Might be wrong, or incomplete, but I think this is to avoid triggering some of the other KernelEvents::RESPONSE listeners like esi, profiler, ... before sending the Response to the client.

As you can see, the priority is set to -1024, and the Response is sent inside the Listener. Which means that any other listener will not have any effect if its priority is over -1024.
So no, it could not be removed.

Not sure I understand @ogizanagi, every kernel.response listener with a priority > -1024 (I assume most if not all) is executed before the StreamedResponseListener.

Right. My assumptions were biased as I inverted the priority. Forget what I said :)

The right answer is here : #2935 (comment)

The response is send by the listener so that the callback is triggered before leaving the request scope, i.e. before leaving the Kernel::handle() call.

I think it's safe to close the issue. @gerryvdm's question is answered in #2935 (comment) and it seems like there's nothing else to do.

@stof @fabpot

The response is send by the listener so that the callback is triggered before leaving the request scope, i.e. before leaving the Kernel::handle() call.

Could you please tell me why is this important at all? Why is that important to trigger callback in Kernel::handle() instead of in send() method in front controller? What's the difference at all? StreamedResponse works exactly the same without this listener and the listener breaks compatibility with tests.

Here, in 2020 with Symfony 4.*, I tried to write tests for request which returns streamed response with PDF document. Wich StreamedResponseListener my terminal is polluted with binary contents. With StreamedResponseListener disabled response of the web application works exactly the same.

The existence of StreamedResponseListener causes two negative things for tests:

• Output of reponse pollutes terminal.
• You can't get content of response from test.

So basically what's the difference? Can't we disable StreamedResponseListener, at least for tests?

You can work around the problem using services_test.yaml with:

services:
    streamed_response_listener:
        class: Symfony\Component\HttpKernel\EventListener\StreamedResponseListener
        tags: ~