The SignalR extension allows to connect and interact with SignalR server directly from html. It establishes connection to the hub, subscribes to the events, allows to send messages to the server, processes incoming messages and swaps the content into your htmx page. In a way, it combines features from SSE and WebSockets extensions, allowing for bi-directional browser-server communication and supporting "channels" ("methods" in SignalR terminology) to distinguish messages between each other.

SignalR is an open-source library that simplifies adding real-time web functionality to apps. Real-time web functionality enables server-side code to push content to clients instantly.

Install the extension by including the script into your page, as well as SignalR library itself.

<script src="https://unpkg.com/@microsoft/signalr@next/dist/browser/signalr.js"></script>
<script src="/js/hx-signalr.js"></script>

Activate the extension by adding hx-ext attribute to your page

<div hx-ext="signalr">...</div>

The extension provides three attributes to create and interact with the connection.

signalr-connect attribute is used to establish connection to SignalR hub. Pass hub URL as a value attribute in any format, supported by SignalR.

<div signalr-connect="/hub"></div>

Other attributes will use the connection from the first parent in page structure.

signalr-send attribute is used to send messages via SignalR connection. The data is serialized in an object, where fields are mapped from input elements and values from hx-include, hx-vals, etc. Additionally, request headers are attached in HEADERS property.

<form signalr-send="echo">
    <input type="text" name="message">
    <button type="submit">Submit</button>
</form>

The form above will be sent as

{
  "message": "<value>",
  "HEADERS": {
    ...
  }
}

That message can be handled by the following hub method

public async Task Echo(EchoRequest request)
{
  await Clients.Caller.SendAsync("echo", $@"<div id=""echo"">{request.message}</div><div hx-swap-oob=""true"" id=""echo-oob-data"">{new Random().Next()}</div>");
}

public record EchoRequest(string message);

❗❗ Please note, that your request object MUST define its fields as strings. The reason for this is the fact that HTML inputs have no distinction between numbers and strings values, but JSON has. Doing client-side conversion will inevitably cause many edge-cases and bugs. Server-side validation and mapping is much easier, although it does introduce a bit of a boilerplate.

signalr-subscribe attribute is used to declare client method and attach message handler to it. Received messages will be swapped into the page body. The default target is innerHTML of the element with that attribute. The target and swapping method can be changed with hx-target and hx-swap attributes. If the message contains OOB elements, they will also be processed as usual.

<!-- Standard subscription -->
<div signalr-subscribe="counter">
</div>

<!-- Changing target and swapping method -->
<div signalr-subscribe="echo" hx-target="#echo-target" hx-swap="beforeend">
</div>
<div id="echo-target"></div>
<!-- OOB swapping is supported -->
<div id="echo-oob-data"></div>

When the message is received, it is processed with similar pipeline as standard htmx responses. That means that extensions that transform response content are supported, and you can, for example, use client side templates to render JSON data on the page.

An element can be subscribed to multiple methods by passing their names as comma separated list. But be careful - the extension has no native way to specify different swap methods or targets for different methods, so messages for different methods can fight over the same target. Best way to use multiple subscriptions is to listen for the events with JS or _hyperscript and handle each method programmatically.

This event is triggered once connection with the hub has been established. This event is raised on signalr-connect element.

• connectionId contains id of SignalR connection object (if present)

Cancelling the event has no effect.

This event is triggered on the elements with active method subscription when a message is received from the hub connection. detail property of the event has a few fields:

• message contains message object as received by the handler. Can be modified by event handler, which will affect next processing steps.
• method contains the name of the method, that received the message. Can be used to filter events when using multiple method subscriptions. Modifying this field has no effect.
• target contains target element. Can be changed to redirect swapping to a different target element. Does not affect OOB swaps.

Cancelling the event will prevent any further processing.

This event is triggered just before sending a message. detail property contains the parameters of the message being sent, which can be modified by the event handler:

• method contains the name of the method being called. Modifying this field will retarget the message to a different handler.
• headers contains the headers, that will be attached into HEADERS field.
• allParameters contains all parameters from the form inputs and hx-vals attributes.
• filteredParameters contains all parameters after filtering is applied. The value from this field will be sent as-is.

Cancelling the event will prevent sending.

This event is triggered just after message was sent. Modifying the data in details property has no effect.

• method contains the name of the method that was called.
• message contains the data that was sent to the hub.

Cancelling this event has no effect.

These events are matching with corresponding SignalR events onreconnecting, and onclose.

• error contains original SignalR error object (if present)

These events are matching with corresponding SignalR event onreconnected

• connectionId contains id of SignalR connection object (if present)

You can use those events to display connection status in the UI and/or gracefully handle connection loss. Here is basic example of showing reconnection status using hx-on attribute

<div id="signalr-indicator" style="display: none;">Connecting to the hub...</div>
<div hx-ext="signalr" signalr-connect="/hub"
     hx-on:htmx:signalr:reconnecting="htmx.find('#signalr-indicator').style.display='block'"
     hx-on:htmx:signalr:reconnected="htmx.find('#signalr-indicator').style.display='none'"></div>

Similar to WebSocket extension, it is possible to provide custom factory method for hub connection. This way you can control all aspects of the connection yourself.

htmx.createHubConnection = function (url) {
  return new signalR.HubConnectionBuilder()
    .withUrl(url)
    .withAutomaticReconnect(Array(100).fill(5000))   // attempt up to 100 reconnections every 5 seconds
    .build()
}

This library is licensed under the terms of MIT License

The implementation is based on the official SSE and WebSockets extensions from htmx by Big Sky Software, which are licensed under the terms of BSD Zero-Clause License.