錯誤狀態：使用firstWhere函式時沒有元素

當我嘗試將從路由中獲得的引數與串列的值進行比較時，我遇到了上述錯誤。

MealDetailScreen 小部件

import 'package:flutter/material.dart';

import '../dummy_data.dart';

class MealDetailScreen extends StatelessWidget {
  static const detailScreenRouteName = '/meal-detail';

  @override
  Widget build(BuildContext context) {
    final mealId = ModalRoute.of(context)!.settings.arguments;
    final selectedMeal = DUMMY_MEALS.firstWhere((meal) => meal.id == mealId);
    return Scaffold(
      appBar: AppBar(title: Text('$mealId')),
      body: Container(
        child: Column(
          children: [
            Container(
              height: 300,
              width: double.infinity,
              child: Image.network(
                selectedMeal.imageUrl,
                fit: BoxFit.cover,
              ),
            )
          ],
        ),
      ),
    );
  }
}

如果我嘗試將可選引數 'orElse' 添加到 firstWhere 函式，我仍然會收到錯誤：這次是回傳型別 'Null' 不是 'Meal'，這是閉包背景關系所要求的。

這是我用來比較 id 的串列。

const DUMMY_MEALS = [
  Meal(
    isVegetarian: false,
    isLactoseFree: false,
    isVegan: false,
    id: 'm1',
    categories: [
      'c1',
      'c2',
    ],
    title: 'Spaghetti with Tomato Sauce',
    affordability: Affordability.Affordable,
    complexity: Complexity.Simple,
    imageUrl: '...',
    duration: 20,
    ingredients: ['4 Tomatoes', '...'],
    steps: ['Cut the tomatoes and the onion into small pieces.', '...'],
    isGlutenFree: false,
  ),
];

這就是我將 id 作為引數傳遞的方式

void selectMeal(BuildContext context) {
    Navigator.of(context)
        .pushNamed(MealDetailScreen.detailScreenRouteName, arguments: {id});
  }

餐型

import 'package:flutter/foundation.dart';

enum Complexity { Simple, Challenging, Hard }

enum Affordability { Affordable, Pricey, Luxurious }

class Meal {
  final String id;
  final List<String> categories;
  final String title;
  final String imageUrl;
  final List<String> ingredients;
  final List<String> steps;
  final int duration;
  final Complexity complexity;
  final Affordability affordability;
  final bool isGlutenFree;
  final bool isLactoseFree;
  final bool isVegan;
  final bool isVegetarian;

  const Meal(
      {required this.id,
      required this.categories,
      required this.title,
      required this.imageUrl,
      required this.ingredients,
      required this.steps,
      required this.duration,
      required this.complexity,
      required this.affordability,
      required this.isGlutenFree,
      required this.isLactoseFree,
      required this.isVegan,
      required this.isVegetarian});
}

uj5u.com熱心網友回復：

路過時arguments: {id}你正在路過_HashSet<String>。但是對于單值id字串就足夠了，否則使用地圖。

在這種情況下，傳遞引數將是

  Navigator.of(context)
        .pushNamed(MealDetailScreen.detailScreenRouteName, arguments: id);

在迭代DUMMY_MEALS串列時，我們可能會得到一個id未包含在DUMMY_MEALS串列中的串列。在這種情況下，您可以在orElse狀態上創建并傳遞 emptyMeal或僅用于try catch處理例外。

    Meal? selectedMeal;

    try {
      final selectedMeal = DUMMY_MEALS.firstWhere((meal) => meal.id == mealId);
    } catch (e) {
      print(e.toString());
    }

雖然selectedMeal可以為空，但我們可以檢查它是否包含膳食。

 return Scaffold(
      appBar: AppBar(title: Text('$mealId')),
      body: selectedMeal == null
          ? Text("Cound not find data")
          : Container(
              child: Column(
                children: [
                  Text(selectedMeal.title),
                ],
              ),
            ),
    );

uj5u.com熱心網友回復：

答案已經給出。但我想提一下，如果您使用 pushNamed 方法，建議您使用 onGenerateRoute 管理傳遞引數。所以你不需要空檢查引數或背景關系。

ModalRoute.of(context)?.settings?.arguments? 

MaterialApp(
  onGenerateRoute: (settings) {
    if (settings.name == PassArgumentsScreen.routeName) {
      final args = settings.arguments as ScreenArguments;
      return MaterialPageRoute(
        builder: (context) {
          return PassArgumentsScreen(
            title: args.title,
            message: args.message,
          );
        },
      );
    }
    assert(false, 'Need to implement ${settings.name}');
    return null;
  },
)

參考：將引數傳遞給命名路由

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