In the tutorial, String categoryTitle; and List displayedMeals; were working fine. In the current version of flutter it won't allow me to just declare as it is. What should I initialize them to? It would be nice to know what is the equivalent in declaring String and List now since thre is a null safety. Also, when I put String? and ListMeal? as declaration and put [!], i get an error of null check operator used on a null value. I can't put late also because I don't what should I initialize these declarations.

import 'package:flutter/material.dart';

import '../widgets/meal_item.dart';
import '../dummy_data.dart';
import '../models/meal.dart';

class CategoryMealsScreen extends StatefulWidget {
  static const routeName = '/category-meals';

  @override
  State<CategoryMealsScreen> createState() => _CategoryMealsScreenState();
}

class _CategoryMealsScreenState extends State<CategoryMealsScreen> {
  // final String categoryId;
  String categoryTitle;
  List<Meal> displayedMeals;
  var _loadedInitData = false;

  @override
  void initState() {
    super.initState();
  }

  @override
  void didChangeDependecies() {
    if (!_loadedInitData) {
      final routeArgs =
          ModalRoute.of(context)!.settings.arguments as Map<String, String>;
      categoryTitle = routeArgs['title']!;
      final categoryId = routeArgs['id'];
      displayedMeals = DUMMY_MEALS.where((meal) {
        return meal.categories.contains(categoryId);
      }).toList();
    }
    _loadedInitData = true;
    super.didChangeDependencies();
  }

  void _removeMeal(String mealId) {
    setState(() {
      displayedMeals!.removeWhere((meal) => meal.id == mealId);
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(categoryTitle!),
      ),
      body: ListView.builder(
        itemBuilder: (ctx, index) {
          return MealItem(
            id: displayedMeals[index].id,
            title: displayedMeals[index].title,
            imageUrl: displayedMeals[index].imageUrl,
            duration: displayedMeals[index].duration,
            affordability: displayedMeals[index].affordability,
            complexity: displayedMeals[index].complexity,
            removedItem: _removeMeal,
          );
        },
        itemCount: displayedMeals.length,
      ),
    );
  }
}

CodePudding user response：

It happens because of null safety. Either you declare your variables as nullable, so they can be null or you use the keyword late in front of the variable like this:

late String categoryTitle;
late List<Meal> displayedMeals;

But then you should also declare the variables in the initState or before you use them, otherwise an exception will be thrown.

CodePudding user response：

Write like that : String? categoryTitle; its nullsafety