docs: create seperate MQTT example

Author: JKRhb

example/mqtt_example.dart

@@ -0,0 +1,105 @@
+// Copyright 2023 Contributors to the Eclipse Foundation. 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
+
+// ignore_for_file: avoid_print
+
+import 'package:dart_wot/dart_wot.dart';
+
+const thingDescriptionJson = '''
+  {
+    "@context": "https://www.w3.org/2022/wot/td/v1.1",
+    "title": "Test Thing",
+    "id": "urn:test",
+    "base": "coap://coap.me",
+    "security": ["auto_sc"],
+    "securityDefinitions": {
+      "auto_sc": {
+        "scheme": "auto"
+      }
+    },
+    "properties": {
+      "status": {
+        "observable": true,
+        "forms": [
+          {
+            "href": "mqtt://test.mosquitto.org:1884",
+            "mqv:filter": "test",
+            "op": ["readproperty", "observeproperty"],
+            "contentType": "text/plain"
+          }
+        ]
+      }
+    },
+    "actions": {
+      "toggle": {
+        "input": {
+          "type": "string"
+        },
+        "forms": [
+          {
+            "href": "mqtt://test.mosquitto.org:1884",
+            "mqv:topic": "test",
+            "mqv:retain": true
+          }
+        ]
+      }
+    }
+  }
+  ''';
+
+final Map<String, BasicCredentials> basicCredentials = {
+  'urn:test': BasicCredentials('rw', 'readwrite'),
+};
+
+Future<BasicCredentials?> basicCredentialsCallback(
+  Uri uri,
+  Form? form, [
+  BasicCredentials? invalidCredentials,
+]) async {
+  final id = form?.thingDescription.identifier;
+
+  return basicCredentials[id];
+}
+
+Future<void> main(List<String> args) async {
+  final servient = Servient(
+    clientFactories: [
+      MqttClientFactory(basicCredentialsCallback: basicCredentialsCallback),
+    ],
+  );
+
+  final wot = await servient.start();
+
+  final thingDescription = ThingDescription(thingDescriptionJson);
+  final consumedThing = await wot.consume(thingDescription);
+  await consumedThing.readAndPrintProperty('status');
+
+  final subscription = await consumedThing.observeProperty(
+    'status',
+    (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();
+
+  await consumedThing.invokeAction('toggle', 'Bye World!');
+  await consumedThing.readAndPrintProperty('status');
+  print('Done!');
+}
+
+extension ReadAndPrintExtension on ConsumedThing {
+  Future<void> readAndPrintProperty(String propertyName) async {
+    final output = await readProperty(propertyName);
+    final value = await output.value();
+    print(value);
+  }
+}