A value converter should implement the IBindingTypeConverter interface. See an example of a globally registered converter type that converts between Boolean and XAML Visibility: BooleanToVisibilityTypeConverter.cs

How GetAffinityForObjects works

Return 0 if you can't convert an object and return anything else if it is possible. The largest value wins.

public int GetAffinityForObjects(Type fromType, Type toType)
{
    if (fromType == typeof(string))
    {
        return 100; // any number other than 0 signifies conversion is possible.
    }
    return 0;
}

How TryConvert works

TryConvert function takes four arguments. The first one is value we are trying to convert, the second one is type to which we are trying to convert that value. conversionHint is like a converter parameter, and result is where we need to store the conversion result. TryConvert function should return true if conversion succeeds and false if conversion fails.

public bool TryConvert(object from, Type toType, object conversionHint, out object result)
{
    try
    {
        result = !string.IsNullOrWhiteSpace((string)from);
    }
    catch (Exception ex)
    {
        this.Log().WarnException("Couldn't convert object to type: " + toType, ex);
        result = null;
        return false;
    }

    return true;
}

Registration

If you'd like to use a custom converter globally, you need to register it using Splat Locator.

Locator.CurrentMutable.RegisterConstant(
    new MyCoolTypeConverter(), 
    typeof(IBindingTypeConverter)
);

Usage

For globally registered converters, usage is automatic. When a binding is created, the converter(s) with the highest affinity are chosen from those registered with Splat. Optionally, you can provide a specific converter to override what the default would have been for either VM to View or View to VM. In this case you don't need to register a converter:

this.Bind(ViewModel, 
    viewModel => viewModel.ViewModelProperty, 
    view => view.Control.Property,
    vmToViewConverterOverride: new CustomTypeConverter());

It's important to specify vmToViewConverterOverride to be sure not to accidentally pass your converter to the conversionHint parameter.

Inline Binding Converters

You can supply inline function methods to Bind. This allows you to quickly supply a conversion method. This avoids having to supply a IBindingTypeConverter for one off cases.

this.WhenActivated(disposables =>
{
    this.Bind(this.ViewModel, 
        viewModel => viewModel.DateTime, 
        view => view.DateTextBox.Text, 
        this.ViewModelToViewConverterFunc, 
        this.ViewToViewModelConverterFunc)
        .DisposeWith(disposables);
});
                    
private string ViewModelToViewConverterFunc(DateTime dateTime)
{
    return dateTime.ToString("O"); // return ISO 8601 Date ime
}

private DateTime ViewToViewModelConverterFunc(string value)
{
    DateTime.TryParse(value, out DateTime returnValue);
    return returnValue;
}