tap_positioned_example.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Offset tapOffset;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Tap example"),
      ),
      body: GestureDetector(
        behavior: HitTestBehavior.opaque,
        onTapDown: (details) {
          setState(() {
            print("offset: ${details.localPosition}");
            tapOffset = details.localPosition;
          });
        },
        child: Stack(
            children: [
              if (tapOffset != null) AnimatedPositioned.fromRect(
                rect: Rect.fromCircle(
                  center: tapOffset,
                  radius: 40.0,
                ),
                child: Container(color: Colors.teal),
                duration: Duration(milliseconds: 400),
                curve: Curves.easeOutCubic,
              ),
            ]
        ),
      ),
    );
  }
}