Add DevToolsExtensionHostInterface and clean up some service management for extensions

packages/devtools_app/lib/src/extensions/embedded/_view_web.dart

@@ -48,7 +48,8 @@ class _EmbeddedExtensionState extends State<EmbeddedExtension> {
 }
 
 class _ExtensionIFrameController extends DisposableController
-    with AutoDisposeControllerMixin {
+    with AutoDisposeControllerMixin
+    implements DevToolsExtensionHostInterface {
   _ExtensionIFrameController(this.embeddedExtensionController);
 
   final EmbeddedExtensionControllerImpl embeddedExtensionController;
@@ -120,29 +121,12 @@ class _ExtensionIFrameController extends DisposableController
     if (e is html.MessageEvent) {
       final extensionEvent = DevToolsExtensionEvent.tryParse(e.data);
       if (extensionEvent != null) {
-        switch (extensionEvent.type) {
-          case DevToolsExtensionEventType.ping:
-          // Ignore. DevTools should not receive/handle ping events.
-          case DevToolsExtensionEventType.pong:
-            if (!_extensionHandlerReady.isCompleted) {
-              _extensionHandlerReady.complete();
-            }
-            break;
-          case DevToolsExtensionEventType.vmServiceConnection:
-            final service = serviceConnection.serviceManager.service;
-            if (service == null) break;
-            _postMessage(
-              DevToolsExtensionEvent(
-                DevToolsExtensionEventType.vmServiceConnection,
-                data: {'uri': service.connectedUri.toString()},
-              ),
-            );
-            break;
-          default:
-            notificationService.push(
-              'Unknown event received from extension: ${e.data}',
-            );
-        }
+        onEventReceived(
+          extensionEvent,
+          onUnknownEvent: () => notificationService.push(
+            'Unknown event received from extension: ${e.data}',
+          ),
+        );
       }
     }
   }
@@ -160,7 +144,7 @@ class _ExtensionIFrameController extends DisposableController
         // Once the extension UI is ready, the extension will receive this
         // [DevToolsExtensionEventType.ping] message and return a
         // [DevToolsExtensionEventType.pong] message, handled in [_handleMessage].
-        _postMessage(DevToolsExtensionEvent.ping);
+        ping();
       });
 
       await _extensionHandlerReady.future.timeout(
@@ -181,4 +165,42 @@ class _ExtensionIFrameController extends DisposableController
     _pollForExtensionHandlerReady?.cancel();
     super.dispose();
   }
+
+  @override
+  void ping() {
+    _postMessage(DevToolsExtensionEvent(DevToolsExtensionEventType.ping));
+  }
+
+  @override
+  void vmServiceConnectionChanged({String? uri}) {
+    _postMessage(
+      DevToolsExtensionEvent(
+        DevToolsExtensionEventType.vmServiceConnection,
+        data: {'uri': uri!},
+      ),
+    );
+  }
+
+  @override
+  void onEventReceived(
+    DevToolsExtensionEvent event, {
+    void Function()? onUnknownEvent,
+  }) {
+    switch (event.type) {
+      case DevToolsExtensionEventType.ping:
+      // Ignore. DevTools should not receive/handle ping events.
+      case DevToolsExtensionEventType.pong:
+        if (!_extensionHandlerReady.isCompleted) {
+          _extensionHandlerReady.complete();
+        }
+        break;
+      case DevToolsExtensionEventType.vmServiceConnection:
+        final service = serviceConnection.serviceManager.service;
+        if (service == null) break;
+        vmServiceConnectionChanged(uri: service.connectedUri.toString());
+        break;
+      default:
+        onUnknownEvent?.call();
+    }
+  }
 }

packages/devtools_app_shared/lib/src/service/connected_app.dart

@@ -147,6 +147,8 @@ class ConnectedApp {
     // Return early if already initialized.
     if (initialized.isCompleted) return;
 
+    assert(serviceManager!.isServiceAvailable);
+
     await Future.wait([isFlutterApp, isProfileBuild, isDartWebApp]);
 
     _operatingSystem = serviceManager!.vm!.operatingSystem ?? unknownOS;

packages/devtools_extensions/lib/src/api/api.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'model.dart';
+
 /// Supported events that can be sent and received over 'postMessage' between
 /// DevTools and a DevTools extension running in an embedded iFrame.
 enum DevToolsExtensionEventType {
@@ -29,3 +31,27 @@ enum DevToolsExtensionEventType {
     return unknown;
   }
 }
+
+/// Interface that a DevTools extension host should implement.
+/// 
+/// This interface is implemented by DevTools itself as well as by a simulated
+/// DevTools environment for simplifying extension development.
+abstract interface class DevToolsExtensionHostInterface {
+  /// This method should send a [DevToolsExtensionEventType.ping] event to the
+  /// DevTools extension to check that it is ready.
+  void ping();
+
+  /// This method should send a [DevToolsExtensionEventType.vmServiceConnection]
+  /// event to the extension to notify it of the vm service uri it should
+  /// establish a connection to.
+  void vmServiceConnectionChanged({String? uri});
+
+  /// Handles events sent by the extension.
+  /// 
+  /// If an unknown event is recevied, this handler should call [onUnknownEvent]
+  /// if non-null.
+  void onEventReceived(
+    DevToolsExtensionEvent event, {
+    void Function()? onUnknownEvent,
+  });
+}

packages/devtools_extensions/lib/src/template/extension_manager.dart

@@ -56,13 +56,7 @@ class ExtensionManager {
             break;
           case DevToolsExtensionEventType.vmServiceConnection:
             final vmServiceUri = extensionEvent.data?['uri'] as String?;
-            unawaited(
-              connectToVmService(vmServiceUri).catchError((e) {
-                // TODO(kenz): post a notification to DevTools for errors
-                // or create an error panel for the extensions screens.
-                print('Error connecting to VM service: $e');
-              }),
-            );
+            unawaited(_connectToVmService(vmServiceUri));
             break;
           case DevToolsExtensionEventType.unknown:
           default:
@@ -80,25 +74,34 @@ class ExtensionManager {
     html.window.parent?.postMessage(event.toJson(), html.window.origin!);
   }
 
-  Future<void> connectToVmService(String? vmServiceUri) async {
+  Future<void> _connectToVmService(String? vmServiceUri) async {
     if (vmServiceUri == null) return;
 
-    final finishedCompleter = Completer<void>();
-    final vmService = await connect<VmService>(
-      uri: Uri.parse(vmServiceUri),
-      finishedCompleter: finishedCompleter,
-      createService: ({
-        // ignore: avoid-dynamic, code needs to match API from VmService.
-        required Stream<dynamic> /*String|List<int>*/ inStream,
-        required void Function(String message) writeMessage,
-        required Uri connectedUri,
-      }) {
-        return VmService(inStream, writeMessage);
-      },
-    );
-    await serviceManager.vmServiceOpened(
-      vmService,
-      onClosed: finishedCompleter.future,
-    );
+    try {
+      final finishedCompleter = Completer<void>();
+      final vmService = await connect<VmService>(
+        uri: Uri.parse(vmServiceUri),
+        finishedCompleter: finishedCompleter,
+        createService: ({
+          // ignore: avoid-dynamic, code needs to match API from VmService.
+          required Stream<dynamic> /*String|List<int>*/ inStream,
+          required void Function(String message) writeMessage,
+          required Uri connectedUri,
+        }) {
+          return VmService(
+            inStream,
+            writeMessage,
+          );
+        },
+      );
+      await serviceManager.vmServiceOpened(
+        vmService,
+        onClosed: finishedCompleter.future,
+      );
+    } catch (e) {
+      // TODO(kenz): post a notification to DevTools for errors
+      // or create an error panel for the extensions screens.
+      print('Unable to connect to VM service at $vmServiceUri: $e');
+    }
   }
 }

packages/devtools_shared/lib/src/service/service.dart

@@ -102,8 +102,8 @@ Future<T> connect<T extends VmService>({
   // Connects to a VM Service but does not verify the connection was fully
   // successful.
   Future<T> connectHelper() async {
-    T service;
-    service = uri.scheme == 'sse' || uri.scheme == 'sses'
+    final useSse = uri.scheme == 'sse' || uri.scheme == 'sses';
+    final T service = useSse
         ? await _connectWithSse<T>(
             uri: uri,
             onError: onError,