Update documentation to show setting response headers with value from `@SchemaMapping` method

[hanyunmin]

As the title suggests, I am trying to set cookie in signin using graphql.

I am using spring mvc and I know that the existing cookie setting method using HttpServletResponse is not available.

Is there no way to set cookie in graphql?

[bclozel]

From where would you like to set this cookie value? A Servlet filter? A @SchemaMapping controller method?
Does the interception section of the reference documentation help?

[hanyunmin]

I want to use it in @schemamapping controller method.
The interception section of the reference documentation doesn't help in my situation.

[bclozel]

Spring for GraphQL is transport-agnostic, meaning that the @SchemaMapping annotated controllers can work on top of HTTP, RSocket, or other transports that don't involve HTTP at all. Once we're in the ExecutionGraphQlService, there is no concept tied to the transport.

The interception section is probably still the best way to achieve this: you can set a value in the GraphQL context and later use it to set response headers.

[rstoyanchev]

I guess the example shows how to take a value from the request, put it in the GraphQLContext, and then access it as an @ContextValue from the controller method. You can also do the reverse, by saving a value in the GraphQLContext from a controller method:

@Controller
class MyController {

    @QueryMapping
    Person person(GraphQLContext context) {
            context.put("cookie", "123");
    }
}

And then access that from an WebGraphQlInterceptor to set the cookie on the response:

class ResponseHeaderInterceptor implements WebGraphQlInterceptor {

	@Override
	public Mono<WebGraphQlResponse> intercept(WebGraphQlRequest request, Chain chain) {
		return chain.next(request).doOnNext(response -> {
			String value = response.getExecutionInput().getGraphQLContext().get("cookie");
			ResponseCookie cookie = ResponseCookie.from("cookieName", value).build();
			response.getResponseHeaders().add(HttpHeaders.SET_COOKIE, cookie.toString());
		});
	}
}

I think we could improve a bit the example in the docs based on this.