Update AppBar title from body widget in Scaffold - exception

Question

I am starting to use flutter and doing the 'Startup Name Generator' tutorial, also modifying the code to do some different things, and have come across an issue I can't solve. I want the AppBar title to show the size of the _suggestions array, but can't get it to work, have tried many different things but to no avail, always getting the exception of trying to use 'setState()' while already in a build.
This is what I currently have:
import 'package:flutter/material.dart';
import 'package:english_words/english_words.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Startup Name Generator',
home: RandomWords(),
);
}
}
class RandomWordsState extends State<RandomWords> {
static final _suggestions = <WordPair>[];
final _biggerFont = const TextStyle(fontSize: 18.0);
ValueNotifier<String> appBarTitleNotifier = ValueNotifier('Startup Name Generator: 0');
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: new AppBar(title: ValueListenableBuilder<String>(
valueListenable: appBarTitleNotifier,
builder: (context,value,child) {
return Text(value);
}
)),
body: _buildSuggestions(),
);
}
Widget _buildSuggestions() {
var blder = ListView.builder(
padding: const EdgeInsets.all(16.0),
itemBuilder: /*1*/ (context, i) {
if (i.isOdd) return Divider(); /*2*/
final index = i ~/2; /*3*/
if (index >= _suggestions.length) {
_suggestions.addAll(generateWordPairs().take(10)); /*4*/
/**** THIS NEXT LINE CAUSES THE ISSUE ****/
appBarTitleNotifier.value = 'Startup Name Generator: ' + _suggestions.length.toString();
}
return _buildRow(_suggestions[index],index);
},
);
return blder;
}
Widget _buildRow(WordPair pair, int i) {
return ListTile(
title: Text(
pair.asPascalCase + ': ' + i.toString(),
style: _biggerFont,
),
);
}
}
class RandomWords extends StatefulWidget {
#override
RandomWordsState createState() => RandomWordsState();
}
I have used a ValueNotifier and listener after doing some research into events, but same exception as when I manually used a setState() call within the ListView itemBuilder (where the issue seems to lie; calling/updating the AppBar title from within the ListView itemBuilder).
I haven't found a solution here so I am asking. Sorry if this has already been answered, but I could not find the solution.

Answer 1

How about using GestureDetector.
onVerticalDragEnd event not executed. but onVerticalDragDown works.
// Copyright 2018 The Flutter team. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'package:english_words/english_words.dart';
import 'package:flutter/material.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Welcome to Flutter',
debugShowCheckedModeBanner: false,
home: Scaffold(
appBar: AppBar(
title: const Text('Welcome to Flutter'),
toolbarHeight: 100.0,
),
body: const Center(
child: RandomWords(),
),
),
);
}
}
class RandomWords extends StatefulWidget {
const RandomWords({Key? key}) : super(key: key);
#override
State<RandomWords> createState() => _RandomWordsState();
}
class _RandomWordsState extends State<RandomWords> {
final _suggestions = <WordPair>[];
final _biggerFont = const TextStyle(fontSize: 18.0);
String title = "Startup Name Generator";
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(title),
),
body: GestureDetector(
onTap: () {
print('tap');
},
onVerticalDragDown : (details) {
print('onVerticalDragDown');
setState(() {
title = "Startup Name Generator ("+_suggestions.length.toString()+")";
});
},
onVerticalDragEnd : (details) {
print('onVerticalDragEnd ');
setState(() {
title = "Startup Name Generator ("+_suggestions.length.toString()+")";
});
},
child:
// Padding(
// padding: const EdgeInsets.all(8.0),
// child: Icon(
// Icons.lightbulb_outline,
// color: Colors.yellow.shade600,
// size: 60,
// ),
// ),
ListView.builder(
padding: const EdgeInsets.all(16.0),
itemBuilder: (context, i) {
if(i.isOdd) return const Divider();
final index = i ~/ 2;
if(index >= _suggestions.length) {
_suggestions.addAll(generateWordPairs().take(10));
}
return ListTile(
title: Text(
// index.toString() + ":" + _suggestions[index].asPascalCase,
_suggestions[index].asPascalCase,
style: _biggerFont,
),
);
},
),
)
);
}
}