Use a multimethod for content-type dispatching

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### New features
+
+* [#548](https://github.com/clojure-emacs/cider-nrepl/pull/548): Make the content-type middleware extensible via multimethod
+
 ## 0.27.4 (2021-12-15)
 
 ### Bugs fixed

doc/modules/ROOT/pages/nrepl-api/ops.adoc

@@ -158,7 +158,9 @@ Returns::
 
 === `content-type-middleware`
 
-Enhances the ``eval`` op by adding ``content-type`` and friends to some responses. Not an op in itself.
+Enhances the ``eval`` op by adding ``content-type`` and ``body`` to certain ``eval`` responses. Not an op in itself.
+
+Depending on the type of the return value of the evaluation this middleware may kick in and include a representation of the result in the response, together with a MIME/Media type to indicate how it should be handled by the client. Comes with implementations for ``URI``, ``URL``, ``File``, and ``java.awt.Image``. More type handlers can be provided by the user by extending the ``cider.nrepl.middleware.content-type/content-type-response`` multimethod. This dispatches using ``clojure.core/type``, so ``:type`` metadata on plain Clojure values can be used to provide custom handling.
 
 Required parameters::
 {blank}
@@ -168,7 +170,9 @@ Optional parameters::
 
 
 Returns::
-{blank}
+* `:body` The rich response document, if applicable.
+* `:content-type` The Media type (MIME type) of the reponse, structured as a pair, `[type {:as attrs}]`
+* `:content-transfer-encoding` The encoding of the response body (Optional, currently only one possible value `"base64"`)
 
 
 === `debug-input`

doc/modules/ROOT/pages/nrepl-api/supplied_middleware.adoc

@@ -27,6 +27,12 @@
 | `complete`
 | Code completion.
 
+| `wrap-content-type`
+| -
+| No
+| `eval`
+| Rich content handling, return multimedia results beyond plain text from `eval`.
+
 | `wrap-debug`
 | -
 | No

src/cider/nrepl/middleware/content_type.clj

@@ -41,7 +41,8 @@
   [2] https://github.com/technomancy/nrepl-discover/blob/master/src/nrepl/discover/samples.clj#L135
   [3] https://tools.ietf.org/html/rfc2045
   [4] https://tools.ietf.org/html/rfc2017"
-  {:authors ["Reid 'arrdem' McKenzie <[email protected]>"]}
+  {:authors ["Reid 'arrdem' McKenzie <[email protected]>"
+             "Arne 'plexus' Brasseur <[email protected]>"]}
   (:require
    [cider.nrepl.middleware.slurp :refer [slurp-reply]])
   (:import
@@ -74,43 +75,45 @@
   (as-url [^URL u]
     u))
 
-(defn response+content-type
-  "Consumes an nREPL response, having a `:value`. If the `:value` is
-  recognized as an AWT Image, a File, or a File URI, rewrite the
-  response to have a `:content-type` being a MIME type of the content,
-  and a `:body` to re-use the RFC term for the message payload."
-  [{:keys [value] :as response}]
-  (cond
-    ;; FIXME (arrdem 2018-04-03):
-    ;;
-    ;;   This could be more generic in terms of tolerating more
-    ;;   protocols / schemes
-
-    ;; RFC-2017 external-body responses for UR[IL]s and things which are just wrappers thereof
-    (or (and (instance? File value)
-             (.exists ^File value))
-        (instance? URI value)
-        (instance? URL value))
-    (assoc response
-           :content-type ["message/external-body"
-                          {"access-type" "URL"
-                           "URL" (.toString ^URL (as-url value))}]
-           :body "")
-
-    ;; FIXME (arrdem 2018-04-03):
-    ;;
-    ;;   This is super snowflakey in terms of only supporting base64
-    ;;   coding this one kind of object.  This could definitely be
-    ;;   more generic / open to extension but hey at least it's
-    ;;   re-using machinery.
-
-    (instance? java.awt.Image value)
-    (with-open [bos (ByteArrayOutputStream.)]
-      (merge response
-             (when (ImageIO/write ^RenderedImage value "png" ^OutputStream bos)
-               (slurp-reply "" ["image/png" {}] (.toByteArray bos)))))
-
-    :else response))
+(defn external-body-response
+  "Partial response map having an external-body content-type referring to the given URL.
+
+  See RFC-2017: Definition of the URL MIME External-Body Access-Type."
+  [value]
+  {:content-type ["message/external-body"
+                  {"access-type" "URL"
+                   "URL" (.toString ^URL (as-url value))}]
+   :body ""})
+
+(defmulti content-type-response
+  "Consumes an nREPL response, having a `:value`. If the `:value` is of a
+  recognized type, then rewrite the response to have a `:content-type` being a
+  MIME type of the content, and a `:body` to re-use the RFC term for the message
+  payload.
+
+  Dispatches on the [[clojure.core/type]] of the value, i.e. the metadata
+  `:type` value, or the class."
+  (comp type :value))
+
+(defmethod content-type-response :default [response]
+  response)
+
+(defmethod content-type-response URI [{:keys [value] :as response}]
+  (merge response (external-body-response value)))
+
+(defmethod content-type-response URL [{:keys [value] :as response}]
+  (merge response (external-body-response value)))
+
+(defmethod content-type-response File [{^File file :value :as response}]
+  (if (.exists file)
+    (merge response (external-body-response file))
+    response))
+
+(defmethod content-type-response java.awt.Image [{^java.awt.Image image :value :as response}]
+  (with-open [bos (ByteArrayOutputStream.)]
+    (ImageIO/write image "png" ^OutputStream bos)
+    (merge response (when (ImageIO/write ^RenderedImage value "png" ^OutputStream bos)
+                      (slurp-reply "" ["image/png" {}] (.toByteArray bos))))))
 
 (defn content-type-transport
   "Transport proxy which allows this middleware to intercept responses
@@ -123,8 +126,8 @@
     (recv [_this timeout]
       (.recv transport timeout))
 
-    (send [_this response]
-      (.send transport (response+content-type response)))))
+    (send [this response]
+      (.send transport (content-type-response response)))))
 
 (defn handle-content-type
   "Handler for inspecting the results of the `eval` op, attempting to