Flutter-方法“ map”在null上被调用

  • Z时代
  • 2024-01-10
  • 分类：问答

尝试使用对象模型从我的API JSON添加元素时收到以下错误 DropdownMenuItem

这是错误：

The method 'map' was called on null.
Receiver: null
Tried calling: map<DropdownMenuItem<Provinces>>(Closure: (Provinces) => DropdownMenuItem<Provinces>)

这是我的飞镖代码：

import 'dart:async';
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:sj/utils/constants.dart';
import 'package:http/http.dart' as http;
import 'package:sj/models/master/provinces.dart';
class IdCardFormPage extends StatefulWidget {
  @override
  _IdCardFormPageState createState() => new _IdCardFormPageState();
}
class _IdCardFormPageState extends State<IdCardFormPage> {
  List<Provinces> listProvinces;
  Provinces _selectedProvince;
  Future<List<Provinces>> getProvinceList() async {
    //final response = await http.get("${APIConstants.API_BASE_URL}api/masters/provincesList.php");
    final response = await http.get("url");
    listProvinces = parseProvinces(response.body);
    return parseProvinces(response.body);
  }
  List<Provinces> parseProvinces(String responseBody) {
    final parsed = json.decode(responseBody).cast<Map<String, dynamic>>();
    return parsed.map<Provinces>((json) => Provinces.fromJson(json)).toList();
  }
  @override
  void initState() {
    getProvinceList();
    super.initState();
  }
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(title: new Text("MASUK"),),
      body: _provinceContainer(),
    );
  }
  Widget _provinceContainer(){
    return new Container(
        child: new InputDecorator(
          decoration: InputDecoration(
              suffixIcon: new Icon(
                Icons.account_balance,
                color: Colors.pink,
              ),
              labelText: LoanEntryField.PD_BANKNAME, labelStyle: TextStyle(fontSize: 16.0)
          ),
          isEmpty: _selectedProvince == null,
          child: new DropdownButtonHideUnderline(
            child: new DropdownButton<Provinces>(
              value: _selectedProvince,
              isDense: true,
              onChanged: (Provinces newValue) {
                setState(() {
                  _selectedProvince = newValue;
                });
              },
              items: listProvinces.map((Provinces value) {
                return new DropdownMenuItem<Provinces>(
                  value: value,
                  child: new Text(value.name, style: new TextStyle(fontSize: 16.0),),
                );
              }).toList(),
            ),
          ),
        ),
        margin: EdgeInsets.only(bottom: 10.0));
  }
}
class Provinces{
  String id;
  String name;
  Provinces({this.id, this.name});
  factory Provinces.fromJson(Map<String, dynamic> json) {
    return Provinces(
      id: json['id'] as String,
      name: json['name'] as String,
    );
  }
}

如何解决这个问题？

回答：

传递一个空容器中，同时listProvices是null：

items: listProvices?.map((Provinces value) {
            return new DropdownMenuItem<Provinces>(
              value: value,
              child: new Text(value.name, style: new TextStyle(fontSize: 16.0),),
            );
          })?.toList() ?? [],

以上是 Flutter-方法“ map”在null上被调用 的全部内容， 来源链接： utcz.com/qa/426968.html

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