將變量從有狀態小部件傳遞到 state

Question

我嘗試傳遞一個變量“int counter = 0;” 在 state 小部件中，但我不知道為什么我無法檢索它，應用程序將我返回為“Null”，但我將所有變量都傳遞給了構造函數（MyHomePage 類擴展了 StatefulWidget）。

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title, this.counter}) : super(key: key);
  final String title;
  int counter= 0;
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  void _incrementCounter() {
    setState(() {
      widget.counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '${widget.counter}',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

謝謝您的幫助

Answer 1

Widget s（包括StatefulWidget s）應該始終是不可變的，不應在其生命周期內改變。 事實上，如果您嘗試向StatefulWidget添加非final變量，您應該會看到警告，因為它是Widget的子類型，具有@immutable注釋。

很少有任何邏輯實際上發生在StatefulWidget子類中。 相反，所有 state 和相關邏輯都應該在State class 中。

您應該像這樣將counter移動到 state class 中：

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

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

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

  void _incrementCounter() {
    setState(() {
      counter++;
    }); 
  }

  // the rest is the same, except using counter instead of widget.counter
}

Answer 2

讓我們首先分析代碼

 home: MyHomePage(title: 'Flutter Demo Home Page'),
 // You did not pass integer to the counter field, dart pass null to counter field


class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title, this.counter
/* here null will be initialized to the counter field */
}) : super(key: key);
  final String title;
  int counter= 0; // now it is null
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

因此，由於這個原因，您會得到 null

如何正確傳遞您的場景：

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title, this.counter=0}) : super(key: key);
  final String title;
  int counter;
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  void _incrementCounter() {
    setState(() {
      widget.counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '${widget.counter}',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}