allowed_http_origins() only works in theme functions.php

Question

The following code works in my theme's functions.php file but not in my plugin's main/entry file:

/** Allow cross origin resource access
 */
function add_cors_http_header(){
  header("Access-Control-Allow-Origin: *");
}
add_action('init','add_cors_http_header');

If put into the main plugin file I see this:

Access to XMLHttpRequest at 'http://localhost/
wptest2/wp-admin/admin-ajax.php' 
from origin 'http://localhost:8080' 
has been blocked by CORS policy: 
No 'Access-Control-Allow-Origin' header 
is present on the requested resource.

I need to setup CORS using my plugin files only so users won't have to make custom mods, is there any way to accomplish this?

Also tried in the plugin entry file (and failed):

function add_allowed_origins(array $origins = []) {
  $origins[] = 'http://localhost:8080';
  return $origins;
}
add_filter('allowed_http_origins', 'add_allowed_origins');

.

function add_cors_http_header() {
  header("Access-Control-Allow-Origin: *");
}
add_action('send_headers', 'add_cors_http_header');

Answer 1

Try using send_headers action hook instead of init. It should work.

add_action( 'send_headers', 'add_cors_http_header' );

function add_cors_http_header()
{
    header("Access-Control-Allow-Origin: *");
}