...)
    {
        return Translator.Translate(key, (int) value);
    }
}

That’s the magic. Add the error handling and more features to get the solution.

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