redux - 学习 Flutter Redux - 这段代码有什么问题？

Question

我刚刚接触 Flutter、Dart 和 Redux。按照 YouTube 视频修改默认的 Flutter 示例以使用 Redux，但它对我来说失败了，我仍然很难理解异常并有效地对它们使用react。这是代码:

import 'package:flutter/material.dart';
import 'package:meta/meta.dart';
import 'package:redux/redux.dart';
import 'package:flutter_redux/flutter_redux.dart';

// following this youtube video: https://youtu.be/X8B-UzqEaWc

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

@immutable
class AppState {
  final int counter;
  AppState(this.counter);
}

// actions
enum Actions { increment }

// pure function
AppState reducer(AppState prev, action) {
  if(action == Actions.increment) {
    return new AppState(prev.counter + 1);
  }
  return prev;
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData.dark(),
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {  

  final store = new Store(reducer, initialState: new AppState(0));

  //print(store.state.counter); <----- Undefined class 'counter'.

  @override
  Widget build(BuildContext context) {
    return new StoreProvider(
      store: store,      
      child: new Scaffold(
        appBar: new AppBar(
          title: new Text("Flutter Redux"),
        ),
        body: new Center(
          child: new Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              new Text(
                'You have pushed the button this many times:',
              ),
              new StoreConnector(
                converter: (store) => store.state.counter,
                builder: (context, counter) => new Text(
                  "$counter",
                  style: Theme.of(context).textTheme.display1,
                  )
              )
            ],
          ),
        ),
        floatingActionButton: new StoreConnector<int, VoidCallback>(
          converter: (store) {
            return () => store.dispatch(Actions.increment);
          },
          builder: (context, callback) => new FloatingActionButton(
            onPressed: callback,
            tooltip: 'Increment',
            child: new Icon(Icons.add),
          ),
        ) 
      ),
    );
  }
}

所以首先，当我尝试运行它时，我收到一个异常，指出“以下 NoSuchMethodError 被抛出构建 StoreConnector(脏): I/flutter (20662): getter 'store' was called on null."。第二个问题是为什么代码中突出显示的打印方法无法识别计数器 getter？谢谢。

Answer 1

问题是我将“dart.previewDart2”设置为 true，我猜想在最新的预览版本中可能出现了问题。将选项设置为 false 后，一切正常。

详细信息:

• 转到文件 -> 首选项 -> 设置。
• 在搜索框中键入“dart”。
• 找到设置 dart.previewDart2 后，点击它左侧的铅笔图标并选择“复制到设置”。
• 在用户设置的右侧，将设置设为 true。