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[widget_previews] [Proposal] Consider providing better errors if widget preview is unconstrained #192712
Description
Use case
People writing widget previews, that accidentally forget that a preview needs a fixed size constraint.
For example:
import 'package:flutter/widget_previews.dart';
import 'package:flutter/widgets.dart';
@Preview(name: 'Oops')
Widget buildPreview() => const Foo();
class Foo extends StatelessWidget {
const Foo();
@override
Widget build(BuildContext context) {
return SizedBox.expand(child: Text('Oops'));
}
}Then you get something like this in stdout of the widget previewer
The following RenderObject was being processed when the exception was fired: RenderConstrainedBox#f9f73 relayoutBoundary=up58 NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE:
creator: SizedBox ← Foo ← @Preview ← EnableWidgetInspectorScope ←
WidgetPreviewTheming ← Container-[GlobalKey#9acff] ← ValueListenableBuilder<bool> ← SizedBox ←
_WidgetPreviewWrapper ← ValueListenableBuilder<bool> ← MediaQuery ←
ValueListenableBuilder<Brightness> ← ⋯
parentData: <none> (can use size)
constraints: BoxConstraints(unconstrained)
size: MISSING
additionalConstraints: BoxConstraints(w=Infinity, h=34.0)
Proposal
Consider intercepting errors that indicate an unconstrained size and fail with a descriptive message.
For example the following is a red flag about the unconstrained layout
constraints: BoxConstraints(unconstrained)
size: MISSING
additionalConstraints: BoxConstraints(w=Infinity, h=34.0)
Activity
[Widget Preview] Provide descriptive error when widget preview is unc…
github-actions commentedon Oct 16, 2026
This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of
flutter doctor -vand a minimal reproduction of the issue.[Widget Preview] Provide descriptive error when widget preview is unc…