如何在Flutter中返回带有地图列表的嵌套结构的值

分类：编程问答

如何解决如何在Flutter中返回带有地图列表的嵌套结构的值

仍然尝试学习地图和列表。

我的目标是从嵌套的地图列表中返回一个列表。函数getMovements（）可以正常工作。它呈现运动列表。然后，我尝试创建另一个函数getVariationsByMovement（），该函数传入运动字符串名称，以尝试返回variantNames列表。在我的用户界面中，我列出了动作列表。用户选择一个动作，下一页将显示一个变化列表。

我陷入了getVariationsByMovement下面的函数中。

class MovementDataSource extends ChangeNotifier{

  List<Map> getAll() => _movement;

  List<String> getMovements()=> _movement
      .map((map) => MovementModel.fromJson(map))
      .map((item) => item.movement)
      .toList();

// I'd  like this to return a list of movement.variation.variationName

  getVariationsByMovement(String movement) => _movement
      .map((map) => MovementModel.fromJson(map))
      .where((item) => item.movement == movement)
      .map((item) => item.**I AM STUCK HERE**)
      .expand((i) => i)
      .toList();


  List _movement = [
    {
      "movement": "Plank","alias": "plank","variation": [
        {"variationName": "High Plank"},{"variationName": "Forearm Plank"},]
    },{
      "movement": "Side Plank","alias": "side plank","variation": [
        {"variationName": "Side Plank Right"},{"variationName": "Side Plank Left"},],},];
}

我的模特

class MovementModel {
  String movement;
  String alias;
  List<VariationModel> variation;

  MovementModel({this.movement,this.alias,this.variation});

  // from firestore
  MovementModel.fromJson(Map<String,dynamic> json) {
    var list = json['variation'] as List;
    List<VariationModel> variationList = list.map((i) => VariationModel.fromJson(i)).toList();
    movement = json['movement'];
    alias = json['alias'];
    variation = variationList;
  }

  // to firestore
  Map<String,dynamic> toJson() {
  final Map<String,dynamic> data = new Map<String,dynamic>();
  data['movement'] = this.movement;
  data['alias'] = this.alias;
  //data['variation'] = VariationModel;
  return data;
  }
}

class VariationModel {
  String variationName;
  String description;

  VariationModel({this.variationName,this.description});

  // from firestore
  VariationModel.fromJson(Map<String,dynamic> json) {
    variationName = json['variationName'];
    description = json['description'];
  }

  // to firestore
  Map<String,dynamic> toJson() {

    return {
      'variationName': variationName,'description': description,};
  }
}

更新 在这种情况下，我试图将结果返回到列表构建器。

  @override
  Widget build(BuildContext context) {
    final _movementDataLocator = locator<MovementDataSource>();
    return ListView.builder(
        scrollDirection: Axis.vertical,shrinkWrap: true,itemCount: _movementDataLocator.getVariationsByMovement(movement).length,itemBuilder: (buildContext,index) {
          return Card(
            child: ListTile (
              title: Text(
                _movementDataLocator.getVariationsByMovement(movement)[index].toString(),style: TextStyle(fontSize: 20),),trailing: Icon(Icons.keyboard_arrow_right),onTap: () async {
                Navigator.push(context,MaterialPageRoute(
                    builder: (_) => SelectDefaultItemPage(variation: _movementDataLocator.getVariationsByMovement(movement)[index])));
              },);
        }
    );
  }
}

解决方法

您可以在下面复制粘贴运行完整代码
您可以使用class MatchingPage extends StatelessWidget { @override Widget build(BuildContext context) { final ad_id = 'my admob ad id'; return Scaffold( appBar: CommonAppBar(text: '매칭'),body: Center( child: Column( children: <Widget>[ Container( height: 60,child: NativeAdmob( adUnitID: ad_id ),),Expanded( child: Container(),CircularProgressIndicator(),SizedBox( height: 30,Text( '예상 대기시간 60초',style: TextStyle(color: Colors.white,fontSize: 16),Text( '???초 경과',CommonButton( text: 'STOP!',onPressed: () {},) ],crossAxisAlignment: CrossAxisAlignment.center,mainAxisAlignment: MainAxisAlignment.center,); } }
代码段

variation: List<VariationModel>.from(json["variation"].map((x) => VariationModel.fromJson(x))

输出

factory MovementModel.fromJson(Map<String,dynamic> json) => MovementModel(
        movement: json["movement"],alias: json["alias"],variation: List<VariationModel>.from(
            json["variation"].map((x) => VariationModel.fromJson(x))),);
      
...
List<VariationModel> vList =
        MovementDataSource().getVariationsByMovement("Plank");

for (int i = 0; i < vList.length; i++) {
      print(vList[i].variationName);
    }

完整代码

I/flutter (21879): High Plank
I/flutter (21879): Forearm Plank

工作演示

enter image description here

完整代码2 

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

List<MovementModel> movementModelFromJson(String str) =>
    List<MovementModel>.from(
        json.decode(str).map((x) => MovementModel.fromJson(x)));
String movementModelToJson(List<MovementModel> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class MovementDataSource extends ChangeNotifier {
  List<Map> getAll() => _movement;

  List<String> getMovements() => _movement
      .map((map) => MovementModel.fromJson(map))
      .map((item) => item.movement)
      .toList();

// I'd  like this to return a list of movement.variation.variationName

  getVariationsByMovement(String movement) => _movement
      .map((map) => MovementModel.fromJson(map))
      .where((item) => item.movement == movement)
      .map((item) => item.variation)
      .expand((i) => i)
      .toList();

  List _movement = [
    {
      "movement": "Plank","alias": "plank","variation": [
        {"variationName": "High Plank"},{"variationName": "Forearm Plank"},]
    },{
      "movement": "Side Plank","alias": "side plank","variation": [
        {"variationName": "Side Plank Right"},{"variationName": "Side Plank Left"},],},];
}

class MovementModel {
  MovementModel({
    this.movement,this.alias,this.variation,});

  String movement;
  String alias;
  List<VariationModel> variation;

  factory MovementModel.fromJson(Map<String,);

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

class VariationModel {
  VariationModel({
    this.variationName,});

  String variationName;

  factory VariationModel.fromJson(Map<String,dynamic> json) => VariationModel(
        variationName: json["variationName"],dynamic> toJson() => {
        "variationName": variationName,};
}

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',theme: ThemeData(
        primarySwatch: Colors.blue,visualDensity: VisualDensity.adaptivePlatformDensity,home: MyHomePage(title: 'Flutter Demo Home Page'),);
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key,this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    List<VariationModel> vList =
        MovementDataSource().getVariationsByMovement("Plank");

    for (int i = 0; i < vList.length; i++) {
      print(vList[i].variationName);
    }
   

    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,children: <Widget>[
            Text(
              'You have pushed the button this many times:',Text(
              '$_counter',style: Theme.of(context).textTheme.headline4,floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,tooltip: 'Increment',child: Icon(Icons.add),);
  }
}

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