如何修复迭代期间的并发修改：“_GrowableList”错误的实例（长度：41）Flutter答案

【问题标题】：How to fix Concurrent modification during iteration: Instance(length:41) of '_GrowableList' error Flutter如何修复迭代期间的并发修改：“_GrowableList”错误的实例（长度：41）Flutter
【发布时间】：2022-01-02 00:43:38
【问题描述】：

我不知道我是否正确地迭代了每个项目。屏幕上什么都没有呈现，我有以下错误说 '迭代期间的并发修改：'_GrowableList'的实例（长度：41）我不确定如何修复此错误并在屏幕上呈现每个项目。有什么想法吗？

import 'package:forecast/models/weather_data.dart';

class ForecastData {
  final List list;

  ForecastData({required this.list});

  factory ForecastData.fromJson(Map<String, dynamic> json) {
    var list = json['list']?.map((e) => e)?.toList(growable: true) ?? [];
    list.forEach((e) {
      WeatherData w = WeatherData(
          placeName: json['city']['name'],
          temperature: e['main']['temp'],
          feels_like: e['main']['temp'],
          pressure: e['main']['temp'],
          humidity: e['main']['temp'],
          wind_speed: e['main']['temp']);

      list.add(w);
      print(list);
    });

    return ForecastData(
      list: list,
    );
  }
}

输出

Restarted application in 4,038ms.
I/flutter (16409): [{dt: 1641081600, main: {temp: 12.94, feels_like: 12.64, temp_min: 12.8, temp_max: 12.94, pressure: 1014, sea_level: 1014, grnd_level: 1010, humidity: 90, temp_kf: 0.14}, weather: [{id: 803, main: Clouds, description: broken clouds, icon: 04n}], clouds: {all: 75}, wind: {speed: 5.32, deg: 211, gust: 12.51}, visibility: 10000, pop: 0.23, sys: {pod: n}, dt_txt: 2022-01-02 00:00:00}, {dt: 1641092400, main: {temp: 12.86, feels_like: 12.53, temp_min: 12.69, temp_max: 12.86, pressure: 1013, sea_level: 1013, grnd_level: 1008, humidity: 89, temp_kf: 0.17}, weather: [{id: 500, main: Rain, description: light rain, icon: 10n}], clouds: {all: 83}, wind: {speed: 4.85, deg: 224, gust: 12.6}, visibility: 10000, pop: 0.62, rain: {3h: 0.94}, sys: {pod: n}, dt_txt: 2022-01-02 03:00:00}, {dt: 1641103200, main: {temp: 12.71, feels_like: 12.18, temp_min: 12.59, temp_max: 12.71, pressure: 1011, sea_level: 1011, grnd_level: 1007, humidity: 82, temp_kf: 0.12}, weather: [{id: 500, main: Rain, description: light rain, icon: 10n}], cl
E/flutter (16409): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Concurrent modification during iteration: Instance(length:41) of '_GrowableList'.
E/flutter (16409): #0      List.forEach (dart:core-patch/growable_array.dart:411:36)
E/flutter (16409): #1      new ForecastData.fromJson (package:forecast/models/forecast_data.dart:10:10)
E/flutter (16409): #2      _MyAppState.loadForecast.<anonymous closure> (package:forecast/main.dart:121:37)
E/flutter (16409): #3      State.setState (package:flutter/src/widgets/framework.dart:1088:30)
E/flutter (16409): #4      _MyAppState.loadForecast (package:forecast/main.dart:120:14)
E/flutter (16409): <asynchronous suspension>

【问题讨论】：

标签： json flutter

【解决方案1】：

错误Concurrent modification during iteration 意味着当您使用forEach 迭代list 时，您也在同时通过添加新元素来修改它，这是不正确的。

您可以在 fromJson 和 forEach 块中拥有另一个 List<WeatherData> 类型的局部变量，只需将新元素添加到此局部变量并在返回中使用它而不是 list。

【讨论】：