How do I get to tap on a CustomPaint path in Flutter using GestureDetect?

Question

I'm very new to flutter and am trying to figure out how to get a gesture detected on a CustomPaint path. I can click on a host of other things but not on paths for some reason... How do I get this to work? My code thus far is below.

import 'package:flutter/material.dart';

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

class MbiraShapes extends StatefulWidget {
    @override
  _MbiraShapesState createState() => _MbiraShapesState();
}

class _MbiraShapesState extends State<MbiraShapes>{

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Mbira Shapes',
        home: PathExample());
  }
}

class PathExample extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CustomPaint(
      painter: KeyPathPainter(),
    );
  }
}

class KeyPathPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    Paint lineDrawer = Paint()
      ..color = Colors.blue
      ..strokeWidth = 8.0;

    Path path = Path()
// Moves to starting point
      ..moveTo(50, 50)
      //draw lines passing through xc, yc to end at x,y
      ..quadraticBezierTo(77, 370, 50, 750)
      ..quadraticBezierTo(100, 775, 150, 750)
      ..quadraticBezierTo(110, 440, 75, 50);
    //close shape from last point
    path.close();
    canvas.drawPath(path, lineDrawer);

    Path path2 = Path()
// Moves to starting point
      ..moveTo(250, 50)
      //draw lines passing through xc, yc to end at x,y
      ..quadraticBezierTo(280, 350, 270, 675)
      ..quadraticBezierTo(290, 750, 350, 750)
      ..quadraticBezierTo(365, 710, 345, 600)
      ..quadraticBezierTo(320, 450, 270, 50);
    //close shape from last point
    path2.close();
    canvas.drawPath(path2, lineDrawer);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) => true;
}

I tried the snippet below with GestureDetector but it does not work. I tried a Listener but get no response for onPointerDown, only for onPointerMove, but I need a tap action, not a move action.

@override
  Widget build(BuildContext context) {

    return Scaffold(
      appBar: AppBar(
  
        title: Text("widget.title)"),
      ),
      
      body: Center(
        child: GestureDetector(
          child: CustomPaint(
            painter: KeyPathPainter(),
            child: Container()),
          onTap: () {
            print("tapped the key");
          },
        ),
      ),
    );
  }

I just want to tap a key and get a response, it will be a sound in the end but for now, I'm just trying to figure out how to get the onTap to work.

Solution 1

You have to override the hitTest() method of your KeyPathPainter class:

@override
bool hitTest(Offset position) {
    // _path - is the same one we built in paint() method;
    return _path.contains(position);
}

That will allow GestureDetector to detect taps inside the path instead of the entire CustomPaint container. Though the _path must be closed since there's no way—at least that I know of—to test hits on curves.

Solution 2

Wrapping your paint with a gesture detector should do the work.

 GestureDetector(
    onTapDown: (details) {
         print("${details.globalPosition.dx}");
         print("${details.globalPosition.dy}");
     },
  ),

Solution 3

I came to this from googling an issue I had. My issue was I was just wrapping the Painter in a gesture detector like:

GestureDetector(
      child: CustomPaint(
        painter: CustomPainter(),
      ),

      onTapDown: _handleTapDown,
);

Where the solution was I needed to have a Container with a color, wrapping the Custom Painter, like so:

return GestureDetector(
      child: Container(
        width: 300,
        height: 300,
        color: Colors.white,
        child: CustomPaint(
          painter: CustomPainter(),
        ),
      ),

      onTapDown: _handleTapDown,
);