Send parameters to setstate?

Question

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.
    );
  }
}

Solution

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;
    });
}

Comments

[Günter Zöchbauer]

What is "this" and what do you mean with "doesn't work"?

[diegoveloper]

what doesn't work? it should work

[rmtmckenzie]

Please post your entire class or ideally entire main.dart if this is a new project. What you've written should work in a stateful widget, but we don't have enough context to tell what's going on there.

[mreig]

Do it, I post the entire class. No error, simply no works, it does nothing

[Günter Zöchbauer]

What are you trying to accomplish? What is not working? What is the expected behavior. To me it's till completely unclear what this question is about.

[mreig]

Ok, So I want to increment the var _counter by a parameter (int i) sended by the floatingActionButton

[mreig]

Ooh! Thanks!!! It works! :D Can you explain me, please, the different between: onPressed: _incrementCounter(2) to onPressed: () => _incrementCounter(2) I want to understand this difference. A lof of thanks! :)

[scottstoll2017]

I'm honestly not sure, that's really a dart question, you probably want to ask with a dart tag.