Document the new Web Host Metadata API for apps

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

developer_manual/app_publishing_maintenance/upgrade-guide.rst

@@ -24,6 +24,11 @@ Last version with database.xml support and migration
 
 Nextcloud 21 is the last major release that supports an app's ``appinfo/database.xml`` to :ref:`define the database schema<database-xml>`. This is your last change to :ref:`automatically convert this deprecated file into the new migration classes<migrate-database-xml>`.
 
+Replaced well-known handler API
+*******************************
+
+There was an old, unused and inofficial mechanism to hook into well-known discovery via config settings. This includes ``host-meta``, ``host-meta.json``, ``nodeinfo`` and ``webfinger``. A :ref:`new public API replaces this mechanism<web-host-metadata>` in Nextcloud 21.
+
 Upgrading to Nextcloud 20
 -------------------------
 

developer_manual/digging_deeper/index.rst

@@ -25,3 +25,4 @@ Digging deeper
    two-factor-provider
    users
    dashboard
+   web_host_metadata

developer_manual/digging_deeper/web_host_metadata.rst

@@ -0,0 +1,149 @@
+.. _web-host-metadata:
+
+=================
+Web Host Metadata
+=================
+
+`RFC6415`_ defines how web hosts can expose their metadata through resources. Starting with Nextcloud 21, it's possible to register handlers for HTTP requests to the ``.well-known/*`` route.
+
+Writing a handler
+-----------------
+
+A well known handler is a simple class that implements the ``\OCP\Http\WellKnown\IHandler`` interface.
+
+.. code-block:: php
+
+    <?php
+
+    declare(strict_types=1);
+
+    namespace OCA\MyApp\Http\WellKnown;
+
+    class Handler implements IHandler {
+
+        public function handle(string $service, IRequestContext $context, ?IResponse $previousResponse): ?IResponse {
+            // the handler-specific logic
+        }
+
+    }
+
+The basic concept is that every handler will be called consecutively. A handler can react to the request and return a new response object or modify the one of the previous handler. The first handler will get a ``$previousResponse`` of null. The second handler gets whatever the first handler returned, so either ``null`` or an instance of ``\OCP\Http\WellKnown\IResponse``.
+
+
+Example generic handler
+^^^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: php
+
+    <?php
+
+    declare(strict_types=1);
+
+    namespace OCA\MyApp\Http\WellKnown;
+
+    use OCP\AppFramework\Http\JSONResponse;
+    use OCP\Http\WellKnown\GenericResponse;
+    use OCP\Http\WellKnown\IHandler;
+    use OCP\Http\WellKnown\IRequestContext;
+    use OCP\Http\WellKnown\IResponse;
+    use OCP\IURLGenerator;
+
+    class GenericHandler implements IHandler {
+
+        public function handle(string $service, IRequestContext $context, ?IResponse $previousResponse): ?IResponse {
+            if ($service !== 'nextcloudtest') {
+                // Not relevant to this handler
+
+                return $previousResponse;
+            }
+
+            return new GenericResponse(
+                new JSONResponse(['message' => 'hello']),
+            );
+        }
+    }
+
+
+Example webfinger handler
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The following example shows how an app could react to `RFC6415`_ webfinger requests:
+
+.. code-block:: php
+
+    <?php
+
+    declare(strict_types=1);
+
+    namespace OCA\MyApp\Http\WellKnown;
+
+    use OCP\Http\WellKnown\IHandler;
+    use OCP\Http\WellKnown\IRequestContext;
+    use OCP\Http\WellKnown\IResponse;
+    use OCP\Http\WellKnown\JrdResponse;
+    use OCP\IURLGenerator;
+
+    class WebFingerHandler implements IHandler {
+
+        /** @var IURLGenerator */
+        private $urlGenerator;
+
+        public function __construct(IURLGenerator $urlGenerator) {
+            $this->urlGenerator = $urlGenerator;
+        }
+
+        public function handle(string $service, IRequestContext $context, ?IResponse $previousResponse): ?IResponse {
+            if ($service !== 'webfinger') {
+                // Not relevant to this handler
+
+                return $previousResponse;
+            }
+
+            $subject = $context->getHttpRequest()->getParam('resource', '');
+            $href = $this->urlGenerator->linkToRouteAbsolute('myapp.example.test');
+
+            // Use the previous response and amend it, if possible
+            $response = $previousResponse;
+            if (!($response instanceof JrdResponse)) {
+                // We override null or any other types
+                $response = new JrdResponse($subject);
+            }
+
+            return $response->addLink('self', 'application/activity+json', $href);
+        }
+    }
+
+Handler registration
+--------------------
+
+The handler class is registered via the :ref:`bootstrap mechanism<Bootstrapping>` of the ``Application`` class.
+
+.. code-block:: php
+
+
+    <?php
+
+    declare(strict_types=1);
+
+    namespace OCA\MyApp\AppInfo;
+
+    use OCA\MyApp\Http\WellKnown\Handler;
+    use OCP\AppFramework\App;
+    use OCP\AppFramework\Bootstrap\IBootContext;
+    use OCP\AppFramework\Bootstrap\IBootstrap;
+    use OCP\AppFramework\Bootstrap\IRegistrationContext;
+
+    class Application extends App implements IBootstrap {
+
+        public function register(IRegistrationContext $context): void {
+            $context->registerWellKnownHandler(Handler::class);
+        }
+
+        public function boot(IBootContext $context): void {}
+
+    }
+
+
+
+.. _`RFC6415`: https://tools.ietf.org/html/rfc6415
+.. _`RFC7033`: https://tools.ietf.org/html/rfc7033