TagHelper specify valid attributes

Question

I am playing around with creating a custom tag helper in MVC 6 / ASP.Net vNext - the taghelper works fine, however is there a way to specify valid asp- attributes to use with the tag so they appear in intellisense? For example, I want asp-ajax and asp-onsuccess to come up in the intellisense list when adding a tag in the view that matches the criteria for my taghelper below:

[TargetElement("form", Attributes = AjaxForm)]
public class UnobtrusiveFormTagHelper : TagHelper
{
    private const string AjaxForm = "asp-ajax";

    public override void Process(TagHelperContext context, TagHelperOutput output)
    {
        base.Process(context, output);

        output.Attributes.Add("data-ajax", true);
        output.Attributes.Add("data-onsuccess", context.AllAttributes["asp-onsuccess"]);

    }

}

Usage:

<form asp-ajax="true" asp-onsuccess="dothis();">

Thanks in advance

Answer 1

With what you have right now (Attributes = AjaxForm) you'll get asp-ajax in your IntelliSense for form tags. If you want to also provide data-onsuccess in IntelliSense on form tags you can add another TargetElement attribute, aka: [TargetElement("form", Attributes = "asp-onsuccess")]. I want to note, that adding Attributes like this only controls "when" the TagHelper runs. Think of it like saying: Only run when these attributes are present on the HTML element.

If you want to consume the values of the attributes AND provide IntelliSense you can add properties:

public bool AspAjax { get; set; }

[HtmlAttributeName("asp-onsuccess")]
public string AspOnSuccess { get; set; }

This approach doesn't control "when" the TagHelper runs but provides a way to input information into your TagHelper. It will enable you to take their values and add them as data- attributes. Note that by adding AspAjax/AspOnSuccess as properties their values no longer exist on output.Attributes so you don't need to remove them.

Hopefully this helps!