CODEWITHSUNDEEP
flutterdart
Denis Ramdan
Denis Ramdan

Reputation: 1954

How do I change the 'state' in a StatefulWidget class but from an external function?

how do I change the 'state' in a StatefulWidget class but from an external function?

Examples are as follows:

in lib/main.dart

import 'package:flutter/material.dart';
import 'increment-count.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}) : super(key: key);
  final String title;

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

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

  void _incrementCounter() {
    setState(() {
      _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(
              '$_counter',
              style: Theme.of(context).textTheme.display1,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

I want to move the _incrementCounter function to another file. for example in 'lib/increment-count.dart'.

in /lib/increment-count.dart

import 'package:flutter/material.dart';

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

but the setState instead displays an error "The function setState isn't defined". can anyone help?

Upvotes: 3

Views: 1310

Answers (2)

LonelyWolf
LonelyWolf

Reputation: 4392

Moving function in another file in this example has no point but here is the solution. Obviously it is a great idea for a bigger function which you want to reuse somewhere else

in your new file increment-count.dart add this 

int incrementCounter(int counter) {
  return counter + 1;
}

and then call it in your main class

 onPressed: () => setState(() {
      _counter = incrementCounter(_counter);
    })

However answer to your question, how to setState from external function, is not possible.

Upvotes: 4

Marcel Dz
Marcel Dz

Reputation: 2714

your'e getting this error "The function setState isn't defined" because the second file is no stateful widget which can take up a setState function. and if you then want to increment the counter you have no access, as it is marked as private and the logic part is in a different file.

Than BloC pattern or providers is what you need. Since I'm also still beginner, I can recommend you using providers, as it is quite easy to learn and it does exactly what you want.

https://pub.dev/packages/provider

with this you can like globally define this function and use it in different files.

This can look like this for example (toggleCounter() would be the global function then):

final counterIncrement = Provider.of<Data>(context).toggleCounter();

Upvotes: 2

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