Custom HTTP Headers

I the "Extension" file under DependencyInjection/*Extension.php, using the ExtensionTrait you can register headers that are accessible via CORS:

Add a header to Access-Control-Expose-Headers

public function prepend(ContainerBuilder $container)
{
    $this->addExposeHeader($container, 'X-Some-Header');
    // ...

Add a header to Access-Control-Allow-Headers

public function prepend(ContainerBuilder $container)
{
    $this->addAllowHeader($container, 'X-Some-Header');
    // ...