在 GestureDetector 我看不到 onScaleStart 和 onTap 之间的区别

在GestureDetector 中，我看不到onScaleStart和onTap之间有任何区别。如果我用两根手指将蓝色方块向两侧展开，那么理论上，方块应该跟随手指改变宽度，而蓝色方块会改变宽度，就像点击时一样。为什么？我注释掉了 onTap。取消注释它并注释掉 onScaleStart没有区别。下面是完整的代码。

主要.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
 @override
 Widget build(BuildContext context) {
  return MaterialApp(
   debugShowCheckedModeBanner: false,
   title: 'Name App',
   theme: ThemeData(
    primarySwatch: Colors.blue,
   ),
   home: Scaffold(
    appBar: AppBar(
     title: Text('Name Page'),
    ),
    body: MyHomePage(),
   ),
  );
 }
}

class MyHomePage extends StatefulWidget {
 @override
 _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

 String _str;
 double _width;
 double _height;
 bool _b;

 @override
 void initState() {
  super.initState();
  _str = '0';
  _width = 100.0;
  _height = 100.0;
  _b = false;
 }

 @override
 void dispose() {
  super.dispose();

 }

 void _f1() {
  setState(() {

  });
 }

 @override
 Widget build(BuildContext context) {
  return Stack(
   children: [
    Align(
     alignment: Alignment(0.0, -0.95),
     child: GestureDetector(

      /*onTap: () {
       setState(() {
        if(_b) {
         _b = false;
         _width = 100.0;
         _str = '2';
        } else {
         _b = true;
         _width = MediaQuery.of(context).size.width / 1.2;
         _str = '1';
        }
       });
      },*/

      onScaleStart: (ScaleStartDetails details) {
       setState(() {
        if(_b) {
         _b = false;
         _width = 100.0;
         _str = '2';
        } else {
         _b = true;
         _width = MediaQuery.of(context).size.width / 1.2;
         _str = '1';
        }
       });
      },

      child: Container(
       //width: 100.0,
       //height: 100.0,
       width: _width,
       height: _height,
       color: Colors.indigoAccent,
      ),
     ),
    ),
    Align(
     alignment: Alignment.center,
     child: Text(
      _str,
      style: TextStyle(
       fontSize: 36.0,
       color: Colors.pink
      ),
     ),
    ),
   ],
  );
 }

}