dart - 我无法添加 Text(installedApps[index] ["app_name"]) 因为 'index' 未定义。我怎样才能添加这个文本？

Question

我正在构建一个 flutter 应用程序。我已经构建了一个带有构造函数的类。我制作了构造函数，因此我可以自定义我的 ListTile

像这样:

class AppList extends StatefulWidget {

  @override
    AppListState createState() => AppListState();

    AppList({Key key, this.child}) : super(key: key);

    final Widget child;

}

class AppListState extends State<AppList> {

  Widget child;

  List<Map<String, String>> _installedApps;

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

  getApps() {
    setState(() {
      installedApps = _installedApps;
      getApp();
    });
  }

  @override
    Widget build(BuildContext context) {

      if (installedApps == null)
        getApps();

      return ListView.builder( 
        itemCount: installedApps == null ? 0 : installedApps.length,
        itemBuilder: (context, index) {
          return child; //This is where the ListTile will go. 
        },
      );
    }

}

//Just in case you were confused, I used a plugin for some of the features

在我构建这个类之后，我将它放在我的示例类中。

示例类:

class Example extends StatefulWidget {

  @override 
    ExampleState createState() => ExampleState();

}

class ExampleState extends State<Example> {

  @override
    Widget build(BuildContext context) {
      return MaterialApp (
        debugShowCheckedModeBanner: false,
        home: Scaffold (
          body: Container (
            color: Colors.black,
            child: AppList (
              child: ListTile ( 
                title: Text(installedApps[index]["app_name"]) //this is the text
              ),
            )
          )  
        ),
      );
    }

} 

然后，我在其中添加了一个 ListTile 和一个文本。但是当我在写文字时，我意识到我无法输入我想要的文字。这是因为 Example 类中没有定义 'index'。

有什么好的方法可以将此文本放入我的示例类中吗？

完整代码:

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

import 'dart:async';
import 'dart:io';

import 'package:flutter_appavailability/flutter_appavailability.dart';

void main() {
  SystemChrome.setEnabledSystemUIOverlays([]);
  runApp(Example());
}

Future<void> getApp() async {

  if (Platform.isAndroid) {

    installedApps = await AppAvailability.getInstalledApps();

    print(await AppAvailability.checkAvailability("com.android.chrome"));

    print(await AppAvailability.isAppEnabled("com.android.chrome"));

  }
  else if (Platform.isIOS) {
    installedApps = iOSApps;

    print(await AppAvailability.checkAvailability("calshow://"));

  }

}

List<Map<String, String>> installedApp;
List<Map<String, String>> installedApps;
List<Map<String, String>> iOSApps = [
  {
    "app_name": "Calendar",
    "package_name": "calshow://"
  },
  {
    "app_name": "Facebook",
    "package_name": "fb://"
  },
  {
    "app_name": "Whatsapp",
    "package_name": "whatsapp://"
  }
];

class Example extends StatefulWidget {

  @override 
    ExampleState createState() => ExampleState();

}

class ExampleState extends State<Example> {

  @override
    Widget build(BuildContext context) {
      return MaterialApp (
        debugShowCheckedModeBanner: false,
        home: Scaffold (
          body: Container (
            color: Colors.black,
            child: AppList ()
          )  
        ),
      );
    }

}

class AppList extends StatefulWidget {

  @override
    AppListState createState() => AppListState();

  AppList({Key key, this.child}) : super(key: key);

  final Widget child;

}

class AppListState extends State<AppList> {

  Widget child;

  List<Map<String, String>> _installedApps;

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

  getApps() {
    setState(() {
      installedApps = _installedApps;
      getApp();
    });
  }

  @override
    Widget build(BuildContext context) {

      if (installedApps == null)
        getApps();

      return ListView.builder( 
        itemCount: installedApps == null ? 0 : installedApps.length,
        itemBuilder: (context, index) {
          return ListTile (
            title: Text(installedApps[index]["app_name"])
          );
        },
     );
   }

}

Answer 1

为什么不将 ListTile 小部件放入 ListView 构建器中？

像这样:

return ListView.builder(
    itemCount: installedApps == null ? 0 : installedApps.length,
    itemBuilder: (BuildContext context, int index){
       return ListTile(
           title: Text(installedApps[index]["app_name"])
           );
)

因为我怀疑即使您获得了索引，您仍然必须找到一种方法来从您的 AppListState 类访问您的 installedApps 变量到您的 ExampleState 类。

您可能还想在 getApps() 方法之外声明 installedApps 变量，就像您在 _installedApps 变量中所做的那样。像这样:

List<Map<String, String>> installedApps;

最后，getApp() 函数的作用是什么？您的 _installedApps 变量如何获取数据？

更新

我修改了您的代码并使用了 FutureBuilder 小部件。

import 'dart:async';
import 'dart:io';

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

import 'package:flutter_appavailability/flutter_appavailability.dart';

void main() {
  SystemChrome.setEnabledSystemUIOverlays([]);
  runApp(Example());
}


class Example extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MaterialApp (
      debugShowCheckedModeBanner: false,
      home: Scaffold (
        body: Container (
          color: Colors.white,
          child: AppList ()
        )  
      ),
    );
  }

}


class AppList extends StatefulWidget {

  AppList({Key key}) : super(key: key);

  @override
  AppListState createState() => AppListState();

}

class AppListState extends State<AppList> {

  //List<Map<String, String>> installedApp;
  List<Map<String, String>> installedApps;
  List<Map<String, String>> iOSApps = [
    {
      "app_name": "Calendar",
      "package_name": "calshow://"
    },
    {
      "app_name": "Facebook",
      "package_name": "fb://"
    },
    {
      "app_name": "Whatsapp",
      "package_name": "whatsapp://"
    }
  ];


  Future<List<Map<String, String>>> getApp() async {

    if (Platform.isAndroid) {

      installedApps = await AppAvailability.getInstalledApps();

      print(await AppAvailability.checkAvailability("com.android.chrome"));

      print(await AppAvailability.isAppEnabled("com.android.chrome"));

    }
    else if (Platform.isIOS) {
      installedApps = iOSApps;

      print(await AppAvailability.checkAvailability("calshow://"));

    }

    return installedApps;
  }


  @override
  Widget build(BuildContext context) {

    return FutureBuilder(
      future: getApp(),
      builder: (BuildContext context, AsyncSnapshot<List<dynamic>> snapshot){

        switch(snapshot.connectionState){
          case ConnectionState.none:
            return Text('None');
          case ConnectionState.active:
          case ConnectionState.waiting:
            return Text('Awaiting result...');
          case ConnectionState.done:
            if (snapshot.hasError)
              return Text('Error: ${snapshot.error}');
            if (!snapshot.hasData)
              return Text('No Data');

            return ListView.builder( 
              itemCount: snapshot.data.length,
              itemBuilder: (BuildContext context, int index) {
                return ListTile (
                  title: Text(snapshot.data[index]["app_name"])
                );
              },
            );   
          }
        }
      );
    }
  }

这是我的 Android 手机上的输出: