measurePopupContent function

Future<Size> measurePopupContent({ required BuildContext context, required Widget builder(BuildContext), required EdgeInsetsGeometry padding, required double maxWidth, })

Measures the rendered size of a popup widget off-screen.

Temporarily inserts the widget into the Overlay at a hidden position, waits one frame for layout, then reads the resulting RenderBox.size. The measurement entry is removed immediately afterward.

Returns the laid-out Size of the content (including padding).

Example usage:

final contentSize = await measurePopupContent(
  context: context,
  builder: (context) => MyPopupContent(),
  padding: const EdgeInsets.all(8.0),
  maxWidth: MediaQuery.of(context).size.width * 0.8,
);

Implementation

Future<Size> measurePopupContent({
  /// The build context used to access the overlay and render tree.
  required BuildContext context,

  /// The widget builder that returns the popup content to measure.
  required Widget Function(BuildContext) builder,

  /// Padding applied around the widget when measuring its size.
  required EdgeInsetsGeometry padding,

  /// The maximum width constraint used during measurement.
  required double maxWidth,
}) async {
  /// Key to locate the render object for size measurement.
  final measureKey = GlobalKey();

  /// Access the nearest [Overlay] for temporary insertion.
  final overlay = Overlay.of(context);

  /// Off-screen overlay entry used purely for layout measurement.
  final measureEntry = OverlayEntry(
    builder: (_) => Positioned(
      left: -10000,
      top: -10000,
      width: maxWidth,
      child: Material(
        color: Colors.transparent,
        child: Padding(
          key: measureKey,
          padding: padding,
          child: builder(context),
        ),
      ),
    ),
  );

  /// Insert temporarily for one frame to trigger layout.
  overlay.insert(measureEntry);

  /// Wait for layout to complete.
  await WidgetsBinding.instance.endOfFrame;

  try {
    /// Read the final laid-out size from the render tree.
    final renderBox =
        measureKey.currentContext!.findRenderObject() as RenderBox;
    return renderBox.size;
  } finally {
    /// Clean up: remove the measurement entry regardless of success or failure.
    measureEntry.remove();
  }
}