TagHelper ; how to modify dynamically added children - razorengine

Question

The asp-items Razor "TagHelper" will add an <option> to a <select> for each value in the SelectList. I want to modify each of those children.
Specifically I want to disable some of them (i.e. add disabled="disabled").
Even more specifically I want to dynamically disable some of them; I'm using angular so I could ng-disabled="{dynamic_boolean_which_determines_disabled}". This means the option could be disabled at first, but after user makes a change, the option could be disabled (without page reload). Angular should take care of this; I think Angular and TagHelpers should work together in theory...
I expected:
I could somehow access an IEnumerable of the children <option> tags that would be created (i.e. one for each item in the SelectList), iterate the children tags, and SetAttribute("disabled") or SetAttribute("ng-disabled")...
I tried:
Creating my own TagHelper which targets the select[asp-items], and tries to GetChildContentAsync() and/or SetContent to reach an IEnumerable <option> tags and iterate them and process each, but I think this will only let me modify the entire InnerHtml as a string; feels hacky to do a String.replace, but I could do it if that's my only option? i.e. ChildrenContent.Replace("<option", "<option disabled=\"...\"")
Creating my own TagHelper which targets the option elements that are children of the select[asp-items], so I can individually process each. This works, but not on the dynamically-added <option> created by asp-items, it only works on "literal" <option> tags that I actually put into my cshtml markup.
I think this'll work but not ideal:
As I said above, I think I can get the result of TagHelper's dynamic asp-items <option></option> <option></option>, as a string, and do a string replace, but I prefer not to work with strings directly...
I suspect (I haven't tried it) that I could just do the work of asp-items myself; i.e. custom-items. But then I'm recreating the wheel by re-doing the work which asp-items could've done for me?

Answer 1

So I hadn't yet read the "AutoLinkHttpTagHelper" in the example which uses string replacement (specifically RegEx replace) to replace every occurrence of a URL, with an <a> pointed at that URL. The cases are slightly different*, but...
Anyway, here's my solution once I learned to stop worrying and love the string modification:
[HtmlTargetElement("select", Attributes = "asp-items")]
public class AspItemsNgDisabledTagHelper : SelectTagHelper
{
//Need it to process *after* the SelectTagHelper
public override int Order { get; } = int.MaxValue;
//https://learn.microsoft.com/en-us/aspnet/core/mvc/views/tag-helpers/authoring#ProcessAsync
public AspItemsNgDisabledTagHelper(IHtmlGenerator gen) : base(gen) {}
public override void Process(TagHelperContext context, TagHelperOutput output)
{
//Notice I'm getting the PostContent;
//SelectTagHelper leaves its literal content (i.e. in your CSHTML, if there is any) alone ; that's Content
//it only **appends** new options specified; that's PostContent
//Makes sense, but I still wasn't expecting it
var generated_options = output.PostContent.GetContent();
//Note you do NOT need to extend SelectTagHelper as I've done here
//I only did it to take advantage of the asp-for property, to get its Name, so I could pass that to the angular function
var select_for = this.For.Name;
//The heart of the processing is a Regex.Replace, just like
//their example https://learn.microsoft.com/en-us/aspnet/core/mvc/views/tag-helpers/authoring#inspecting-and-retrieving-child-content
var ng_disabled_generated_options = Regex.Replace(
generated_options,
"<option value=\"(\\w+)\">",
$"<option value=\"$1\" ng-disabled=\"is_disabled('{select_for}', '$1')\">");
//Finally, you Set your modified Content
output.PostContent.SetHtmlContent(ng_disabled_generated_options);
}
}
Few learning opportunities:
Was thinking I'd find AspForTagHelper and AspItemsTagHelper, (angular background suggested that the corresponding attributes; asp-for and asp-items, would be separate "directives" aka TagHelper).
In fact, TagHelper "matching" focuses on the element name (unlike angular which can match element name... attribute... class... CSS selector)
Therefore I found what I was looking for in SelectTagHelper, which has For and Items as properties. Makes sense.
As I said above, I extend SelectTagHelper, but that's not necessary to answer my original question. It's only necessary if you want access to the this.For.Name as I've done, but there may even be a way around that (i.e. re-bind its own For property here?)
I got on a distraction thinking I would need to override the SelectTagHelper's behavior to achieve my goals; i.e. Object-Oriented Thinking. In fact, even if I did extend SelectTagHelper, that doesn't stop a separate instance of the base SelectTagHelper from matching and processing the element. In other words, element processing happens in a pipeline.
This explains why extending and calling base.Process(), will result in Select doing its job twice; once when your instance matches, and again when the base instance matched.
(I suppose could've prevented SelectTagHelper from matching by creating a new element name like <asp-items-select>? But, not necessary... I just avoid calling base.Process(). So unless that's a bad practice...)
*Different in this way:
They want to create a tag where none exists, whereas I want to add an attribute a tag which is already there; i.e. the <option>
Though the <option> "tag" is generated by the SelectTagHelper in its PostContent (was expecting to find it in Content), and I don't think tags-generated-in-strings-by-content-mods can be matched with their corresponding TagHelper -- so maybe we really are the same in that we're just dealing with plain old strings
Their "data" aka "model" is implied in the text itself; they find a URL and that URL string becomes a unit of meaning they use. In my case, there is an explicit class for Modeling; the SelectList (<select>) which consists of some SelectListItem (<option>) -- but that class doesn't help me either.
That class only gives me attributes like public bool Disabled (remember, this isn't sufficient for me because the value of disabled could change to true or false within browser; i.e. client-side only), and public SelectListGroup Group -- certainly nothing as nonstandard as ng-disabled, nor a "catch-all" property like Attributes which could let me put arbitrary attributes (ng-disabled or anything else) in there.