How to add a Multi Checkbox List to the editor of a Form-based Element? - orchardcms

Question

I'm trying to build a custom Typeahead Form Element. I have predefined multiple datasets to draw autocomplete suggestions from. When users add my field to a form, I want them to be able to select one or more of these datasets through checkboxes.
Orchard does not seem to have any Shape like this out of the box. By looking at other Form Elements in Orchard.DynamicForms and the SelectList Shape defined in Orchard.Forms.Shapes.EditorShapes.cs, I was able to piece together this working code for the Driver:
public class TypeaheadFieldElementDriver : FormsElementDriver<TypeaheadField> {
private readonly ITokenizer _tokenizer;
private readonly IEnumerable<IDataSet> _dataSets;
public TypeaheadFieldElementDriver(IFormsBasedElementServices formsServices, ITokenizer tokenizer, IEnumerable<IDataSet> _dataSets)
: base(formsServices)
{
_tokenizer = tokenizer;
this._dataSets = _dataSets;
}
...
protected override void DescribeForm(DescribeContext context) {
context.Form("TypeaheadField", factory =>
{
var shape = (dynamic)factory;
var form = shape.Fieldset(
Id: "TypeaheadField",
_Value: shape.Textbox(
Id: "Value",
Name: "Value",
Title: "Value",
Classes: new[] { "text", "medium", "tokenized" },
Description: T("The value of this typeahead field.")),
_DataSetIds: shape.SelectList(
Id: "DataSetIds",
Name: "DataSetIds",
Title: "Remote Datasets",
Multiple: true,
Description: T("The remote datasets to fetch suggestions from.")));
foreach (var dataSet in _dataSets)
{
form._DataSetIds.Items.Add(new SelectListItem { Text = T(dataSet.Description).Text, Value = dataSet.Id });
}
return form;
});
...
}
}
This successfully renders a multiple select field and sort of works for my purposes but I'm not able to completely unselect every option because the one that was selected keeps getting posted. For this reason and just general ease of use, I would prefer to render a list of checkboxes instead but I'm not sure how to proceed.