Adds ai_recipe_generation sample (#2242)

Adding the demo app from my I/O talk. Because AI.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

ai_recipe_generation/lib/features/prompt/prompt_view_model.dart

@@ -0,0 +1,168 @@
+import 'package:ai_recipe_generation/services/gemini.dart';
+import 'package:flutter/cupertino.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:google_generative_ai/google_generative_ai.dart';
+import 'package:image_picker/image_picker.dart';
+
+import '../../services/firestore.dart';
+import '../../util/filter_chip_enums.dart';
+import '../recipes/recipe_model.dart';
+import 'prompt_model.dart';
+
+class PromptViewModel extends ChangeNotifier {
+  PromptViewModel({
+    required this.multiModalModel,
+    required this.textModel,
+  });
+
+  final GenerativeModel multiModalModel;
+  final GenerativeModel textModel;
+  bool loadingNewRecipe = false;
+
+  PromptData userPrompt = PromptData.empty();
+  TextEditingController promptTextController = TextEditingController();
+
+  String badImageFailure =
+      "The recipe request either does not contain images, or does not contain images of food items. I cannot recommend a recipe.";
+
+  Recipe? recipe;
+  String? _geminiFailureResponse;
+  String? get geminiFailureResponse => _geminiFailureResponse;
+  set geminiFailureResponse(String? value) {
+    _geminiFailureResponse = value;
+    notifyListeners();
+  }
+
+  void notify() => notifyListeners();
+
+  void addImage(XFile image) {
+    userPrompt.images.insert(0, image);
+    notifyListeners();
+  }
+
+  void addAdditionalPromptContext(String text) {
+    final existingInputs = userPrompt.additionalTextInputs;
+    userPrompt.copyWith(additionalTextInputs: [...existingInputs, text]);
+  }
+
+  void removeImage(XFile image) {
+    userPrompt.images.removeWhere((el) => el.path == image.path);
+    notifyListeners();
+  }
+
+  void resetPrompt() {
+    userPrompt = PromptData.empty();
+    notifyListeners();
+  }
+
+  // Creates an ephemeral prompt with additional text that the user shouldn't be
+  // concerned with to send to Gemini, such as formatting.
+  PromptData buildPrompt() {
+    return PromptData(
+      images: userPrompt.images,
+      textInput: mainPrompt,
+      basicIngredients: userPrompt.selectedBasicIngredients,
+      cuisines: userPrompt.selectedCuisines,
+      dietaryRestrictions: userPrompt.selectedDietaryRestrictions,
+      additionalTextInputs: [format],
+    );
+  }
+
+  Future<void> submitPrompt() async {
+    loadingNewRecipe = true;
+    notifyListeners();
+    // Create an ephemeral PromptData, preserving the user prompt data without
+    // adding the additional context to it.
+    var model = userPrompt.images.isEmpty ? textModel : multiModalModel;
+    final prompt = buildPrompt();
+
+    try {
+      final content = await GeminiService.generateContent(model, prompt);
+
+      // handle no image or image of not-food
+      if (content.text != null && content.text!.contains(badImageFailure)) {
+        geminiFailureResponse = badImageFailure;
+      } else {
+        recipe = Recipe.fromGeneratedContent(content);
+      }
+    } catch (error) {
+      geminiFailureResponse = 'Failed to reach Gemini. \n\n$error';
+      if (kDebugMode) {
+        print(error);
+      }
+      loadingNewRecipe = false;
+    }
+
+    loadingNewRecipe = false;
+    resetPrompt();
+    notifyListeners();
+  }
+
+  void saveRecipe() {
+    FirestoreService.saveRecipe(recipe!);
+  }
+
+  void addBasicIngredients(Set<BasicIngredientsFilter> ingredients) {
+    userPrompt.selectedBasicIngredients.addAll(ingredients);
+    notifyListeners();
+  }
+
+  void addCategoryFilters(Set<CuisineFilter> categories) {
+    userPrompt.selectedCuisines.addAll(categories);
+    notifyListeners();
+  }
+
+  void addDietaryRestrictionFilter(
+      Set<DietaryRestrictionsFilter> restrictions) {
+    userPrompt.selectedDietaryRestrictions.addAll(restrictions);
+    notifyListeners();
+  }
+
+  String get mainPrompt {
+    return '''
+You are a Cat who's a chef that travels around the world a lot, and your travels inspire recipes.
+
+Recommend a recipe for me based on the provided image.
+The recipe should only contain real, edible ingredients.
+If there are no images attached, or if the image does not contain food items, respond exactly with: $badImageFailure
+
+Adhere to food safety and handling best practices like ensuring that poultry is fully cooked.
+I'm in the mood for the following types of cuisine: ${userPrompt.cuisines},
+I have the following dietary restrictions: ${userPrompt.dietaryRestrictions}
+Optionally also include the following ingredients: ${userPrompt.ingredients}
+Do not repeat any ingredients.
+
+After providing the recipe, add an descriptions that creatively explains why the recipe is good based on only the ingredients used in the recipe.  Tell a short story of a travel experience that inspired the recipe.
+List out any ingredients that are potential allergens.
+Provide a summary of how many people the recipe will serve and the the nutritional information per serving.
+
+${promptTextController.text.isNotEmpty ? promptTextController.text : ''}
+''';
+  }
+
+  final String format = '''
+Return the recipe as valid JSON using the following structure:
+{
+  "id": \$uniqueId,
+  "title": \$recipeTitle,
+  "ingredients": \$ingredients,
+  "description": \$description,
+  "instructions": \$instructions,
+  "cuisine": \$cuisineType,
+  "allergens": \$allergens,
+  "servings": \$servings,
+  "nutritionInformation": {
+    "calories": "\$calories",
+    "fat": "\$fat",
+    "carbohydrates": "\$carbohydrates",
+    "protein": "\$protein",
+  },
+}
+  
+uniqueId should be unique and of type String. 
+title, description, cuisine, allergens, and servings should be of String type. 
+ingredients and instructions should be of type List<String>.
+nutritionInformation should be of type Map<String, String>.
+''';
+}