Is there any way to paint a Widget at a given position on a Canvas?
More specifically, I want to paint the child widgets of Marker's related to a FlutterMap on a separate Canvas in front of the actual FlutterMap widget. Here's an attempt at creating a CustomPainter that would do that, but I can't figure out how to actually paint the widgets on the canvas. Using the RenderObject requires a PaintingContext, which I don't know how to create/retrieve:
class MarkerPainter extends CustomPainter {
MapController mc;
BuildContext context;
List<Marker> markers;
MarkerPainter(this.context, this.mc, this.markers);
@override
void paint(Canvas canvas, Size size) {
if( markers != null && markers.isNotEmpty ){
for(int i=0; i<markers.length; i++){
Marker marker = markers[i];
Offset o = myCalculateOffsetFromLatLng(marker.point, mc, context);
// Won't work, this needs a PaintingContext...
marker.builder(context).createElement().renderObject.paint(context, o);
}
}
}
@override
bool shouldRepaint(MarkerPainter oldDelegate) => oldDelegate.markers != markers;
}
You cannot do this with a CustomPainter.
This class is only a simplification of the real deal: RenderObject, which has access to everything related to painter (and layout+more).
What you should do is, instead of a CustomPainter, create a RenderBox (a 2d RenderObject)
In your case, what you want is to draw a list of widgets. In that situation, you will need to create:
children to drawWrapping up, a widget used this way:
MyExample(
children: [
Text('foo'),
Text('bar'),
],
),
would be written this way:
import 'package:flutter/rendering.dart';
import 'package:flutter/material.dart';
class MyExample extends MultiChildRenderObjectWidget {
MyExample({
Key key,
@required List<Widget> children,
}) : super(key: key, children: children);
@override
RenderMyExample createRenderObject(BuildContext context) {
return RenderMyExample();
}
}
class MyExampleParentData extends ContainerBoxParentData<RenderBox> {}
class RenderMyExample extends RenderBox
with ContainerRenderObjectMixin<RenderBox, MyExampleParentData> {
@override
void setupParentData(RenderObject child) {
if (child.parentData is! MyExampleParentData) {
child.parentData = MyExampleParentData();
}
}
@override
void performLayout() {
size = constraints.biggest;
for (var child = firstChild; child != null; child = childAfter(child)) {
child.layout(
// limit children to a max height of 50
constraints.copyWith(maxHeight: 50),
);
}
}
@override
void paint(PaintingContext context, Offset offset) {
// Paints all children in order vertically, separated by 50px
var verticalOffset = .0;
for (var child = firstChild; child != null; child = childAfter(child)) {
context.paintChild(child, offset + Offset(0, verticalOffset));
verticalOffset += 50;
}
}
}
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