feat: update CoAP discovery example

example/coap_discovery.dart

@@ -16,27 +16,46 @@ extension PrintExtension on InteractionOutput {
   }
 }
 
+Future<void> handleThingDescription(
+  WoT wot,
+  ThingDescription thingDescription,
+) async {
+  final consumedThing = await wot.consume(thingDescription);
+  await consumedThing.writeProperty(propertyName, 'Hello World!');
+  var output = await consumedThing.readProperty(propertyName);
+  await output.printValue();
+  await consumedThing.writeProperty(propertyName, 'Bye World!');
+  output = await consumedThing.readProperty(propertyName);
+  await output.printValue();
+}
+
 Future<void> main(List<String> args) async {
   final servient = Servient()..addClientFactory(CoapClientFactory());
 
   final wot = await servient.start();
-
-  await for (final thingDescription in wot.discover(
-    ThingFilter(
-      url: Uri.parse('coap://plugfest.thingweb.io:5683/testthing'),
-    ),
-  )) {
-    final consumedThing = await wot.consume(thingDescription);
-
-    try {
-      await consumedThing.writeProperty(propertyName, 'Hello World!');
-      var output = await consumedThing.readProperty(propertyName);
-      await output.printValue();
-      await consumedThing.writeProperty(propertyName, 'Bye World!');
-      output = await consumedThing.readProperty(propertyName);
-      await output.printValue();
-    } on Exception catch (e) {
-      print(e);
+  final thingFilter = ThingFilter(
+    url: Uri.parse('coap://plugfest.thingweb.io:5683/testthing'),
+  );
+
+  // Example using for-await-loop
+  try {
+    await for (final thingDescription in wot.discover(thingFilter)) {
+      await handleThingDescription(wot, thingDescription);
     }
+    print('Discovery with "await for" has finished.');
+  } on Exception catch (error) {
+    print(error);
   }
+
+  // Example using the .listen() method, allowing for error handling
+  //
+  // Notice how the "onDone" callback is called before the result is passed
+  // to the handleThingDescription function.
+  wot.discover(thingFilter).listen(
+    (thingDescription) async {
+      await handleThingDescription(wot, thingDescription);
+    },
+    onError: (error) => print('Encountered an error: $error'),
+    onDone: () => print('Discovery with "listen" has finished.'),
+  );
 }