feat: introduce new MQTT discovery method

lib/src/binding_mqtt/mqtt_client.dart

@@ -21,7 +21,7 @@ import "mqtt_subscription.dart";
 /// [ProtocolClient] for supporting the MQTT protocol.
 ///
 /// Currently, only MQTT version 3.1.1 is supported.
-final class MqttClient extends ProtocolClient {
+final class MqttClient extends ProtocolClient with MqttDiscoverer {
   /// Constructor.
   MqttClient({
     MqttConfig? mqttConfig,
@@ -197,4 +197,46 @@ final class MqttClient extends ProtocolClient {
 
     return MqttSubscription(form, client, complete, next: next, error: error);
   }
+
+  @override
+  Stream<Content> performMqttDiscovery(
+    Uri brokerUri, {
+    required String discoveryTopic,
+    required String expectedContentType,
+    required Duration discoveryTimeout,
+  }) async* {
+    final client = await _connect(brokerUri, null);
+
+    // TODO: Revisit QoS value and subscription check
+    if (client.subscribe(discoveryTopic, MqttQos.atLeastOnce) == null) {
+      throw MqttBindingException(
+        "Subscription to topic $discoveryTopic failed",
+      );
+    }
+
+    final receivedMessageStream = client.updates;
+    if (receivedMessageStream == null) {
+      throw MqttBindingException(
+        "Subscription to topic $discoveryTopic failed",
+      );
+    }
+
+    Timer(
+      discoveryTimeout,
+      () async {
+        client.disconnect();
+      },
+    );
+
+    await for (final receivedMessageList in receivedMessageStream) {
+      for (final receivedMessage in receivedMessageList) {
+        final mqttMessage = receivedMessage.payload;
+        if (mqttMessage is MqttPublishMessage) {
+          final messagePayload = mqttMessage.payload.message;
+
+          yield Content(expectedContentType, Stream.value(messagePayload));
+        }
+      }
+    }
+  }
 }

lib/src/core/implementation/discovery/discovery_configuration.dart

@@ -138,6 +138,45 @@ final class ExploreDirectoryConfiguration extends DiscoveryConfiguration {
   final int? limit;
 }
 
+/// Experimental [DiscoveryConfiguration] that is used to perform discovery with
+/// the MQTT protocol.
+@experimental
+final class MqttDiscoveryConfiguration extends DiscoveryConfiguration {
+  /// Instantiates a new [DiscoveryConfiguration] for MQTT.
+  const MqttDiscoveryConfiguration(
+    this.brokerUri, {
+    this.discoveryTopic = "wot/td/#",
+    this.expectedContentType = "application/td+json",
+    this.discoveryTimeout = const Duration(seconds: 5),
+  });
+
+  /// [Uri] of the broker the
+  final Uri brokerUri;
+
+  /// The topic that will be used for performing the discovery process.
+  ///
+  /// If a wildcard topic is used, then the discovery process may return more
+  /// than one TD.
+  ///
+  /// Defaults to `wot/td/#`.
+  final String discoveryTopic;
+
+  /// The Thing Description content type that is expected during the discovery
+  /// process.
+  ///
+  /// Data that is received during the discovery process that is not
+  /// deserializable using the content type provided here will be ignored.
+  ///
+  /// Defaults to `application/td+json`.
+  final String expectedContentType;
+
+  /// Time period after which the MQTT discovery process is going to be
+  /// cancelled.
+  ///
+  /// Defaults to five seconds.
+  final Duration discoveryTimeout;
+}
+
 /// Base class for configuring discovery mechanisms that involve a two-step
 /// approach.
 ///

lib/src/core/implementation/thing_discovery.dart

@@ -75,6 +75,18 @@ class ThingDiscovery extends Stream<ThingDescription>
             thingFilter: thingFilter,
           );
           yield* thingDiscoveryProcess;
+        case MqttDiscoveryConfiguration(
+            :final brokerUri,
+            :final discoveryTopic,
+            :final expectedContentType,
+            :final discoveryTimeout,
+          ):
+          yield* _performMqttDiscovery(
+            brokerUri,
+            discoveryTopic,
+            expectedContentType,
+            discoveryTimeout,
+          );
       }
     }
   }
@@ -250,6 +262,34 @@ class ThingDiscovery extends Stream<ThingDescription>
     );
   }
 
+  Stream<ThingDescription> _performMqttDiscovery(
+    Uri brokerUri,
+    String discoveryTopic,
+    String expectedContentType,
+    Duration discoveryTimeout,
+  ) async* {
+    final uriScheme = brokerUri.scheme;
+    final client = _clientForUriScheme(uriScheme);
+
+    if (client is! MqttDiscoverer) {
+      yield* Stream.error(
+        DiscoveryException(
+          "Client for URI scheme $uriScheme does not support MQTT Discovery.",
+        ),
+      );
+      return;
+    }
+
+    final contentStream = client.performMqttDiscovery(
+      brokerUri,
+      discoveryTopic: discoveryTopic,
+      expectedContentType: expectedContentType,
+      discoveryTimeout: discoveryTimeout,
+    );
+
+    yield* _transformContentStreamToThingDescriptions(contentStream);
+  }
+
   @override
   Future<void> stop() async {
     final stopFutures = _clients.values.map((client) => client.stop());

lib/src/core/protocol_interfaces/protocol_discoverer.dart

@@ -45,3 +45,22 @@ base mixin CoreLinkFormatDiscoverer on ProtocolClient {
   @experimental
   Stream<DiscoveryContent> discoverWithCoreLinkFormat(Uri uri);
 }
+
+/// Interface for performing experimental discovery using the MQTT protocol.
+@experimental
+base mixin MqttDiscoverer on ProtocolClient {
+  /// Performs discovery of Thing Descriptions using the MQTT protocol via the
+  /// given [brokerUri].
+  ///
+  /// By default, the [discoveryTopic] `wot/td/#` will be used as discussed in
+  /// [this issue].
+  ///
+  /// [this issue]: https://github.com/w3c/wot-discovery/issues/134
+  @experimental
+  Stream<Content> performMqttDiscovery(
+    Uri brokerUri, {
+    required String discoveryTopic,
+    required String expectedContentType,
+    required Duration discoveryTimeout,
+  });
+}