如何使用构建器函数？ - 颤振

Question

我问了一个名为 'How to inject a Widget into a custom child Widget and use the child Widgets iteration index?' 的问题，我已经得到了答案，但是有一个问题。

答案说我应该：

“与其将 Widget 传递给您的自定义 AppList，不如传递一个构建器函数，该函数 returns 一个 Widget 并根据需要获取参数，例如索引和所需的任何配置。类似于以下内容：

函数定义：

typedef Widget MyListTileBuilder(String tileText);

然后更改以下内容：

final Widget child;

至

final MyListTileBuilder childBuilder;

当然你需要在示例中实现你的构建器方法Class:

Widget MyListTileBuilderImplementation (int index) {
  return ListTile ( 
                title: Text(installedApps[index]["app_name"]) //this is the text
              ),
}

当您在示例中构建 AppList 时 class 您传递了方法

AppList (
              childBuilder: MyListTileBuilderImplementation 
)

最后在 AppList 中调用构建器，而不是添加子小部件”：

itemBuilder: (context, index) {
  return childBuilder(index); //This is where the ListTile will go. 
},

所以，我尝试编辑我的代码。但由于某种原因，它不起作用。我认为我做错了。有人知道如何解决这个问题吗？（如果您能编辑我的完整代码，那将非常非常有帮助。）

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

这是您的示例，其中进行了更改：

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

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

//import 'package:flutter_appavailability/flutter_appavailability.dart';

typedef Widget MyListTileBuilder(int index);

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=[
  {"app_name":"app1"},
  {"app_name":"app2"},
  {"app_name":"app3"},
];
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> {

  Widget MyListTileBuilderImplementation (int index) {
    return ListTile ( 
                  title: Text(installedApps[index]["app_name"] + "  index:" + index.toString()) //this is the text
                );
  }

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

}

class AppList extends StatefulWidget {

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

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

  final MyListTileBuilder childBuilder;

}

class AppListState extends State<AppList> {

  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  widget.childBuilder(index);
        },
     );
   }

}

如何使用构建器函数？ - 颤振

How do I use a builder function? - flutter

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custom-widgets