How to Calculate the Coordinates of touch in Flutter?

Question

I want show a popup at touch Coordinates. I am using Stack and Positioned widgets to place the popup.

Answer 1

You can add a GestureDetector as parent of stack, and register onTapDownDetails listener. This should call your listener on every tapdown event, with global offset of the tap in TapDownDetails parameter of the your listener.

Here is sample code demonstrating the same.

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      home: new MyHomePage(),
    );
  }
}

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

class MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        appBar: new AppBar(
          title: new Text('Popup Demo'),
        ),
        body: new MyWidget());
  }
}

class MyWidget extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return new MyWidgetState();
  }
}

class MyWidgetState extends State<MyWidget> {
  double posx = 100.0;
  double posy = 100.0;

  void onTapDown(BuildContext context, TapDownDetails details) {
    print('${details.globalPosition}');
    final RenderBox box = context.findRenderObject();
    final Offset localOffset = box.globalToLocal(details.globalPosition);
    setState(() {
      posx = localOffset.dx;
      posy = localOffset.dy;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new GestureDetector(
      onTapDown: (TapDownDetails details) => onTapDown(context, details),
      child: new Stack(fit: StackFit.expand, children: <Widget>[
        // Hack to expand stack to fill all the space. There must be a better
        // way to do it.
        new Container(color: Colors.white),
        new Positioned(
          child: new Text('hello'),
          left: posx,
          top: posy,
        )
      ]),
    );
  }
}