feat(example): per-step model selection for the Voice Loop (STT / LLM / TTS)

packages/flutter_gemma/example/lib/home_screen.dart

@@ -8,7 +8,7 @@ import 'package:flutter_gemma_example/stt_models_screen.dart';
 import 'package:flutter_gemma_example/translate_models_screen.dart';
 import 'package:flutter_gemma_example/tts_models_screen.dart';
 import 'package:flutter_gemma_example/utils/installed_model_lookup.dart';
-import 'package:flutter_gemma_example/voice_screen.dart';
+import 'package:flutter_gemma_example/voice_setup_screen.dart';
 
 class HomeScreen extends StatefulWidget {
   const HomeScreen({super.key});
@@ -148,7 +148,7 @@ class _HomeScreenState extends State<HomeScreen> {
               subtitle: 'Speak → on-device STT → LLM → TTS → hear the reply',
               icon: Icons.record_voice_over,
               color: Colors.teal,
-              onTap: () => _push(const VoiceScreen()),
+              onTap: () => _push(const VoiceSetupScreen()),
             ),
             const SizedBox(height: 16),
             _NavigationCard(

packages/flutter_gemma/example/lib/model_selection_screen.dart

@@ -22,7 +22,21 @@ enum SortType {
 }
 
 class ModelSelectionScreen extends StatefulWidget {
-  const ModelSelectionScreen({super.key});
+  /// When set, the screen is in "pick a model" mode: tapping an entry invokes
+  /// this and pops (returning the choice to the caller, e.g.
+  /// `VoiceSetupScreen`) instead of navigating into the download/chat flow.
+  final ValueChanged<Model>? onSelected;
+
+  /// Optional eligibility filter — when set, only models satisfying it are
+  /// listed. A pick-mode caller whose install path can't handle every catalog
+  /// entry uses this to offer only installable models. The Voice Loop LLM step
+  /// installs via `installModel(...).fromNetwork(url)` in `VoiceScreen`, which
+  /// can handle neither OS built-in models (no file) nor `localModel` asset
+  /// entries (their `url` is an `assets/...` path), so it passes
+  /// `(m) => !m.isBuiltIn && !m.localModel`.
+  final bool Function(Model model)? modelFilter;
+
+  const ModelSelectionScreen({super.key, this.onSelected, this.modelFilter});
 
   @override
   State<ModelSelectionScreen> createState() => _ModelSelectionScreenState();
@@ -95,6 +109,14 @@ class _ModelSelectionScreenState extends State<ModelSelectionScreen> {
     // Show all models on all platforms
     var models = Model.values.toList();
 
+    // A pick-mode caller restricts the list to models its install path can
+    // actually handle (e.g. the Voice Loop LLM step excludes built-in + local
+    // asset models it cannot network-install).
+    final modelFilter = widget.modelFilter;
+    if (modelFilter != null) {
+      models = models.where(modelFilter).toList();
+    }
+
     // Platform-filter the OS built-in models: Gemini Nano is Android-only (ML
     // Kit GenAI / AICore), Apple Foundation Models are iOS/macOS-only. Both
     // carry localModel: true (so they escape the web/network filters and stay
@@ -335,7 +357,7 @@ class _ModelSelectionScreenState extends State<ModelSelectionScreen> {
                 itemCount: models.length,
                 itemBuilder: (context, index) {
                   final model = models[index];
-                  return ModelCard(model: model);
+                  return ModelCard(model: model, onSelected: widget.onSelected);
                 },
               ),
             ),
@@ -348,8 +370,9 @@ class _ModelSelectionScreenState extends State<ModelSelectionScreen> {
 
 class ModelCard extends StatefulWidget {
   final Model model;
+  final ValueChanged<Model>? onSelected;
 
-  const ModelCard({super.key, required this.model});
+  const ModelCard({super.key, required this.model, this.onSelected});
 
   @override
   State<ModelCard> createState() => _ModelCardState();
@@ -502,6 +525,14 @@ class _ModelCardState extends State<ModelCard> {
             ),
             trailing: Icon(Icons.arrow_forward_ios, color: Colors.grey[400]),
             onTap: () {
+              // Selection mode (e.g. picking the LLM step in VoiceSetupScreen):
+              // return the model to the caller instead of navigating into the
+              // download/chat flow.
+              if (widget.onSelected != null) {
+                widget.onSelected!(widget.model);
+                Navigator.pop(context);
+                return;
+              }
               // Built-in OS models: route through the download screen so its
               // builtIn short-circuit runs (instant bundled install +
               // BuiltInAi.ensureReady), surfacing availability errors before

packages/flutter_gemma/example/lib/stt_models_screen.dart

@@ -4,10 +4,17 @@ import 'package:flutter_gemma_example/stt_screen.dart';
 
 /// STT model selection screen — mirrors [EmbeddingModelsScreen]. Lists the
 /// [SttModel] catalog; picking a supported entry pushes [SttScreen], which
-/// installs it (idempotent) and sets it active. Unsupported entries (need a
+/// installs it (idempotent) and sets it active — unless [onSelected] is set
+/// (pick-a-model mode), which returns the choice and pops instead. Unsupported
+/// entries (need a
 /// log-mel frontend, see [SttModel.unsupportedReason]) are shown but disabled.
 class SttModelsScreen extends StatelessWidget {
-  const SttModelsScreen({super.key});
+  /// When set, the screen is in "pick a model" mode: tapping a supported entry
+  /// invokes this and pops (returning the choice to the caller, e.g.
+  /// `VoiceSetupScreen`) instead of navigating into [SttScreen].
+  final ValueChanged<SttModel>? onSelected;
+
+  const SttModelsScreen({super.key, this.onSelected});
 
   @override
   Widget build(BuildContext context) {
@@ -41,7 +48,7 @@ class SttModelsScreen extends StatelessWidget {
                 itemCount: SttModel.values.length,
                 itemBuilder: (context, index) {
                   final model = SttModel.values[index];
-                  return _SttModelCard(model: model);
+                  return _SttModelCard(model: model, onSelected: onSelected);
                 },
               ),
             ),
@@ -54,8 +61,9 @@ class SttModelsScreen extends StatelessWidget {
 
 class _SttModelCard extends StatelessWidget {
   final SttModel model;
+  final ValueChanged<SttModel>? onSelected;
 
-  const _SttModelCard({required this.model});
+  const _SttModelCard({required this.model, this.onSelected});
 
   @override
   Widget build(BuildContext context) {
@@ -114,12 +122,20 @@ class _SttModelCard extends StatelessWidget {
             ? Icon(Icons.arrow_forward_ios, color: Colors.grey[400])
             : const Icon(Icons.lock_outline, color: Colors.white24),
         onTap: model.isSupported
-            ? () => Navigator.push(
-                context,
-                MaterialPageRoute<void>(
-                  builder: (context) => SttScreen(model: model),
-                ),
-              )
+            ? () {
+                final cb = onSelected;
+                if (cb != null) {
+                  cb(model);
+                  Navigator.pop(context);
+                } else {
+                  Navigator.push(
+                    context,
+                    MaterialPageRoute<void>(
+                      builder: (context) => SttScreen(model: model),
+                    ),
+                  );
+                }
+              }
             : null,
       ),
     );

packages/flutter_gemma/example/lib/tts_models_screen.dart

@@ -4,12 +4,19 @@ import 'package:flutter_gemma_example/tts_screen.dart';
 
 /// TTS model selection screen — mirrors [SttModelsScreen]. Lists the
 /// [TtsModel] catalog; picking a supported entry pushes [TtsScreen], which
-/// installs it (idempotent) and sets it active. Unsupported entries (need
+/// installs it (idempotent) and sets it active — unless [onSelected] is set
+/// (pick-a-model mode), which returns the choice and pops instead. Unsupported
+/// entries (need
 /// their own `TtsModelProfile`, see [TtsModel.unsupportedReason]) are shown
 /// but disabled. Model selection lives here — the standard list screen —
 /// not in an in-screen dropdown, so TTS matches STT / Inference / Translate.
 class TtsModelsScreen extends StatelessWidget {
-  const TtsModelsScreen({super.key});
+  /// When set, the screen is in "pick a model" mode: tapping a supported entry
+  /// invokes this and pops (returning the choice to the caller, e.g.
+  /// `VoiceSetupScreen`) instead of navigating into [TtsScreen].
+  final ValueChanged<TtsModel>? onSelected;
+
+  const TtsModelsScreen({super.key, this.onSelected});
 
   @override
   Widget build(BuildContext context) {
@@ -43,7 +50,7 @@ class TtsModelsScreen extends StatelessWidget {
                 itemCount: TtsModel.values.length,
                 itemBuilder: (context, index) {
                   final model = TtsModel.values[index];
-                  return _TtsModelCard(model: model);
+                  return _TtsModelCard(model: model, onSelected: onSelected);
                 },
               ),
             ),
@@ -56,8 +63,9 @@ class TtsModelsScreen extends StatelessWidget {
 
 class _TtsModelCard extends StatelessWidget {
   final TtsModel model;
+  final ValueChanged<TtsModel>? onSelected;
 
-  const _TtsModelCard({required this.model});
+  const _TtsModelCard({required this.model, this.onSelected});
 
   @override
   Widget build(BuildContext context) {
@@ -134,12 +142,20 @@ class _TtsModelCard extends StatelessWidget {
             ? Icon(Icons.arrow_forward_ios, color: Colors.grey[400])
             : const Icon(Icons.lock_outline, color: Colors.white24),
         onTap: model.isSupported
-            ? () => Navigator.push(
-                context,
-                MaterialPageRoute<void>(
-                  builder: (context) => TtsScreen(model: model),
-                ),
-              )
+            ? () {
+                final cb = onSelected;
+                if (cb != null) {
+                  cb(model);
+                  Navigator.pop(context);
+                } else {
+                  Navigator.push(
+                    context,
+                    MaterialPageRoute<void>(
+                      builder: (context) => TtsScreen(model: model),
+                    ),
+                  );
+                }
+              }
             : null,
       ),
     );