Allow Media Type and Encoding re-use

Add the Media Type Object and Encoding Object to the Components Object,
and allow a Reference Object anywhere they are allowed.

To ensure that re-usable Objects can be documented clearly, add
a `description` field to both Objects.

Author: handrews

src/oas.md

@@ -627,6 +627,8 @@ All objects defined within the Components Object will have no effect on the API
 | <a name="components-links"></a> links | Map[`string`, [Link Object](#link-object) \| [Reference Object](#reference-object)] | An object to hold reusable [Link Objects](#link-object). |
 | <a name="components-callbacks"></a> callbacks | Map[`string`, [Callback Object](#callback-object) \| [Reference Object](#reference-object)] | An object to hold reusable [Callback Objects](#callback-object). |
 | <a name="components-path-items"></a> pathItems | Map[`string`, [Path Item Object](#path-item-object)] | An object to hold reusable [Path Item Objects](#path-item-object). |
+| <a name="components-media-types"></a> mediaTypes | Map[`string`, [Media Type Objects](#media-type-object) \| [Reference Object](#reference-object)] | An object to hold reusable [Media Type Objects](#media-type-object). |
+| <a name="components-encodings"></a> encodings | Map[`string`, [Encoding Objects](#encoding-object) \| [Reference Object](#reference-object)] | An object to hold reusable [Encoding Objects](#encoding-object). |
 
 This object MAY be extended with [Specification Extensions](#specification-extensions).
 
@@ -1008,7 +1010,7 @@ For use with `in: "querystring"` and `application/x-www-form-urlencoded`, see [E
 
 | Field Name | Type | Description |
 | ---- | :----: | ---- |
-| <a name="parameter-content"></a>content | Map[`string`, [Media Type Object](#media-type-object)] | A map containing the representations for the parameter. The key is the media type and the value describes it. The map MUST only contain one entry. |
+| <a name="parameter-content"></a>content | Map[`string`, [Media Type Object](#media-type-object) \| [Reference Object](#reference-object)] | A map containing the representations for the parameter. The key is the media type and the value describes it. The map MUST only contain one entry. |
 
 ##### Style Values
 
@@ -1198,7 +1200,7 @@ Describes a single request body.
 | Field Name | Type | Description |
 | ---- | :----: | ---- |
 | <a name="request-body-description"></a>description | `string` | A brief description of the request body. This could contain examples of use. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation. |
-| <a name="request-body-content"></a>content | Map[`string`, [Media Type Object](#media-type-object)] | **REQUIRED**. The content of the request body. The key is a media type or [media type range](https://www.rfc-editor.org/rfc/rfc9110.html#appendix-A) and the value describes it. The map SHOULD have at least one entry; if it does not, the behavior is implementation-defined. For requests that match multiple keys, only the most specific key is applicable. e.g. `"text/plain"` overrides `"text/*"` |
+| <a name="request-body-content"></a>content | Map[`string`, [Media Type Object](#media-type-object) \| [Reference Object](#reference-object)] | **REQUIRED**. The content of the request body. The key is a media type or [media type range](https://www.rfc-editor.org/rfc/rfc9110.html#appendix-A) and the value describes it. The map SHOULD have at least one entry; if it does not, the behavior is implementation-defined. For requests that match multiple keys, only the most specific key is applicable. e.g. `"text/plain"` overrides `"text/*"` |
 | <a name="request-body-required"></a>required | `boolean` | Determines if the request body is required in the request. Defaults to `false`. |
 
 This object MAY be extended with [Specification Extensions](#specification-extensions).
@@ -1249,11 +1251,12 @@ See [Working With Examples](#working-with-examples) for further guidance regardi
 
 | Field Name | Type | Description |
 | ---- | :----: | ---- |
+| <a name="media-type-description"></a>description | `string` | A brief description of the media type. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation. |
 | <a name="media-type-schema"></a>schema | [Schema Object](#schema-object) | A schema describing the complete content of the request, response, parameter, or header. |
 | <a name="media-type-item-schema"></a>itemSchema | [Schema Object](#schema-object) | A schema describing each item within a [sequential media type](#sequential-media-types). |
 | <a name="media-type-example"></a>example | Any | Example of the media type; see [Working With Examples](#working-with-examples). |
 | <a name="media-type-examples"></a>examples | Map[ `string`, [Example Object](#example-object) \| [Reference Object](#reference-object)] | Examples of the media type; see [Working With Examples](#working-with-examples). |
-| <a name="media-type-encoding"></a>encoding | Map[`string`, [Encoding Object](#encoding-object)] | A map between a property name and its encoding information, as defined under [Encoding Usage and Restrictions](#encoding-usage-and-restrictions).  The `encoding` field SHALL only apply when the media type is `multipart` or `application/x-www-form-urlencoded`. If no Encoding Object is provided for a property, the behavior is determined by the default values documented for the Encoding Object. |
+| <a name="media-type-encoding"></a>encoding | Map[`string`, [Encoding Object](#encoding-object) \| [Reference Object](#reference-object)] | A map between a property name and its encoding information, as defined under [Encoding Usage and Restrictions](#encoding-usage-and-restrictions).  The `encoding` field SHALL only apply when the media type is `multipart` or `application/x-www-form-urlencoded`. If no Encoding Object is provided for a property, the behavior is determined by the default values documented for the Encoding Object. |
 
 This object MAY be extended with [Specification Extensions](#specification-extensions).
 
@@ -1639,6 +1642,7 @@ These fields MAY be used either with or without the RFC6570-style serialization
 
 | Field Name | Type | Description |
 | ---- | :----: | ---- |
+| <a name="encoding-description"></a>description | `string` | A brief description of the media type. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation. |
 | <a name="encoding-content-type"></a>contentType | `string` | The `Content-Type` for encoding a specific property. The value is a comma-separated list, each element of which is either a specific media type (e.g. `image/png`) or a wildcard media type (e.g. `image/*`). Default value depends on the property type as shown in the table below. |
 | <a name="encoding-headers"></a>headers | Map[`string`, [Header Object](#header-object) \| [Reference Object](#reference-object)] | A map allowing additional information to be provided as headers. `Content-Type` is described separately and SHALL be ignored in this section. This field SHALL be ignored if the media type is not a `multipart`. |
 
@@ -1929,7 +1933,7 @@ Describes a single response from an API operation, including design-time, static
 | <a name="response-summary"></a>summary | `string` | A short summary of the meaning of the response. |
 | <a name="response-description"></a>description | `string` | A description of the response. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation. |
 | <a name="response-headers"></a>headers | Map[`string`, [Header Object](#header-object) \| [Reference Object](#reference-object)] | Maps a header name to its definition. [RFC9110](https://www.rfc-editor.org/rfc/rfc9110.html#section-5.1) states header names are case insensitive. If a response header is defined with the name `"Content-Type"`, it SHALL be ignored. |
-| <a name="response-content"></a>content | Map[`string`, [Media Type Object](#media-type-object)] | A map containing descriptions of potential response payloads. The key is a media type or [media type range](https://www.rfc-editor.org/rfc/rfc9110.html#appendix-A) and the value describes it. For responses that match multiple keys, only the most specific key is applicable. e.g. `"text/plain"` overrides `"text/*"` |
+| <a name="response-content"></a>content | Map[`string`, [Media Type Object](#media-type-object) \| [Reference Object](#reference-object)] | A map containing descriptions of potential response payloads. The key is a media type or [media type range](https://www.rfc-editor.org/rfc/rfc9110.html#appendix-A) and the value describes it. For responses that match multiple keys, only the most specific key is applicable. e.g. `"text/plain"` overrides `"text/*"` |
 | <a name="response-links"></a>links | Map[`string`, [Link Object](#link-object) \| [Reference Object](#reference-object)] | A map of operations links that can be followed from the response. The key of the map is a short name for the link, following the naming constraints of the names for [Component Objects](#components-object). |
 
 This object MAY be extended with [Specification Extensions](#specification-extensions).
@@ -2449,7 +2453,7 @@ Using `content` with a `text/plain` media type is RECOMMENDED for headers where
 
 | Field Name | Type | Description |
 | ---- | :----: | ---- |
-| <a name="header-content"></a>content | Map[`string`, [Media Type Object](#media-type-object)] | A map containing the representations for the header. The key is the media type and the value describes it. The map MUST only contain one entry. |
+| <a name="header-content"></a>content | Map[`string`, [Media Type Object](#media-type-object) \| [Reference Object](#reference-object)] | A map containing the representations for the header. The key is the media type and the value describes it. The map MUST only contain one entry. |
 
 ##### Header Object Example
 

src/schemas/validation/schema.yaml

@@ -199,8 +199,16 @@ $defs:
         type: object
         additionalProperties:
           $ref: '#/$defs/path-item'
+      mediaTypes:
+        type: object
+        additionalProperties:
+          $ref: '#/$defs/media-type-or-reference'
+      encodings:
+        type: object
+        additionalProperties:
+          $ref: '#/$defs/encoding-or-reference'
     patternProperties:
-      '^(schemas|responses|parameters|examples|requestBodies|headers|securitySchemes|links|callbacks|pathItems)$':
+      '^(schemas|responses|parameters|examples|requestBodies|headers|securitySchemes|links|callbacks|pathItems|mediaTypes|encodings)$':
         $comment: Enumerating all of the property names in the regex above is necessary for unevaluatedProperties to work as expected
         propertyNames:
           pattern: '^[a-zA-Z0-9._-]+$'
@@ -517,31 +525,45 @@ $defs:
     $comment: https://spec.openapis.org/oas/v3.2#fixed-fields-10
     type: object
     additionalProperties:
-      $ref: '#/$defs/media-type'
+      $ref: '#/$defs/media-type-or-reference'
     propertyNames:
       format: media-range
 
   media-type:
     $comment: https://spec.openapis.org/oas/v3.2#media-type-object
     type: object
     properties:
+      description:
+        type: string
       schema:
         $dynamicRef: '#meta'
       itemSchema:
         $dynamicRef: '#meta'
       encoding:
         type: object
         additionalProperties:
-          $ref: '#/$defs/encoding'
+          $ref: '#/$defs/encoding-or-reference'
     allOf:
       - $ref: '#/$defs/specification-extensions'
       - $ref: '#/$defs/examples'
     unevaluatedProperties: false
 
+  media-type-or-reference:
+    if:
+      type: object
+      required:
+        - $ref
+    then:
+      $ref: '#/$defs/reference'
+    else:
+      $ref: '#/$defs/media-type'
+
   encoding:
     $comment: https://spec.openapis.org/oas/v3.2#encoding-object
     type: object
     properties:
+      description:
+        type: string
       contentType:
         type: string
         format: media-range
@@ -566,6 +588,16 @@ $defs:
       - $ref: '#/$defs/styles-for-form'
     unevaluatedProperties: false
 
+  encoding-or-reference:
+    if:
+      type: object
+      required:
+        - $ref
+    then:
+      $ref: '#/$defs/reference'
+    else:
+      $ref: '#/$defs/encoding'
+
   responses:
     $comment: https://spec.openapis.org/oas/v3.2#responses-object
     type: object

tests/schema/pass/media-type-examples.yaml

@@ -3,6 +3,25 @@ openapi: 3.2.0
 info:
   title: API
   version: 1.0.0
+components:
+  mediaTypes:
+    StreamingPets:
+      description: |
+        Streaming sequence of JSON pet representations,
+        suitable for use with any of the streaming JSON
+        media types.
+      itemSchema:
+        $ref: '#components/schemas/Pet'
+  encodings:
+    ReservedNoExplode:
+      description: |
+        This allows multiple values under the same name,
+        and disables reserved percent-encoding.
+        Clients are responsible for adequately percent-encoding
+        the values in advance.
+      style: form
+      explode: false
+      allowReserved: true
 paths:
   /something:
     put:
@@ -31,8 +50,9 @@ paths:
               frog:
                 $ref: '#/components/examples/frog-example'
           application/jsonl:
-            itemSchema:
-              $ref: '#components/schemas/Pet'
+            $ref: '#/components/mediaTypes/StreamingPets'
+          application/x-ndjson:
+            $ref: '#/components/mediaTypes/StreamingPets'
           application/xml:
             schema:
               type: object
@@ -132,9 +152,7 @@ paths:
                     schema:
                       type: integer
               forCoverage:
-                style: form
-                explode: false
-                allowReserved: true
+                $ref: '#/components/encodings/ReservedNoExplode'
               forCoverage2:
                 style: spaceDelimited
                 explode: true