How can i set Offset to specific value - flutter

Question

i have found this simple full code
import 'package:flutter/material.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return const Exp3();
}
}
typedef OffsetBuilder = Offset Function(Size size);
class Exp3 extends StatefulWidget {
const Exp3({Key? key}) : super(key: key);
#override
State<Exp3> createState() => _Exp3State();
}
class _Exp3State extends State<Exp3> {
OffsetBuilder _offsetBuilder = (_) => Offset.zero;
#override
Widget build(BuildContext context) {
return MaterialApp(
home: Builder(builder: (context) {
return Container( // parent container
color: Colors.red,
child: GestureDetector(
onPanUpdate: (details) {
final currentBuilder = _offsetBuilder;
_offsetBuilder = (Size containerSize) {
return currentBuilder.call(containerSize) + details.delta;
};
setState(() {});
},
child: CustomSingleChildLayout(
delegate: MyCustomSingleChildLayoutDelegate(
canGoOffParentBounds: false,
padding: const EdgeInsets.all(8.0),
offsetBuilder: _offsetBuilder,
),
child: Container(
width: 100,
height: 100,
color: Colors.yellow,
),
),
),
);
}
),
);
}
}
now my widget located at Offset.zero; for first snapshot by it's variable OffsetBuilder _offsetBuilder = (_) => Offset.zero;
Question: How i set OffsetBuilder _offsetBuilder = (_) => Offset.zero; to located at following value by first snapshot or first initialize
last point on the screen (max buttom) - 300
and here the class i use for SingleChildLayoutDelegate
class MyCustomSingleChildLayoutDelegate extends SingleChildLayoutDelegate {
final Offset Function(Size childSize) offsetBuilder;
final EdgeInsets padding;
final bool canGoOffParentBounds;
MyCustomSingleChildLayoutDelegate({
required this.offsetBuilder,
required this.padding,
required this.canGoOffParentBounds,
});
#override
BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
return BoxConstraints.loose(constraints.biggest).deflate(padding);
}
#override
Offset getPositionForChild(Size size, Size childSize) {
Offset childTopLeft = offsetBuilder.call(childSize);
if (canGoOffParentBounds) {
// no more checks on the position needed
return childTopLeft;
}
if (childTopLeft.dx + childSize.width > size.width - padding.right) {
final distance = -(childTopLeft.dx - (size.width - padding.right - childSize.width));
childTopLeft = childTopLeft.translate(distance, 0);
}
if (childTopLeft.dx < padding.left) {
final distance = padding.left - childTopLeft.dx;
childTopLeft = childTopLeft.translate(distance, 0);
}
if (childTopLeft.dy + childSize.height > size.height - padding.bottom) {
final distance = -(childTopLeft.dy - (size.height - padding.bottom - childSize.height));
childTopLeft = childTopLeft.translate(0, distance);
}
if (childTopLeft.dy < padding.top) {
final distance = padding.top - childTopLeft.dy;
childTopLeft = childTopLeft.translate(0, distance);
}
return childTopLeft;
}
#override
bool shouldRelayout(MyCustomSingleChildLayoutDelegate oldDelegate) {
return oldDelegate.offsetBuilder != offsetBuilder;
}
}