Support reconnects on page refreshes for SSE client connections

If we treat reloads as parts of an app lifecycle, it seems natural to
preserve an app instance id on full reloads, and looks like a session
storage is a good fit.

Fixes #2726.

Author: iinozemtsev

dwds/lib/src/handlers/dev_handler.dart

@@ -636,21 +636,11 @@ class DevHandler {
       readyToRunMainCompleter.future,
     );
 
-    // We can take over a connection if there is no connectedInstanceId (this
-    // means the client completely disconnected), or if the existing
-    // AppConnection is in the KeepAlive state (this means it disconnected but
-    // is still waiting for a possible reconnect - this happens during a page
-    // reload).
-    final canReconnect =
-        services != null &&
-        (services.connectedInstanceId == null ||
-            existingConnection?.isInKeepAlivePeriod == true);
-
-    if (canReconnect) {
+    if (_canReconnect(existingConnection, message)) {
       // Disconnect any old connection (eg. those in the keep-alive waiting
       // state when reloading the page).
       existingConnection?.shutDown();
-      final chromeProxy = services.proxyService;
+      final chromeProxy = services!.proxyService;
       if (chromeProxy is ChromeProxyService) {
         chromeProxy.destroyIsolate();
       }
@@ -703,24 +693,10 @@ class DevHandler {
       readyToRunMainCompleter.future,
     );
 
-    // Allow connection reuse for page refreshes and same instance reconnections
-    final isSameInstance =
-        existingConnection?.request.instanceId == message.instanceId;
-    final isKeepAliveReconnect =
-        existingConnection?.isInKeepAlivePeriod == true;
-    final hasNoActiveConnection = services?.connectedInstanceId == null;
-    final noExistingConnection = existingConnection == null;
-
-    final canReconnect =
-        services != null &&
-        (isSameInstance ||
-            (isKeepAliveReconnect && hasNoActiveConnection) ||
-            (noExistingConnection && hasNoActiveConnection));
-
-    if (canReconnect) {
+    if (_canReconnect(existingConnection, message)) {
       // Reconnect to existing service.
       await _reconnectToService(
-        services,
+        services!,
         existingConnection,
         connection,
         message,
@@ -749,6 +725,30 @@ class DevHandler {
     return connection;
   }
 
+  /// Allow connection reuse for page refreshes and same instance reconnections
+  bool _canReconnect(
+    AppConnection? existingConnection,
+    ConnectRequest message,
+  ) {
+    final services = _servicesByAppId[message.appId];
+
+    if (services == null) {
+      return false;
+    }
+
+    if (existingConnection?.request.instanceId == message.instanceId) {
+      return true;
+    }
+
+    final isKeepAliveReconnect =
+        existingConnection?.isInKeepAlivePeriod == true;
+    final hasNoActiveConnection = services.connectedInstanceId == null;
+    final noExistingConnection = existingConnection == null;
+
+    return (isKeepAliveReconnect && hasNoActiveConnection) ||
+        (noExistingConnection && hasNoActiveConnection);
+  }
+
   /// Handles reconnection to existing services for web-socket mode.
   Future<void> _reconnectToService(
     AppDebugServices services,

dwds/lib/src/injected/client.js

@@ -47,7 +47,22 @@ Future<void>? main() {
     () async {
       // Set the unique id for this instance of the app.
       // Test apps may already have this set.
-      dartAppInstanceId ??= const Uuid().v1();
+      const dartAppInstanceIdKey = 'dartAppInstanceId';
+      if (dartAppInstanceId == null) {
+        // Check the session storage for the instance id.
+        final storedInstanceId = window.sessionStorage.getItem(
+          dartAppInstanceIdKey,
+        );
+        if (storedInstanceId != null) {
+          dartAppInstanceId = storedInstanceId;
+        } else {
+          dartAppInstanceId = const Uuid().v1();
+          window.sessionStorage.setItem(
+            dartAppInstanceIdKey,
+            dartAppInstanceId!,
+          );
+        }
+      }
 
       final fixedPath = _fixProtocol(dwdsDevHandlerPath);
       final fixedUri = Uri.parse(fixedPath);