How am I able to pass setState in a stateless widget from a statefull widget

Question

In the below code I have passed set state as a function call in a stateless widget how is this possible I know there is nothing wrong with flutter, but its something about the functions in general, I am not getting the basics right can someone plz explain me this concept. Comments are provided for assistance.

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {    //STATEFUL WIDGET
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          Demo(function: () {
            setState(() {});      //PASSING SET STATE
          });
        },
      ),
    );
  }
}

class Demo extends StatelessWidget {  //STATE  LESS WIDGET
  final Function function;
  Demo({this.function});

  @override
  Widget build(BuildContext context) {
    return Container();
  }
}

Solution

setState() is a method inside the State class. In your code you have the following:

   onPressed: () {
          Demo(function: () {
            setState(() {});      //PASSING SET STATE
          });
        },

Here you create an instance of Demo and give it a callback as an argument. Inside that callback, you are calling setState which is a method inside the State class.

Comments

[Ravindra Kushwaha]

Refer this demo example for passing the value either in State or stateless stackoverflow.com/a/61743422/3946958

[UTKARSH Sharma]

no, the problem is how can I pass set state call in a stateless widget as stateless widget doesn't know what is set state so how can I pass it as an argument. I know I can pass but (why) is my question

[pskink]

so is Demo(function: () { setState(() {}); //PASSING SET STATE }); working or you have any problem with passing such a callback to some other widget?

[UTKARSH Sharma]

it's working fine even I have developed a full-fledged app but the question is (how ).

[pskink]

have you used any RaisedButton / OutlineButton / FlatButton? they have onPressed property - this is exactly the same - you are passing some function to that button and when the click is performed tha button calls that callback function

[UTKARSH Sharma]

I am not getting the callback thing plz elaborate a bit how it works in this case.

[Peter Haddad]

@UTKARSHSharma check this stackoverflow.com/a/59504089/7015400

[Peter Haddad]

@UTKARSHSharma @UTKARSHSharma for ex in javascript function doHomework(subject, callback) { alert(Starting my ${subject} homework.); callback(); } doHomework('math', function() { alert('Finished my homework'); }); Here, the function doHomework takes a subject variable and function reference callback. After the alert appears and you know the subject, then the function (callback) will be called. So basically a callback is a function that is called as a response of something occurring. It can be a response of a click on a button or as a response of retrieving data from the db

[Peter Haddad]

@UTKARSHSharma so a callback is not called immediately, it is a function that is called in response to something occurring. Thats why its called "callback", you can think about it as call me later. All buttons in flutter, use callback functions and these functions are called in response to when the user clicks the button. All asynchronous code in flutter uses callback and these callbacks are called when the data is retrieved