add (very rough) gizmo, restructure Dart package into library, add EntityListWidget

lib/filament/scene.dart

@@ -0,0 +1,118 @@
+import 'dart:async';
+import 'package:flutter_filament/filament/entities/gizmo.dart';
+import 'package:flutter_filament/filament/filament_controller.dart';
+
+///
+/// For now, this class just holds the entities that have been loaded (though not necessarily visible in the Filament Scene).
+///
+class SceneImpl extends Scene {
+  final Gizmo _gizmo;
+  Gizmo get gizmo => _gizmo;
+
+  SceneImpl(this._gizmo);
+
+  @override
+  FilamentEntity? selected;
+
+  final _onUpdatedController = StreamController<bool>.broadcast();
+  @override
+  Stream<bool> get onUpdated => _onUpdatedController.stream;
+
+  final _onLoadController = StreamController<FilamentEntity>.broadcast();
+  @override
+  Stream<FilamentEntity> get onLoad => _onLoadController.stream;
+
+  final _onUnloadController = StreamController<FilamentEntity>.broadcast();
+  @override
+  Stream<FilamentEntity> get onUnload => _onUnloadController.stream;
+
+  final _lights = <FilamentEntity>{};
+  final _entities = <FilamentEntity>{};
+
+  void registerLight(FilamentEntity entity) {
+    _lights.add(entity);
+    _onLoadController.sink.add(entity);
+    _onUpdatedController.add(true);
+  }
+
+  void unregisterLight(FilamentEntity entity) {
+    if (selected == entity) {
+      selected = null;
+      _gizmo.detach();
+    }
+    _lights.remove(entity);
+    _onUnloadController.add(entity);
+    _onUpdatedController.add(true);
+  }
+
+  void unregisterEntity(FilamentEntity entity) {
+    if (selected == entity) {
+      selected = null;
+      _gizmo.detach();
+    }
+    _entities.remove(entity);
+    _onUnloadController.add(entity);
+    _onUpdatedController.add(true);
+  }
+
+  void registerEntity(FilamentEntity entity) {
+    _entities.add(entity);
+    _entities.add(entity);
+    _onLoadController.sink.add(entity);
+    _onUpdatedController.add(true);
+  }
+
+  void clearLights() {
+    for (final light in _lights) {
+      if (selected == light) {
+        selected = null;
+        _gizmo.detach();
+      }
+      _onUnloadController.add(light);
+    }
+
+    _lights.clear();
+    _onUpdatedController.add(true);
+  }
+
+  void clearEntities() {
+    for (final entity in _entities) {
+      if (selected == entity) {
+        selected = null;
+        _gizmo.detach();
+      }
+      _onUnloadController.add(entity);
+    }
+    _entities.clear();
+    _onUpdatedController.add(true);
+  }
+
+  ///
+  /// Lists all entities currently loaded (not necessarily active in the scene).
+  ///
+  Iterable<FilamentEntity> listLights() {
+    return _lights;
+  }
+
+  @override
+  Iterable<FilamentEntity> listEntities() {
+    return _entities;
+  }
+
+  void registerSelected(FilamentEntity entity) {
+    selected = entity;
+    _onUpdatedController.add(true);
+  }
+
+  void unregisterSelected() {
+    selected = null;
+    _onUpdatedController.add(true);
+  }
+
+  @override
+  void select(FilamentEntity entity) {
+    selected = entity;
+    _gizmo.attach(entity);
+    _onUpdatedController.add(true);
+  }
+}