Flutter - 来自 json api 的数据将列在下拉列表中

Question

我正在尝试从 json api 获取数据并列在下拉列表中，但我遇到 The method 'map' was called on null 错误。

import "package:flutter/material.dart";
import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart' as http;

void main() => runApp(MaterialApp(
      title: "Hospital Management",
      home: MyApp(),
    ));

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  String _mySelection;

  final String url = "http://webmyls.com/php/getdata.php";

  List data;

  Future<String> getSWData() async {
    var res = await http
        .get(Uri.encodeFull(url), headers: {"Accept": "application/json"});
    var resBody = json.decode(res.body);

    setState(() {
      data = resBody;
    });

    print(resBody);

    return "Sucess";
  }

  @override
  void initState() {
    super.initState();
    this.getSWData();
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: AppBar(
        title: Text("Hospital Management"),
      ),
      body: new Center(
        child: new DropdownButton(
          items: data.map((item) {
            return new DropdownMenuItem(
              child: new Text(item['item_name']),
              value: item['id'].toString(),
            );
          }).toList(),
          onChanged: (newVal) {
            setState(() {
              _mySelection = newVal;
            });
          },
          value: _mySelection,
        ),
      ),
    );
  }
}

调试控制台的错误

flutter: The method 'map' was called on null.
flutter: Receiver: null
flutter: Tried calling: map<DropdownMenuItem<String>>(Closure: 
(dynamic) => DropdownMenuItem<String>)
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #0      Object.noSuchMethod 
(dart:core/runtime/libobject_patch.dart:48:5)

我希望在下拉菜单中显示来自 json api 的数据列表。我是飞镖和颤振的初学者。帮我解决这个错误。

Answer 1

data 未初始化。它设置在getSWData 上，即async。

一种可能的解决方案是将data 作为emptyList 作为初始值。

List data = List();

如果你想根据数据显示DropdownButton，你可以使用FutureBuilder并显示loader之类的东西，直到数据来自getSWData

Answer 2

您的 Api http://webmyls.com/php/getdata.php 返回列表而不是地图。因此，它不能具有直接属性advisor_report。考虑删除它。

数据应该在setState内部改变。

你从来没有打电话给getSWData()

下面是更多改进的例子。

import "package:flutter/material.dart";
import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart' as http;

void main() => runApp(MaterialApp(
      title: "Hospital Management",
      home: MyApp(),
    ));

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  String _mySelection;

  final String url = "http://webmyls.com/php/getdata.php";

  List data;

  Future<String> getSWData() async {
    var res = await http
        .get(Uri.encodeFull(url), headers: {"Accept": "application/json"});
    var resBody = json.decode(res.body);

    setState(() {
      data = resBody;
    });

    print(resBody);

    return "Sucess";
  }

  @override
  void initState() {
    super.initState();
    this.getSWData();
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: AppBar(
        title: Text("Hospital Management"),
      ),
      body: new Center(
        child: new DropdownButton(
          items: data.map((item) {
            return new DropdownMenuItem(
              child: new Text(item['item_name']),
              value: item['id'].toString(),
            );
          }).toList(),
          onChanged: (newVal) {
            setState(() {
              _mySelection = newVal;
            });
          },
          value: _mySelection,
        ),
      ),
    );
  }
}