flutter - Flutter : For 循环和 LayoutBuilder

Question

List<Widget> _temp = [];    

for(int i = 0; i < 10; i++) {
   if(i == 0) { string = "Hello"; } else { string = "World"; }
   _temp.add(Container(
      child: LayoutBuilder(
        builder: (context, constraint) { 
          return Text(string); //This return line makes problem with the for loop
        })
   ));
}

return _temp;

它只打印世界。如果我使用 Text 小部件作为没有 LayoutBuilder 的 Container 小部件的直接子级，则会打印 Hello 和 World。

我认为return关键字是问题所在。解决办法是什么？

标签： flutterdart

Answer 1

将容器提取到另一个函数返回小部件时工作
您可以在下面复制粘贴运行完整代码

代码片段

Widget _textWdiget(string)
  {
    return Container(
        child: LayoutBuilder(
        builder: (context, constraint) {
      return Text(string); //This return line makes problem with the for loop
    }));
  }

  Widget getTextWidgets()
  {
    String string;
    List<Widget> _temp = [];

    for(int i = 0; i < 10; i++) {
      if(i == 0) { string = "Hello"; } else { string = "World";}
      _temp.add(_textWdiget(string));
    }
    return Column(children: _temp);
  }

演示

完整代码

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child:  getTextWidgets(),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }

  Widget _textWdiget(string)
  {
    return Container(
        child: LayoutBuilder(
        builder: (context, constraint) {
      return Text(string); //This return line makes problem with the for loop
    }));
  }

  Widget getTextWidgets()
  {
    String string;
    List<Widget> _temp = [];

    for(int i = 0; i < 10; i++) {
      if(i == 0) { string = "Hello"; } else { string = "World";}
      _temp.add(_textWdiget(string));
    }
    return Column(children: _temp);
  }
}