feat: add first version of MQTT binding

example/example.dart

@@ -8,23 +8,44 @@
 
 import 'package:dart_wot/dart_wot.dart';
 
+final Map<String, BasicCredentials> basicCredentials = {
+  'urn:test': BasicCredentials('rw', 'readwrite')
+};
+
+Future<BasicCredentials?> basicCredentialsCallback(
+  Uri uri,
+  Form? form, [
+  BasicCredentials? invalidCredentials,
+]) async {
+  final id = form?.interactionAffordance.thingDescription.identifier;
+
+  return basicCredentials[id];
+}
+
 Future<void> main(List<String> args) async {
   final CoapClientFactory coapClientFactory = CoapClientFactory();
   final HttpClientFactory httpClientFactory = HttpClientFactory();
-  final servient = Servient()
+  final MqttClientFactory mqttClientFactory = MqttClientFactory();
+  final servient = Servient(
+    clientSecurityProvider: ClientSecurityProvider(
+      basicCredentialsCallback: basicCredentialsCallback,
+    ),
+  )
     ..addClientFactory(coapClientFactory)
-    ..addClientFactory(httpClientFactory);
+    ..addClientFactory(httpClientFactory)
+    ..addClientFactory(mqttClientFactory);
   final wot = await servient.start();
 
   const thingDescriptionJson = '''
   {
     "@context": "http://www.w3.org/ns/td",
     "title": "Test Thing",
+    "id": "urn:test",
     "base": "coap://coap.me",
-    "security": ["nosec_sc"],
+    "security": ["auto_sc"],
     "securityDefinitions": {
-      "nosec_sc": {
-        "scheme": "nosec"
+      "auto_sc": {
+        "scheme": "auto"
       }
     },
     "properties": {
@@ -34,6 +55,28 @@ Future<void> main(List<String> args) async {
             "href": "/hello"
           }
         ]
+      },
+      "status2": {
+        "observable": true,
+        "forms": [
+          {
+            "href": "mqtt://test.mosquitto.org:1884",
+            "mqv:filter": "test",
+            "op": ["readproperty", "observeproperty"],
+            "contentType": "text/plain"
+          }
+        ]
+      }
+    },
+    "actions": {
+      "toggle": {
+        "forms": [
+          {
+            "href": "mqtt://test.mosquitto.org:1884",
+            "mqv:topic": "test",
+            "mqv:retain": true
+          }
+        ]
       }
     }
   }
@@ -44,6 +87,19 @@ Future<void> main(List<String> args) async {
   final status = await consumedThing.readProperty('status');
   final value = await status.value();
   print(value);
+  final subscription = await consumedThing.observeProperty(
+    'status2',
+    (data) async {
+      final value = await data.value();
+      print(value);
+    },
+  );
+
+  await consumedThing.invokeAction('toggle', 'Hello World!');
+  await consumedThing.invokeAction('toggle', 'Hello World!');
+  await consumedThing.invokeAction('toggle', 'Hello World!');
+  await consumedThing.invokeAction('toggle', 'Hello World!');
+  await subscription.stop();
 
   final thingUri = Uri.parse(
     'https://raw.githubusercontent.com/w3c/wot-testing'
@@ -61,5 +117,15 @@ Future<void> main(List<String> args) async {
     );
   }
 
+  await consumedThing.invokeAction('toggle', 'Bye World!');
+  await consumedThing.readAndPrintProperty('status2');
   print('Done!');
 }
+
+extension ReadAndPrintExtension on ConsumedThing {
+  Future<void> readAndPrintProperty(String propertyName) async {
+    final output = await readProperty(propertyName);
+    final value = await output.value();
+    print(value);
+  }
+}

lib/binding_mqtt.dart

@@ -0,0 +1,13 @@
+// Copyright 2022 The NAMIB Project Developers. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+// SPDX-License-Identifier: BSD-3-Clause
+
+/// Protocol binding for the Message Queue Telemetry Transport (MQTT). Follows
+/// the latest [WoT Binding Templates Specification][spec link] for MQTT.
+///
+/// [spec link]: https://w3c.github.io/wot-binding-templates/bindings/protocols/mqtt
+
+export 'src/binding_mqtt/mqtt_client_factory.dart';
+export 'src/binding_mqtt/mqtt_config.dart';

lib/dart_wot.dart

@@ -9,6 +9,7 @@ library dart_wot;
 
 export 'binding_coap.dart';
 export 'binding_http.dart';
+export 'binding_mqtt.dart';
 export 'core.dart';
 export 'definitions.dart';
 export 'scripting_api.dart';

lib/src/binding_mqtt/constants.dart

@@ -0,0 +1,51 @@
+// Copyright 2022 The NAMIB Project Developers. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+// SPDX-License-Identifier: BSD-3-Clause
+
+/// The URI scheme for unsecured MQTT (using TCP).
+///
+/// Note that this scheme is not standardized yet, as there is an ongoing debate
+/// about URI schemes in the context of MQTT.
+const mqttUriScheme = 'mqtt';
+
+/// The default port number used for the [mqttUriScheme].
+const defaultMqttPort = 1883;
+
+/// The URI scheme for secure MQTT (using TCP and TLS).
+///
+/// Note that this scheme is not standardized yet, as there is an ongoing debate
+/// about URI schemes in the context of MQTT.
+const mqttSecureUriScheme = 'mqtts';
+
+/// The default port number used for the [mqttSecureUriScheme].
+const defaultMqttSecurePort = 8883;
+
+/// URI pointing to the MQTT vocabulary.
+///
+/// Used for resolving MQTT-related compact URIs (CURIEs) in TDs. Note that
+/// the MQTT vocabulary is not standardized yet, so this URI will change in
+/// future versions of this library.
+const mqttContextUri = 'http://www.example.org/mqtt-binding#';
+
+/// The default prefix used in MQTT-related compact URIs (CURIEs) in TDs.
+const defaultMqttPrefix = 'mqv';
+
+/// Default timeout length used for reading properties and discovering TDs.
+const defaultTimeout = Duration(seconds: 10);
+
+/// Default duration MQTT connections are kept alive in seconds.
+const defaultKeepAlivePeriod = 20;
+
+/// Default content type when returning a `Content` object from the MQTT
+/// binding.
+///
+/// Evaluates to `'application/octet-stream'.
+const defaultContentType = 'application/octet-stream';
+
+/// Content type used for the Content objects returned by discovery using MQTT.
+///
+/// Evaluates to `application/td+json`.
+// TODO: Should probably be redefined globally
+const discoveryContentType = 'application/td+json';

lib/src/binding_mqtt/mqtt_binding_exception.dart

@@ -0,0 +1,16 @@
+// Copyright 2022 The NAMIB Project Developers. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+// SPDX-License-Identifier: BSD-3-Clause
+
+/// An [Exception] that is thrown if an error occurs within the MQTT binding.
+class MqttBindingException implements Exception {
+  /// Constructor.
+  MqttBindingException(this._message);
+
+  final String _message;
+
+  @override
+  String toString() => 'MqttBindingException: $_message';
+}