颤音:尝试将数据添加到地图列表中时出错

Question

我对颤抖非常新。我正在开发一个测试应用程序，而打击是我的代码。

main.dart

import 'package:flutter/material.dart';
import './pages/homepage.dart';
void main() {
  runApp(MyApp());
}
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return MaterialApp(
      theme: ThemeData(primaryColor: Colors.green),
      home: Homepage(),
    );
  }
}

homepage.dart

import 'package:flutter/material.dart';
class Homepage extends StatelessWidget {
  List<Map<String, String>> productMap;
  Homepage() {
    productMap.add({"title": "Chocolate", "imageUrl": ""});
  }
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
      appBar: AppBar(
        title: Text("Choco Factory"),
      ),
      body: HomepageUI(),
    );
  }
}
class HomepageUI extends StatefulWidget {
  HomepageUI() {}
  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return _HomepageUIBuilder();
  }
}
class _HomepageUIBuilder extends State<HomepageUI> {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Expanded(
      child: Column(
        children: <Widget>[
          ListView.builder(
            itemBuilder: _listBuilder,
          )
        ],
      ),
    );
  }
  Widget _listBuilder(BuildContext context, int index) {
    return Card(
      child: Column(
        children: <Widget>[
          Image.asset("name"),
          Text("data"),
          RaisedButton(
            onPressed: () {},
            child: Text("Details"),
            color: ThemeData().primaryColor,
          )
        ],
      ),
    );
  }
}

我会收到以下错误，我不知道为什么。我来自Java背景，对我来说，代码似乎还可以。另外，我还有一个问题，在Java Map中，我们可以在Map中添加许多键值对，但是在Flutter中，似乎只有一个键值对？

Launching libmain.dart on A37f in debug mode...
Built buildappoutputsapkdebugapp-debug.apk.
I/Choreographer(11935): Skipped 87 frames!  The application may be doing too much work on its main thread.
I/flutter (11935): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter (11935): The following NoSuchMethodError was thrown building MyApp(dirty):
I/flutter (11935): The method 'add' was called on null.
I/flutter (11935): Receiver: null
I/flutter (11935): Tried calling: add(_LinkedHashMap len:2)
I/flutter (11935): When the exception was thrown, this was the stack:
I/flutter (11935): #0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
I/flutter (11935): #1      new Homepage 
I/flutter (11935): #2      MyApp.build 
I/flutter (11935): #3      StatelessElement.build 
I/flutter (11935): #4      ComponentElement.performRebuild 
I/flutter (11935): #5      Element.rebuild 
I/flutter (11935): #6      ComponentElement._firstBuild 
I/flutter (11935): #7      ComponentElement.mount 
I/flutter (11935): #8      Element.inflateWidget 
I/flutter (11935): #9      Element.updateChild 
I/flutter (11935): #10     RenderObjectToWidgetElement._rebuild 
I/flutter (11935): #11     RenderObjectToWidgetElement.mount 
I/flutter (11935): #12     RenderObjectToWidgetAdapter.attachToRenderTree.<anonymous closure> 
I/flutter (11935): #13     BuildOwner.buildScope 
I/flutter (11935): #14     RenderObjectToWidgetAdapter.attachToRenderTree 
I/flutter (11935): #15     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.attachRootWidget 
I/flutter (11935): #16     runApp 
I/flutter (11935): #17     main 
I/flutter (11935): #18     _runMainZoned.<anonymous closure>.<anonymous closure> (dart:ui/hooks.dart:189:25)
I/flutter (11935): #23     _runMainZoned.<anonymous closure> (dart:ui/hooks.dart:180:5)
I/flutter (11935): #24     _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:300:19)
I/flutter (11935): #25     _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:171:12)
I/flutter (11935): (elided 4 frames from package dart:async)
I/flutter (11935): ════════════════════════════════════════════════════════════════════════════════════════════════════

Answer 1

List<Map<String, String>> productMap;

应该是

List<Map<String, String>> productMap = [];

否则productMap仅声明，但不开始初始化（null）

Answer 2

您尚未初始化List。

请更改

List<Map<String, String>> productMap;

to

List<Map<String, String>> productMap = <Map<String, String>>[];

希望这有帮助！