update quickstart to use new API

examples/flutter/quickstart/lib/main.dart

@@ -4,7 +4,6 @@ import 'package:thermion_flutter/thermion_flutter.dart';
 import 'package:vector_math/vector_math_64.dart' as v;
 import 'dart:math';
 
-
 void main() {
   runApp(const MyApp());
 }
@@ -35,40 +34,59 @@ class MyHomePage extends StatefulWidget {
 
 class _MyHomePageState extends State<MyHomePage> {
   bool _loaded = false;
-  late ThermionFlutterPlugin _thermionFlutterPlugin;
+  ThermionViewer? _thermionViewer;
-  late Future<ThermionViewer> _thermionViewer;
 
   @override
   void initState() {
     super.initState();
-    _thermionFlutterPlugin = ThermionFlutterPlugin();
+  }
-    _thermionViewer = _thermionFlutterPlugin.initialize();
+
+  ThermionFlutterTexture? _texture;
+
+  Future _load() async {
+    var viewer = await ThermionFlutterPlugin.createViewer();
+    _thermionViewer = viewer;
+    _thermionViewer!.loadSkybox("assets/default_env_skybox.ktx");
+    _thermionViewer!.loadGlb("assets/cube.glb");
+
+    _thermionViewer!.setCameraPosition(0, 1, 10);
+    _thermionViewer!.setCameraRotation(
+        v.Quaternion.axisAngle(v.Vector3(1, 0, 0), -30 / 180 * pi) *
+            v.Quaternion.axisAngle(v.Vector3(0, 1, 0), 15 / 180 * pi));
+    _thermionViewer!.addLight(LightType.SUN, 7500, 50000, 0, 0, 0, 1, -1, -1);
+    _thermionViewer!.setRendering(true);
+    _loaded = true;
+
+    setState(() {});
+  }
+
+  Future _unload() async {
+    await ThermionFlutterPlugin.destroyTexture(_texture!);
+    var viewer = _thermionViewer!;
+    _thermionViewer = null;
+    setState(() {});
+    await viewer.dispose();
+    _loaded = false;
+    setState(() {});
+  }
+
+  Widget _loadButton() {
+    return Center(
+        child: ElevatedButton(child: const Text("Load"), onPressed: _load));
+  }
+
+  Widget _unloadButton() {
+    return Center(
+        child: ElevatedButton(child: const Text("Unload"), onPressed: _unload));
   }
 
   @override
   Widget build(BuildContext context) {
     return Stack(children: [
-      Positioned.fill(child: ThermionWidget(plugin: _thermionFlutterPlugin)),
+      if (_thermionViewer != null)
-      if (!_loaded)
+        Positioned.fill(child: ThermionWidget(viewer: _thermionViewer!)),
-        Center(
+      if (!_loaded) _loadButton(),
-            child: ElevatedButton(
+      if (_loaded) _unloadButton(),
-                child: const Text("Load"),
-                onPressed: () async {
-                  var viewer = await _thermionViewer;
-                  await viewer.loadSkybox("assets/default_env_skybox.ktx");
-                  await viewer.loadGlb("assets/cube.glb");
-
-                  await viewer.setCameraPosition(0, 1, 10);
-                  await viewer.setCameraRotation(v.Quaternion.axisAngle(
-                          v.Vector3(1, 0, 0), -30 / 180 * pi) *
-                      v.Quaternion.axisAngle(
-                          v.Vector3(0, 1, 0), 15 / 180 * pi));
-                  await viewer.addLight(
-                      LightType.SUN, 7500, 50000, 0, 0, 0, 1, -1, -1);
-                  await viewer.setRendering(true);
-                  _loaded = true;
-                  setState(() {});
-                }))
     ]);
   }
 }