Htmx.TagHelpers 1.8.1

Htmx.Net

This is a package designed to add server side helper methods for HttpRequest and HttpResponse. This makes working with htmx server-side concepts simpler. You should also consider reading about Hyperscript, an optional companion project for HTMX.

If you're new to HTMX, checkout out this series on getting started with HTMX for ASP.NET Core developer which also includes a sample project and patterns that you might find helpful.

Htmx Extension Methods

Getting Started

Install the Htmx NuGet package to your ASP.NET Core project.

dotnet add package Htmx

HttpRequest

Using the HttpRequest, we can determine if the request was initiated by Htmx on the client.

httpContext.Request.IsHtmx()

This can be used to either return a full page response or a partial page render.

// in a Razor Page
return Request.IsHtmx()
    ? Partial("_Form", this)
    : Page();

We can also retrieve the other header values htmx might set.

Request.IsHtmx(out var values);

Read more about the other header values on the official documentation page.

Browser Caching

As a special note, please be mindful that if your server can render different content for the same URL depending on some other headers, you need to use the Vary response HTTP header. For example, if your server renders the full HTML when Request.IsHtmx() is false, and it renders a fragment of that HTML when Request.IsHtmx() is true, you need to add Vary: HX-Request. That causes the cache to be keyed based on a composite of the response URL and the HX-Request request header — rather than being based just on the response URL.

// in a Razor Page
if (Request.IsHtmx())
{
  Response.Headers.Add("Vary", "HX-Request");
  return Partial("_Form", this)
}

return Page();

HttpResponse

We can set Http Response headers using the Htmx extension method, which passes an action and HtmxResponseHeaders object.

Response.Htmx(h => {
    h.PushUrl("/new-url")
     .WithTrigger("cool")
});

Read more about the HTTP response headers at the official documentation site.

Triggering Client-Side Events

You can trigger client side events with HTMX using the HX-Trigger header. Htmx.Net provides a WithTrigger helper method to configure one or more events that you wish to trigger.

Response.Htmx(h => {
    h.WithTrigger("yes")
     .WithTrigger("cool", timing: HtmxTriggerTiming.AfterSettle)
     .WithTrigger("neat", new { valueForFrontEnd= 42, status= "Done!" }, timing: HtmxTriggerTiming.AfterSwap);
});

CORS Policy

By default, all Htmx requests and responses will be blocked in a cross-origin context.

If you configure your application in a cross-origin context, then setting a CORS policy in ASP.NET Core also allows you to define specific restrictions on request and response headers, enabling fine-grained control over the data that can be exchanged between your web application and different origins.

This library provides a simple approach to exposing Htmx headers to your CORS policy:

var  MyAllowSpecificOrigins = "_myAllowSpecificOrigins";

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddCors(options =>
{
    options.AddPolicy(name: MyAllowSpecificOrigins,
                      policy  =>
                      {
                          policy.WithOrigins("http://example.com", "http://www.contoso.com")
                                   .WithHeaders(HtmxRequestHeaders.Keys.All)  // Add htmx request headers
                                   .WithExposedHeaders(HtmxResponseHeaders.Keys.All)  // Add htmx response headers
                      });
});

Htmx.TagHelpers

Getting Started

Install the Htmx.TagHelpers NuGet package to your ASP.NET Core project. Targets .NET Core 3.1+ projects.

dotnet add package Htmx.TagHelpers

Make the Tag Helpers available in your project by adding the following line to your _ViewImports.cshtml:

@addTagHelper *, Htmx.TagHelpers

You'll generally need URL paths pointing back to your ASP.NET Core backend. Luckily, Htmx.TagHelpers mimics the url generation included in ASP.NET Core. This makes linking HTMX with your ASP.NET Core application a seamless experience.

<div hx-target="this">
    <button hx-get
            hx-page="Index"
            hx-page-handler="Snippet"
            hx-swap="outerHtml">
        Click Me (Razor Page w/ Handler)
    </button>
</div>

<div hx-target="this">
    <button hx-get
            hx-controller="Home"
            hx-action="Index"
            hx-route-id="1">
        Click Me (Controller)
    </button>
</div>

<div hx-target="this">
    <button hx-post
            hx-route="named">
        Click Me (Named)
    </button>
</div>

Htmx.Config

An additional htmx-config tag helper is included that can be applied to a meta element in your page's head that makes creating HTMX configuration simpler. For example, below we can set the historyCacheSize, default indicatorClass, and whether to include ASP.NET Core's anti-forgery tokens as an additional element on the HTMX configuration.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta name="htmx-config" 
          historyCacheSize="20"
          indicatorClass="htmx-indicator"
          includeAspNetAntiforgeryToken="true"
          />
    
</head>

The resulting HTML will be.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta name="htmx-config" content='{"indicatorClass":"htmx-indicator","historyCacheSize":20,"antiForgery":{"formFieldName":"__RequestVerificationToken","headerName":"RequestVerificationToken","requestToken":"<token>"}}' />
    
</head>

HTMX and Anti-forgery Tokens

You can set the attribute includeAspNetAntiforgerToken on the htmx-config element. Then you'll need to include this additional JavaScript in your web application. We include the attribute __htmx_antiforgery to track the event listener was added already. This keeps us from accidentally re-registering the event listener.

if (!document.body.attributes.__htmx_antiforgery) {
    document.addEventListener("htmx:configRequest", evt => {
        let httpVerb = evt.detail.verb.toUpperCase();
        if (httpVerb === 'GET') return;
        let antiForgery = htmx.config.antiForgery;
        if (antiForgery) {
            // already specified on form, short circuit
            if (evt.detail.parameters[antiForgery.formFieldName])
                return;

            if (antiForgery.headerName) {
                evt.detail.headers[antiForgery.headerName]
                    = antiForgery.requestToken;
            } else {
                evt.detail.parameters[antiForgery.formFieldName]
                    = antiForgery.requestToken;
            }
        }
    });
    document.addEventListener("htmx:afterOnLoad", evt => {
        if (evt.detail.boosted) {
            const parser = new DOMParser();
            const html = parser.parseFromString(evt.detail.xhr.responseText, 'text/html');
            const selector = 'meta[name=htmx-config]';
            const config = html.querySelector(selector);
            if (config) {
                const current = document.querySelector(selector);
                // only change the anti-forgery token
                const key = 'antiForgery';
                htmx.config[key] = JSON.parse(config.attributes['content'].value)[key];
                // update DOM, probably not necessary, but for sanity's sake
                current.replaceWith(config);
            }
        }
    });
    document.body.attributes.__htmx_antiforgery = true;
}

You can access the snippet in two ways. The first is to use the HtmxSnippet static class in your views.

<script>
@Html.Raw(HtmxSnippets.AntiforgeryJavaScript)
</script>

A simpler way is to use the HtmlExtensions class that extends IHtmlHelper.

@Html.HtmxAntiforgeryScript()

This html helper will result in a <script> tag along with the previously mentioned JavaScript. Note: You can still register multiple event handlers for htmx:configRequest, so having more than one is ok.

Note that if the hx-[get|post|put] attribute is on a <form ..> tag and the <form> element has a method="post" (and also an empty or missing action="") attribute, the ASP.NET Tag Helpers will add the Anti-forgery Token as an input element and you do not need to further configure your requests as above. You could also use hx-include pointing to a form, but this all comes down to a matter of preference.

Additionally, and the recommended approach is to use the HtmxAntiforgeryScriptEndpoint, which will let you map the JavaScript file to a specific endpoint, and by default it will be _htmx/antiforgery.js.

app.UseAuthorization();
// registered here
app.MapHtmxAntiforgeryScript();
app.MapRazorPages();
app.MapControllers();

You can now configure this endpoint with caching, authentication, etc. More importantly, you can use the script in your head tag now by applying the defer tag, which is preferred to having JavaScript at the end of a body element.

<head>
    <meta charset="utf-8"/>
    <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
    <meta
        name="htmx-config"
        historyCacheSize="20"
        indicatorClass="htmx-indicator"
        includeAspNetAntiforgeryToken="true"/>
    <title>@ViewData["Title"] - Htmx.Sample</title>
    <link rel="stylesheet" href="~/lib/bootstrap/dist/css/bootstrap.min.css"/>
    <link rel="stylesheet" href="~/css/site.css" asp-append-version="true"/>
    <script src="~/lib/jquery/dist/jquery.min.js" defer></script>
    <script src="~/lib/bootstrap/dist/js/bootstrap.bundle.min.js" defer></script>
    <script src="https://unpkg.com/htmx.org@@1.9.2" defer></script>
    
    <script src="@HtmxAntiforgeryScriptEndpoints.Path" defer></script>
</head>

License

Copyright © 2022 Khalid Abuhakmeh

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.