Flutter：将列表另存为收藏

看看我做的例子。以下是我添加的json，我只是将数据更改为您的理解

[
   {
       "city": "Canada",
       "attractions": [
           "Niagara Falls: An Elegant View",
           "Whistler: Your Perfect Ski Resort"
       ]
   },
   {
       "city": "Germany",
       "attractions": [
           "Berlin",
           "Munich"
       ]
   }
]

您提供的json中的模型类：

// To parse this JSON data, do
//
//     final cities = citiesFromJson(jsonString);

import 'dart:convert';

List<Cities> citiesFromJson(String str) => List<Cities>.from(json.decode(str).map((x) => Cities.fromJson(x)));

String citiesToJson(List<Cities> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class Cities {
    Cities({
        this.city,
        this.attractions,
    });

    String city;
    List<String> attractions;

    factory Cities.fromJson(Map<String, dynamic> json) => Cities(
        city: json["city"],
        attractions: List<String>.from(json["attractions"].map((x) => x)),
    );

    Map<String, dynamic> toJson() => {
        "city": city,
        "attractions": List<dynamic>.from(attractions.map((x) => x)),
    };
}

这是主要的用户界面和实现

import 'dart:convert';

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Index(),
    );
  }
}

class Index extends StatefulWidget {
  @override
  _IndexState createState() => _IndexState();
}

List data;
List<Cities> citylist = List();
List<Cities> citysavedlist = List();
int index;

class _IndexState extends State<Index> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: listView(),
    );
  }

  Future<String> fetchData() async {
    String data =
        await DefaultAssetBundle.of(context).loadString("json/parse.json");
    final jsonResult = json.decode(data);
    this.setState(() {
      jsonResult
          .forEach((element) => citylist.add(new Cities.fromJson(element)));
    });
    return "Success!";
  }

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

  listView() {
    return ListView.builder(
      itemCount: citylist == null ? 0 : citylist.length,
      itemBuilder: (context, index) {
        return Column(
          children: <Widget>[_buildRow(index, citylist)],
        );
      },
    );
  }

  Widget _buildRow(index, citylist) {
    final bool alreadySaved = citysavedlist.contains(citylist[index]);
    return Padding(
      padding: const EdgeInsets.only(top: 5.0, left: 5.0, right: 5.0),
      child: Card(
        child: ListTile(
            title: Text(citylist[index].city, style: TextStyle(fontSize: 22.0)),
            trailing: IconButton(
              icon: Icon(
                alreadySaved ? Icons.star : Icons.star_border,
                color: alreadySaved ? Colors.blue : Colors.blue,
              ),
              onPressed: () {
                setState(() {
                  if (alreadySaved) {
                    citysavedlist.remove(citylist[index]);
                  } else {
                    citysavedlist.add(citylist[index]);
                  }
                });
              },
            ), //subtitle: Text(subtitle),
            onTap: () {
              Navigator.push(
                  context,
                  MaterialPageRoute(
                      builder: (context) => Detail(citylist[index])));
            }),
      ),
    );
  }

  void _pushSaved() {
    Navigator.of(context).push(
      MaterialPageRoute<void>(
        builder: (BuildContext context) {
          final Iterable<ListTile> tiles = citysavedlist.map(
            (Cities pair) {
              return ListTile(
                  title: Text(
                    pair.city,
                  ),
                  onTap: () {
                    Navigator.push(context,
                        MaterialPageRoute(builder: (context) => Detail(pair)));
                  });
            },
          );

          final List<Widget> divided = ListTile.divideTiles(
            context: context,
            tiles: tiles,
          ).toList();
          return Scaffold(
            appBar: AppBar(
              title: const Text('Saved Suggestions'),
            ),
            body: ListView(children: divided),
          );
        },
      ),
    );
  }
}

class Detail extends StatefulWidget {
  final Cities cities;
  Detail(this.cities);
  @override
  _DetailState createState() => _DetailState();
}

class _DetailState extends State<Detail> {
  List<String> attractionsSavedList = List();

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

    getData();
  }

  /* 

  This is done using the shared Prefrences where the saving structure is String

  This is where key is the unique user id, this is because if you logout with another user then this will differentiate the user and get the string based on it. 

  I have saved  a Map<String,List<String>> as String encoded it and while using it decode the string which will give you the map.
   */

  getData() async {
    SharedPreferences sharedPreferences = await SharedPreferences.getInstance();
    var stringvalue = sharedPreferences.getString("Id");
    print(stringvalue);
    if (stringvalue != null) {
      Map<String, dynamic> newMap = json.decode(stringvalue);
      print('This is the city selected : ${newMap[widget.cities.city]}');

      var newlist = newMap[widget.cities.city];
      if (newlist != null) {
        newlist.forEach((element) {
          print(element);
          attractionsSavedList.add(element);
        });
      }
    } else {
      print('No values to show');
    }

    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.cities.city),
      ),
      body: Container(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            Row(
              mainAxisAlignment: MainAxisAlignment.end,
              children: <Widget>[
                Padding(
                  padding: const EdgeInsets.all(15.0),
                  child: GestureDetector(
                    onTap: () {
                      Navigator.push(
                          context,
                          MaterialPageRoute(
                              builder: (context) => SavedAttractions()));
                    },
                    child: Text(
                      'Saved Attractions',
                      style: TextStyle(
                        fontSize: 18,
                        decoration: TextDecoration.underline,
                      ),
                    ),
                  ),
                )
              ],
            ),
            Padding(
              padding: const EdgeInsets.all(8.0),
              child: Text(
                widget.cities.city,
                style: TextStyle(fontSize: 20),
              ),
            ),
            Expanded(
              child: ListView.builder(
                itemCount: widget.cities.attractions.length,
                itemBuilder: (BuildContext context, int index) {
                  final bool attractionsExists = attractionsSavedList
                      .contains(widget.cities.attractions[index]);
                  return Card(
                      child: Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Row(
                      mainAxisAlignment: MainAxisAlignment.spaceBetween,
                      children: <Widget>[
                        Text(widget.cities.attractions[index]),
                        IconButton(
                          onPressed: () async {
                            SharedPreferences _prefs =
                                await SharedPreferences.getInstance();

                            setState(() {
                              if (attractionsExists) {
                                attractionsSavedList
                                    .remove(widget.cities.attractions[index]);

                                String uniqueid = _prefs.getString("Id");

                                Map<String, dynamic> mapvalue =
                                    json.decode(uniqueid);

                                List list = mapvalue[widget.cities.city];

                                String stringtoRemove;

                                list.forEach((element) {
                                  if (element ==
                                      widget.cities.attractions[index]) {
                                    stringtoRemove =
                                        widget.cities.attractions[index];
                                  }
                                });

                                if (attractionsSavedList.length == 0) {
                                  list.remove(stringtoRemove);

                                  if (list.length == 0) {
                                    mapvalue.remove(stringtoRemove);
                                    _prefs.setString(
                                        "Id", json.encode(mapvalue));
                                  }
                                } else {
                                  list.remove(stringtoRemove);
                                  mapvalue.remove(stringtoRemove);
                                  
                                  mapvalue[widget.cities.city] = list;

                                  _prefs.setString("Id", json.encode(mapvalue));
                                }
                              } else {
                                print(
                                    'It does not exist in the list ${attractionsSavedList.length}');
                                attractionsSavedList
                                    .add(widget.cities.attractions[index]);

                                var uniqueId = _prefs.getString("Id");
                                if (uniqueId != null) {
                                  Map<String, dynamic> newMap =
                                      json.decode(uniqueId);
                                  if (attractionsSavedList.length > 0) {
                                    print(widget.cities.city);

                                    newMap[widget.cities.city] =
                                        attractionsSavedList;
                                    _prefs.setString("Id", json.encode(newMap));
                                  }
                                } else {
                                  if (attractionsSavedList.length > 0) {
                                    Map newMap = Map();
                                    newMap[widget.cities.city] =
                                        attractionsSavedList;
                                    _prefs.setString("Id", json.encode(newMap));
                                  }
                                }

                                /*  {
                                  widget.cities.city: attractionsSavedList
                                }; */

                                attractionsSavedList.forEach((element) {
                                  print(
                                      'This is the list after adding  $element');
                                });
                              }
                            });
                          },
                          icon: Icon(
                            attractionsExists ? Icons.star : Icons.star_border,
                            color:
                                attractionsExists ? Colors.blue : Colors.blue,
                          ),
                        )
                      ],
                    ),
                  ));
                },
              ),
            ),
          ],
        ),
      ),
    );
  }
}

class SavedAttractions extends StatefulWidget {
  @override
  _SavedAttractionsState createState() => _SavedAttractionsState();
}

class _SavedAttractionsState extends State<SavedAttractions> {
  List<String> attractionsList = List();
  bool _isLoading = false;

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

  getSavedAttractions() async {
    setState(() {
      _isLoading = true;
    });
    SharedPreferences preferences = await SharedPreferences.getInstance();

    String value = preferences.getString('Id');

    Map<String, dynamic> newMapVAlue = json.decode(value);

    newMapVAlue.forEach((key, value) {
      value.forEach((elements) {
        attractionsList.add(elements);
      });
    });

    setState(() {
      _isLoading = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: _isLoading
          ? Center(
              child: CircularProgressIndicator(),
            )
          : ListView.builder(
              itemCount: attractionsList.length,
              shrinkWrap: true,
              itemBuilder: (context, index) {
                return Card(
                  child: Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Text(attractionsList[index]),
                  ),
                );
              }),
    );
  }
}

List<Cities> citiesFromJson(String str) =>
    List<Cities>.from(json.decode(str).map((x) => Cities.fromJson(x)));
String citiesToJson(List<Cities> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class Cities {
  Cities({
    this.city,
    this.attractions,
  });

  String city;
  List<String> attractions;

  factory Cities.fromJson(Map<String, dynamic> json) => Cities(
        city: json["city"],
        attractions: List<String>.from(json["attractions"].map((x) => x)),
      );

  Map<String, dynamic> toJson() => {
        "city": city,
        "attractions": List<dynamic>.from(attractions.map((x) => x)),
      };
}