Can't Use () {} (AKA: Anonymous Function) in flutter

Question

I did created a brand-new project just now and I encountered this problem.

I Can't Use Buttons because I can't Assign a function to them!

Here is my code:

import 'package:flutter/material.dart';
import 'package:fluttertoast/fluttertoast.dart';
import 'package:url_launcher/url_launcher.dart';

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.white,
      child: const Align(
          alignment: Alignment.center,
          child: TextButton(onPressed: () {}, child: Text('LAUNCH'))), //PROBLEM IS HERE
    );
  }

  Future<void> _launchUrl(String urlString) async {
    if (await canLaunch(urlString)) {
      await launch(
        urlString,
        forceWebView: true,
      );
    } else {
      Fluttertoast.showToast(
          msg: "Try Again",
          toastLength: Toast.LENGTH_LONG,
          gravity: ToastGravity.CENTER,
          timeInSecForIosWeb: 1,
          backgroundColor: Colors.red,
          textColor: Colors.white,
          fontSize: 16.0);
    }
  }
}

As you can see I can't give () {} to the onPressed method of TextButton. This is the root of my problem.

The Error that Flutter shows for this is: Invalid constant value.

Also the packages I use:

dependencies:
  fluttertoast: ^8.0.8
  url_launcher: ^6.0.17
  animated_splash_screen: ^1.1.0

See my note at stackoverflow.com/a/69214531: Allowing lambdas to be `const` wouldn't be free. — jamesdlin, Dec 19 '21 at 20:38

score 1 · Accepted Answer · answered Dec 17 '21 at 17:43

To solve this issue please remove const infront of align.

From this:

child: const Align(
          alignment: Alignment.center,
          child: TextButton(onPressed: () {}, child: Text('LAUNCH'))),

To this:

child: Align(
          alignment: Alignment.center,
          child: TextButton(onPressed: () {}, child: Text('LAUNCH'))),

Can't Use () {} (AKA: Anonymous Function) in flutter

1 Answers1