Flutter Measure Size widget

measure_size_widget.dart

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

class MeasureSizeRenderObject extends RenderProxyBox {
  MeasureSizeRenderObject(this.onChanged);

  Size? oldSize;
  final ValueChanged<Size> onChanged;

  @override
  void performLayout() {
    super.performLayout();

    final newSize = child!.size;
    if (oldSize == newSize) {
      return;
    }

    oldSize = newSize;
    WidgetsBinding.instance?.addPostFrameCallback((_) {
      onChanged(newSize);
    });
  }
}

class MeasureSize extends SingleChildRenderObjectWidget {
  const MeasureSize({
    required this.onChanged,
    required Widget child,
    Key? key,
  }) : super(key: key, child: child);

  final ValueChanged<Size> onChanged;

  @override
  RenderObject createRenderObject(BuildContext context) {
    return MeasureSizeRenderObject(onChanged);
  }
}