app: Support multiple resource representations

The `VariedResponse` class essentially turns the website into a REST
API. When a controller returns a response object that is an instance of
this class, the response representation sent to the user-agent will vary
based on the value of the `Accept` request header. Specifically,
requests containing `Accept: application/json` will receive a
JSON-encoded object, while typical browser requests will get an (X)HTML
document.

src/dcow/app.py

@@ -1,7 +1,12 @@
 from milla.dispatch import routing
-from . import gallery, thumbnails
-import os
+from . import (
+    base,
+    gallery,
+    thumbnails,
+)
+import functools
 import milla.util
+import os
 
 
 DEFAULT_CONFIG = {
@@ -30,3 +35,21 @@ class Application(milla.Application):
 
         r.add_route('/', gallery.GalleryController())
         r.add_route('/thumbnails/{image}', thumbnails.ThumbnailController())
+
+    def make_request(self, environ):
+        request = super(Application, self).make_request(environ)
+        default_format = base.VariedResponse.default_content_type
+        offers = [
+            'application/json',
+            'application/xhtml+xml',
+            'text/html',
+        ]
+        want_fmt = request.accept.best_match(offers, default_format)
+        if want_fmt == 'application/json':
+            request.want = 'json'
+        elif want_fmt == 'text/html':
+            request.want = 'html'
+        else:
+            request.want = 'xhtml'
+        request.ResponseClass = functools.partial(base.VariedResponse, request)
+        return request

src/dcow/base.py

@@ -0,0 +1,49 @@
+import jinja2
+import json
+import milla
+
+
+class Template(object):
+
+    LOADER = jinja2.PackageLoader(__name__.rsplit('.', 1)[0])
+
+    def __init__(self, name):
+        self.env = jinja2.Environment(loader=self.LOADER)
+        self.tmpl = self.env.get_template(name)
+
+    def render(self, context):
+        return self.tmpl.render(**context)
+
+
+class VariedResponse(milla.Response):
+
+    def __init__(self, request, *args, **kwargs):
+        super(VariedResponse, self).__init__(*args, **kwargs)
+        self.request = request
+
+    def set_payload(self, template, context=None):
+        if not self.vary:
+            self.vary = ['Accept']
+        elif 'accept' not in (v.lower() for v in self.vary):
+            self.vary = self.vary + ('Accept',)
+        if context is None:
+            context = {}
+        if template is None or self.request.want == 'json':
+            self.content_type = str('application/json')
+            json.dump(context, self.body_file)
+        else:
+            if self.request.want == 'xhtml':
+                self.content_type = str('application/xhtml+xml')
+            self.render(template, context)
+
+    def render(self, template, context=None):
+        if context is None:
+            context = {}
+        t = Template(template)
+        t.env.globals.update(
+            url=self.request.create_href,
+            static=self.request.static_resource,
+        )
+        if hasattr(self.request, 'user'):
+            t.env.globals['user'] = self.request.user
+        self.text = t.render(context)