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lib/rfb/child_size_notifier_widget.dart

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+import 'package:flutter/widgets.dart';
+
+/// Widget that exposes its child's size via a [ValueNotifier].
+///
+/// Inspired by: https://stackoverflow.com/a/58004112/373138
+class SizeTrackingWidget extends StatefulWidget {
+  final Widget _child;
+  final ValueNotifier<Size> _sizeValueNotifier;
+
+  const SizeTrackingWidget({
+    super.key,
+    required final ValueNotifier<Size> sizeValueNotifier,
+    required final Widget child,
+  })  : _child = child,
+        _sizeValueNotifier = sizeValueNotifier;
+
+  @override
+  State<StatefulWidget> createState() => _SizeTackingState();
+}
+
+class _SizeTackingState extends State<SizeTrackingWidget> {
+  @override
+  void initState() {
+    super.initState();
+    WidgetsBinding.instance.addPostFrameCallback((final _) {
+      widget._sizeValueNotifier.value =
+          (context.findRenderObject() as RenderBox?)!.size;
+    });
+  }
+
+  @override
+  Widget build(final BuildContext context) => widget._child;
+}