Flutter GestureDetector isn't working at all

Question

I am trying to make a Text widget zoomable with GestureDetector but it isn't working at all and i get no error even...

Note that i tried many things like wrapping the scaffold itself with the GestureDetector..

main.dart

import 'package:flutter/material.dart';
import 'package:testy/zoomable.dart';

void main() => runApp(
  MaterialApp(
    debugShowCheckedModeBanner: false,
    title: 'Testy',
    theme: ThemeData(
      primarySwatch: Colors.green,
      primaryColor: Colors.green
    ),
    home: Zoomable(),
  )
);

zoomable.dart

import 'package:flutter/material.dart';

class Zoomable extends StatefulWidget {
  @override
  _ZoomableState createState() => _ZoomableState();
}

class _ZoomableState extends State<Zoomable> {
  @override
  Widget build(BuildContext context) {
    double size = 70;
    return Scaffold(
      appBar: AppBar(
        title: Text('Zoomable'),
      ),
      body: GestureDetector(
        onScaleStart: (details) {},
        onScaleUpdate: (ScaleUpdateDetails details) {
          if (size < 150.0 && size > 50.0) {
            setState(() {
              size = size + details.scale;
            });
          }
        },
        onTap: () {
          if (size < 150.0 && size > 50.0) {
            setState(() {
              size = size + 1;
            });
          }
        },
        child: Text(
          'Zoomable',
          style: TextStyle(fontSize: size),
        ),
      ),
    );
  }
}

Answer 1

It's because you are calling double size = 70; within the build() function. When you are calling setState() , the build() function is called and the size is set back to 70. Simply move the size to outside of the build() function and it will work.