Using EventStream doesn't work because MimeType from Controller Action is always text/html

Question

I'm trying to listen to SSE events from a controller action.

I am setting the headers in the controller action with:

  Craft::$app->response->headers
      ->set('Content-Type: text/event-stream')
      ->set('Cache-Control: no-cache');

The EventSource listener always gives me:

EventSource's response has a MIME type ("text/html") that is not "text/event-stream". Aborting the connection.

Checking the URL (controller action URL) manually, you can see indeed that the headers are "text/html" and I cannot change them.

What is a good way to send SSE events from CraftCMS?

Answer 1

I have finally found out that you need to set the RAW on the Response.

$this->response->format = Response::FORMAT_RAW;

So you can overwrite the response format with your own headers.