Btukfyl

How can I send parameters to setState()?

For example, the first example of flutter (the increment counter):

_incrementCounter(int i) {

setState(() {

    _counter=i;

  });

}

Why doesn't this work?

The entire class:

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;



  //I deleted the void

  _incrementCounter(int i) {

    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=_counter+i;

    });

  }



  @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: Column(

          // Column is also layout widget. It takes a list of children and

          // arranges them vertically. By default, it sizes itself to fit its

          // children horizontally, and tries to be as tall as its parent.

          //

          // Invoke "debug painting" (press "p" in the console, choose the

          // "Toggle Debug Paint" action from the Flutter Inspector in Android

          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)

          // to see the wireframe for each widget.

          //

          // Column has various properties to control how it sizes itself and

          // how it positions its children. Here we use mainAxisAlignment to

          // center the children vertically; the main axis here is the vertical

          // axis because Columns are vertical (the cross axis would be

          // horizontal).

          mainAxisAlignment: MainAxisAlignment.center,

          children: <Widget>[

            Text(

              'You have pushed the button this many times:',

            ),

            Text(

              '$_counter',

              style: Theme.of(context).textTheme.display1,

            ),

          ],

        ),

      ),

      floatingActionButton: FloatingActionButton(

        onPressed: _incrementCounter(2),

        tooltip: 'Increment',

        child: Icon(Icons.add),

      ), // This trailing comma makes auto-formatting nicer for build methods.

    );

  }

}

Don't get rid of the void. I'm assuming you did that in order to try to deal with the following error:

Error: The argument type '(dart.core::int) → void' can't be assigned
to the parameter type '() → void'. Try changing the type of the
parameter, or casting the argument to '() → void'.

I think the root of your problem was that you needed to change your call to a lambda:

onPressed: () => _incrementCounter(2),

That works when you call the new version of _incrementCounter here:

void _incrementCounter(int i) {

    setState(() {

        _counter = _counter + i;

    });

}