Make it possible to return PSGI-style streaming response

MANIFEST

@@ -268,6 +268,7 @@ t/12_response/08_drop_content.t
 t/12_response/09_headers_to_array.t
 t/12_response/10_error_dumper.t
 t/12_response/11_CVE-2012-5572.t
+t/12_response/12_psgi_streaming.t
 t/13_engines/00_load.t
 t/13_engines/02_template_init.t
 t/14_serializer/01_helpers.t

lib/Dancer/Cookbook.pod

@@ -302,6 +302,33 @@ The following routes will be generated for us:
 
 
 
+=head2 Low-level access to PSGI interface
+
+You may return a PSGI-formatted response from your Dancer route if needed.
+This ability is useful if you are building a streaming app. Note that you will
+need a PSGI-compliant server with streaming support.
+
+    get '/streaming' => sub {
+        # use my app infrastructure to make database lookups, session checks etc.
+        # then return a callback-style response
+        return sub {
+            my $respond = shift;
+            my $writer = $respond->(
+                [ 200, [ 'Content-Type', 'text/event-stream' ] ]
+            );
+            subscribe_user($writer); # set up events in your app
+            $writer->write(
+                HTTP::ServerEvent->as_string(
+                    data    => 'connected'
+                )
+            );
+        }
+    };
+
+See L<PSGI> for more details.
+
+
+
 =head1 MUSCLE MEMORY: STORING DATA
 
 =head2 Handling sessions

lib/Dancer/Handler.pm

@@ -145,7 +145,16 @@ sub render_response {
 
     my $content = $response->content;
 
-    unless ( ref($content) eq 'GLOB' ) {
+    if (ref($content) eq 'CODE') {
+        unless (
+            Dancer::SharedData->request
+            && Dancer::SharedData->request->env->{'psgi.streaming'}
+        ) {
+            raise core => 'Sorry, streaming is not supported on this server.';
+        }
+        return $content;
+    }
+    elsif ( ref($content) ne 'GLOB' ) {
         my $charset = setting('charset');
         my $ctype   = $response->header('Content-Type');
 

lib/Dancer/Serializer.pm

@@ -40,7 +40,7 @@ sub process_response {
 
     my $content = $response->{content};
 
-    if (ref($content) && (ref($content) ne 'GLOB')) {
+    if (ref($content) && ref($content) ne 'GLOB' && ref($content) ne 'CODE') {
         local $@;
         eval { $content = engine->serialize($content) };
 

t/12_response/12_psgi_streaming.t

@@ -0,0 +1,25 @@
+package main;
+use strict;
+use warnings;
+use Test::More tests => 2, import => ['!pass'];
+use Dancer::Test;
+
+{
+    use Dancer;
+    set environment => 'psgi.streaming';    # it is required in a real-world app
+                                            # but here it makes no difference
+    get '/psgi' => sub {
+        sub {
+            # implementation is irrelevant
+        }
+    };
+}
+
+note "Testing PSGI-streaming response";
+my $req = [ GET => '/psgi' ];
+route_exists $req;
+my $res = dancer_response @$req;
+is(ref($res->content), 'CODE', 'reponse contains coderef');
+
+1;
+