How to get offset of a current widget - dart

Question

I am trying to draw a widget whenever a user presses the screen.
Currently I am doing this by storing a list of widgets and when ontapup is fired on the gesture i am adding to a list of widgets.
Widget build(BuildContext context) {
Widget draw = new Text("A");
List<Widget> children = new List<Widget>();
return new Scaffold(
appBar: new AppBar(
title: const Text('Heading'),
leading: new Icon(Icons.question_answer),
),
body: new GestureDetector(
onTapUp: (details) {
setState(() {
children.add(new Positioned(
left: details.globalPosition.dx,
top: details.globalPosition.dy,
child: draw,
));
});
},
child: new Stack(children: children)
...
So my code is working I am drawing the widget when I click but my problem is that when adding the new Positioned() to stack the position is based on the screen which does not include the appbar offset. Is there a way to get the stacks initial x/y position? Or is there a way to get the appbars height? How do I get a widgets position or height/width?

Answer 1

Ok for anyone else who has the same issue I needed to create my own widget and use
context.findRenderObject()
and
globalToLocal()
Just FYI global to local did not work while in the one solution I needed to make it its own widget.

Answer 2

To get the offset of a widget, you must get the renderObject, cast it as a RenderBox, and then convert it's local position to a global position. Like this:
#override
Widget build(BuildContext context) {
RenderBox renderBox = context.findRenderObject();
Offset widgetOffset = renderBox.localToGlobal(Offset.zero);
print("X: ${widgetOffset.dx}");
print("Y: ${widgetOffset.dy}");
}
If you need the position of a child widget, you can wrap that child in a LayoutBuilder, like this:
Container(
child: LayoutBuilder(
builder: (BuildContext context, BoxConstraints box) {
RenderBox renderBox = context.findRenderObject();
Offset widgetOffset = renderBox.localToGlobal(Offset.zero);
print("X: ${widgetOffset.dx}");
print("Y: ${widgetOffset.dy}");
}
)
)