Make connectUrl lazy and once initialized

lib/stomp_config.dart

@@ -70,10 +70,13 @@ class StompConfig {
   /// Callback for debug messages
   final StompDebugCallback onDebugMessage;
 
+  /// The transport url of the WebSocket to connect to
   String get connectUrl =>
-      useSockJS ? SockJsUtils().generateTransportUrl(url) : url;
+      _connectUrl ??= useSockJS ? SockJsUtils().generateTransportUrl(url) : url;
 
-  const StompConfig({
+  String? _connectUrl;
+
+  StompConfig({
     required this.url,
     this.reconnectDelay = const Duration(seconds: 5),
     this.heartbeatIncoming = const Duration(seconds: 5),
@@ -157,6 +160,9 @@ class StompConfig {
     );
   }
 
+  /// Resets the transport URL
+  void resetSession() => _connectUrl = null;
+
   static void _noOp([_, __]) {}
 
   static Future<void> _noOpFuture() => Future.value();

lib/stomp_handler.dart

@@ -53,7 +53,7 @@ class StompHandler {
   void start() async {
     _isActive = true;
     try {
-      _channel = await platform.connect(config);
+      _channel = await platform.connect(config..resetSession());
       // It can happen that dispose was called while the future above hasn't completed yet
       // To prevent lingering connections we need to make sure that we disconnect cleanly
       if (!_isActive) {

test/stomp_config_test.dart

@@ -0,0 +1,51 @@
+import 'dart:async';
+
+import 'package:test/test.dart';
+import 'package:web_socket_channel/web_socket_channel.dart';
+
+import 'package:stomp_dart_client/stomp_config.dart';
+import 'package:stomp_dart_client/src/_connect_api.dart'
+    if (dart.library.html) '../lib/src/_connect_html.dart'
+    if (dart.library.io) '../lib/src/_connect_io.dart' as platform;
+
+void main() {
+  group('StompConfig', () {
+    test('Generate session URL once per connection', () async {
+      final config = StompConfig.sockJS(
+        url: 'http://localhost',
+        reconnectDelay: Duration(milliseconds: 500),
+      );
+
+      final connectUrls = <String, String>{};
+
+      void connect() async {
+        try {
+          await platform.connect(config..resetSession());
+        } on WebSocketChannelException catch (_) {
+          // Save subsequent calls of `connectUrl`.
+          connectUrls.addAll({config.connectUrl: config.connectUrl});
+          if (connectUrls.length == 1) {
+            // On 1st connect we expect that the current stored values are equal
+            expect(connectUrls.entries.first.key,
+                equals(connectUrls.entries.first.value));
+          } else if (connectUrls.length == 2) {
+            // On 2nd connect we expect that the current stored values are equal
+            expect(connectUrls.entries.last.key,
+                equals(connectUrls.entries.last.value));
+            // But they are different from the values saved on 1st connect
+            expect(connectUrls.entries.first.key,
+                isNot(equals(connectUrls.entries.last.key)));
+            expect(connectUrls.entries.first.value,
+                isNot(equals(connectUrls.entries.last.value)));
+          }
+          Timer(config.reconnectDelay, connect);
+        }
+      }
+
+      connect();
+
+      // Wait until exit
+      await Future.delayed(Duration(seconds: 3));
+    });
+  });
+}