IDG: Add first elements of discoverability & extensibility API

packages/devtools_app/lib/src/extensibility/discoverable.dart

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+import 'dart:async';
+
+import '../primitives/simple_items.dart';
+import '../screens/memory/memory_controller_discoverable.dart';
+import '../screens/performance/performance_controller_discoverable.dart';
+import '../shared/globals.dart';
+import 'services_proxy.dart';
+
+export '../screens/memory/memory_controller_discoverable.dart';
+export '../screens/performance/performance_controller_discoverable.dart';
+
+class StructuredLogEvent {
+  StructuredLogEvent(this.type, {this.data});
+  final String type;
+  final Object? data;
+}
+
+class VMEvent extends StructuredLogEvent {
+  VMEvent(type, {data}) : super(type, data: data);
+}
+
+class DevToolsUserEvent extends StructuredLogEvent {
+  DevToolsUserEvent(type, {data}) : super(type, data: data);
+}
+
+/// An event manager class. Clients can listen for classes of events, optionally
+/// filtered by a string type. This can be used to decouple events sources and
+/// event listeners.
+class EventsManager {
+  EventsManager() {
+    _controller = StreamController.broadcast();
+    setGlobal(EventsManager, this);
+  }
+
+  late StreamController<StructuredLogEvent> _controller;
+
+  /// Listen for events. Clients can pass in an optional [type]
+  /// which filters the events to only those specific ones.
+  /// To stop listening to events, keep a reference to the resulting
+  /// [StreamSubscription] and cancel it.
+  Stream<StructuredLogEvent> onEvent({String? type}) {
+    if (type == null) {
+      return _controller.stream;
+    } else {
+      return _controller.stream.where(
+        (StructuredLogEvent event) =>
+            event.type == type || event.type.startsWith(type),
+      );
+    }
+  }
+
+  /// Add an event to the event bus.
+  void addEvent(StructuredLogEvent event) {
+    _controller.add(event);
+  }
+
+  /// Close (destroy) this [StructuredLogEventsManager]. This is generally not used
+  /// outside of a testing context. All stream listeners will be closed and the
+  /// bus will not fire any more events.
+  void close() {
+    unawaited(_controller.close());
+  }
+}
+
+class DiscoverableDevToolsApp {
+  DiscoverableDevToolsApp() {
+    vmServicesProxy = VMServicesProxy();
+    setGlobal(DiscoverableDevToolsApp, this);
+    frameworkController.onPageChange.listen((event) {
+      _selectedPageId = event.id;
+    });
+  }
+
+  late VMServicesProxy vmServicesProxy;
+
+  /// Get the available screen ids from [ScreenIds] class.
+  void selectPage(String pageId) {
+    frameworkController.notifyShowPageId(pageId);
+  }
+
+  String? _selectedPageId;
+  String? get selectedPageId => _selectedPageId;
+
+  DiscoverableMemoryPage? memoryPage;
+  DiscoverablePerformancePage? performancePage;
+  // TODO: add of the other pages here
+
+  static const pageChangedEventKeyPrefix = 'page-changed.';
+  static final memoryPageChangedEventKey =
+      '$pageChangedEventKeyPrefix${DiscoverableMemoryPage.id}';
+  static final performancePageChangedEventKey =
+      '$pageChangedEventKeyPrefix${DiscoverablePerformancePage.id}';
+}
+
+abstract class DiscoverablePage {}