How do I use ValueConverters in MvvmCross data binding

Understanding Value Converters

Value Converters are classes that implement the `IMvxValueConverter` interface. This interface provides two key methods: `Convert` and `ConvertBack`. The `Convert` method transforms values from the ViewModel to the View, while the `ConvertBack` method does the opposite. In many cases, especially for non-editable UI elements like labels or images, you might only need to implement the `Convert` method, leaving `ConvertBack` unimplemented or throwing a `NotImplementedException`[1][3].

Implementing a Value Converter

To create a Value Converter, you need to implement the `IMvxValueConverter` interface. Here's an example of a simple converter that converts a string to its length:

csharp
public class LengthValueConverter : MvxValueConverter
{
    protected override int Convert(string value, Type targetType, object parameter, CultureInfo cultureInfo)
    {
        if (value == null)
            return 0;
        return value.Length;
    }

    protected override int ConvertBack(int value, Type targetType, object parameter, CultureInfo cultureInfo)
    {
        throw new NotImplementedException();
    }
}

In this example, the `Convert` method returns the length of the input string, and `ConvertBack` is not implemented because it's not needed for this one-way conversion.

Registering Value Converters

To use a Value Converter in your MvvmCross application, you need to register it. This is typically done in the `Setup.cs` file. You can register converters individually or register all converters within an assembly. Here's how you might register a converter individually:

csharp
protected override void FillValueConverters(IMvxValueConverterRegistry registry)
{
    base.FillValueConverters(registry);
    registry.AddOrOverwrite("LengthConverter", new LengthValueConverter());
}

Using Value Converters in Bindings

Once registered, you can use Value Converters in your bindings. The syntax for using converters varies depending on the platform and binding syntax you're using (e.g., Swiss, Fluent, or Tibet binding). For example, in a Tibet binding, you might use a converter like this:

csharp
local:MvxBind="Text LengthConverter(MyProperty)"

This binds the `Text` property of a UI element to the `MyProperty` property in your ViewModel, using the `LengthConverter` to convert the value.

Additional Parameters

Value Converters can also accept additional parameters that can be passed from the binding declaration. These parameters can be used to customize the conversion logic. For instance, you might want to use a parameter to specify a format string for a date conversion.

csharp
public class CustomDateConverter : MvxValueConverter
{
    protected override string Convert(DateTime value, Type targetType, object parameter, CultureInfo cultureInfo)
    {
        if (parameter is string format)
            return value.ToString(format);
        return value.ToString();
    }
}

You can then pass a format string in your binding:

csharp
local:MvxBind="Text CustomDateConverter(MyDate, 'yyyy-MM-dd')"

This will convert the `MyDate` property to a string using the specified format.

Conclusion

Value Converters are a crucial part of the MvvmCross data binding system, allowing you to transform data between your ViewModel and View in a flexible and reusable way. By implementing the `IMvxValueConverter` interface, registering your converters, and using them in your bindings, you can create powerful and dynamic user interfaces that reflect the state of your application's data.