Flutter - Creating a custom control with Flutter - dart

Question

I need to create a custom control which allows a user to drag a pointer within a bounded rectangle. Very like this joystick control here :https://github.com/zerokol/JoystickView
I have managed to cobble something together using a CustomPainter to draw the control point and a GestureDetector to track where the user drags the pointer on the view. However, I can't get this to capture the panning input. I can't get it to capture any input at all. I don't know if what I am doing is the best approach. I could be on the totally wrong track. Here is the code.
import 'package:flutter/material.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/rendering.dart';
void main() {
runApp(new TouchTest());
}
class TouchTest extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Touch Test',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new Scaffold(
appBar: new AppBar(
title: const Text('Test'),
),
body: new Container(
decoration: new BoxDecoration(
color: Colors.white,
border: new Border.all(
color: Colors.black,
width: 2.0,
),
),
child: new Center(
child: new TouchControl()
),
),
)
);
}
}
class TouchControl extends StatefulWidget {
final double xPos;
final double yPos;
final ValueChanged<Offset> onChanged;
const TouchControl({Key key,
this.onChanged,
this.xPos:0.0,
this.yPos:0.0}) : super(key: key);
#override
TouchControlState createState() => new TouchControlState();
}
/**
* Draws a circle at supplied position.
*
*/
class TouchControlState extends State<TouchControl> {
double xPos = 0.0;
double yPos = 0.0;
GestureDetector _gestureDetector;
TouchControl() {
_gestureDetector = new GestureDetector(
onPanStart:_handlePanStart,
onPanEnd: _handlePanEnd,
onPanUpdate: _handlePanUpdate);
}
void onChanged(Offset offset) {
setState(() {
widget.onChanged(offset);
xPos = offset.dx;
yPos = offset.dy;
});
}
#override
bool hitTestSelf(Offset position) => true;
#override
void handleEvent(PointerEvent event, BoxHitTestEntry entry) {
if (event is PointerDownEvent ) {
// ??
}
}
void _handlePanStart(DragStartDetails details) {
onChanged(details.globalPosition);
}
void _handlePanEnd(DragEndDetails details) {
// TODO
}
void _handlePanUpdate(DragUpdateDetails details) {
onChanged(details.globalPosition);
}
#override
Widget build(BuildContext context) {
return new Center(
child: new CustomPaint(
size: new Size(xPos, yPos),
painter: new TouchControlPainter(xPos, yPos),
),
);
}
}
class TouchControlPainter extends CustomPainter {
static const markerRadius = 10.0;
final double xPos;
final double yPos;
TouchControlPainter(this.xPos, this.yPos);
#override
void paint(Canvas canvas, Size size) {
final paint = new Paint()
..color = Colors.blue[400]
..style = PaintingStyle.fill;
canvas.drawCircle(new Offset(xPos, yPos), markerRadius, paint);
}
#override
bool shouldRepaint(TouchControlPainter old) => xPos != old.xPos && yPos !=old.yPos;
}

Answer 1

Your code isn't using the GestureDetector anywhere.
You should use it to wrap the CustomPaint the build() function of TouchControlState.
import 'package:flutter/material.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/rendering.dart';
void main() {
runApp(new TouchTest());
}
class TouchTest extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Touch Test',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new Scaffold(
appBar: new AppBar(
title: const Text('Test'),
),
body: new Container(
decoration: new BoxDecoration(
color: Colors.white,
border: new Border.all(
color: Colors.black,
width: 2.0,
),
),
child: new Center(
child: new TouchControl()
),
),
)
);
}
}
class TouchControl extends StatefulWidget {
final double xPos;
final double yPos;
final ValueChanged<Offset> onChanged;
const TouchControl({Key key,
this.onChanged,
this.xPos:0.0,
this.yPos:0.0}) : super(key: key);
#override
TouchControlState createState() => new TouchControlState();
}
/**
* Draws a circle at supplied position.
*
*/
class TouchControlState extends State<TouchControl> {
double xPos = 0.0;
double yPos = 0.0;
GlobalKey _painterKey = new GlobalKey();
void onChanged(Offset offset) {
final RenderBox referenceBox = context.findRenderObject();
Offset position = referenceBox.globalToLocal(offset);
if (widget.onChanged != null)
widget.onChanged(position);
setState(() {
xPos = position.dx;
yPos = position.dy;
});
}
#override
bool hitTestSelf(Offset position) => true;
#override
void handleEvent(PointerEvent event, BoxHitTestEntry entry) {
if (event is PointerDownEvent ) {
// ??
}
}
void _handlePanStart(DragStartDetails details) {
onChanged(details.globalPosition);
}
void _handlePanEnd(DragEndDetails details) {
print('end');
// TODO
}
void _handlePanUpdate(DragUpdateDetails details) {
onChanged(details.globalPosition);
}
#override
Widget build(BuildContext context) {
return new ConstrainedBox(
constraints: new BoxConstraints.expand(),
child: new GestureDetector(
behavior: HitTestBehavior.opaque,
onPanStart:_handlePanStart,
onPanEnd: _handlePanEnd,
onPanUpdate: _handlePanUpdate,
child: new CustomPaint(
size: new Size(xPos, yPos),
painter: new TouchControlPainter(xPos, yPos),
),
),
);
}
}
class TouchControlPainter extends CustomPainter {
static const markerRadius = 10.0;
final double xPos;
final double yPos;
TouchControlPainter(this.xPos, this.yPos);
#override
void paint(Canvas canvas, Size size) {
final paint = new Paint()
..color = Colors.blue[400]
..style = PaintingStyle.fill;
canvas.drawCircle(new Offset(xPos, yPos), markerRadius, paint);
}
#override
bool shouldRepaint(TouchControlPainter old) => xPos != old.xPos && yPos !=old.yPos;
}