smart_measurer 0.1.2

Measure a widget's real size and react to it, with frame-accurate precision tracking and a zero-delay, size-aware decoration painter for pure-drawing use cases.

smart_measurer

Measure a widget's real size in Flutter and react to it — with a clean estimate during the one-frame measurement delay, and a zero-delay variant for when you only need to draw.

Why this package

Flutter always runs build before layout within a frame. It is structurally impossible to know a widget's size when building it for the first time — any solution that returns a new Widget based on a measured size necessarily has a one-frame delay. smart_measurer doesn't hide this constraint: it gives you an explicit isPrecise flag so you know, every frame, whether the displayed size is measured or estimated.

For cases where you only need to paint a size-dependent decoration (not build new widgets), MeasuredDecoration avoids that delay entirely by staying inside the RenderObject layer.

The three widgets

Widget	Delay	Use case
SmartMeasurer	1 frame	Full control, access to measuredChild
SimpleMeasurer	1 frame	Simplified variant (auto-overlaid child)
MeasuredDecoration	None	Pure Canvas drawing based on child size

Installation

Add the package to your pubspec.yaml:

dependencies:
  smart_measurer: ^0.1.0

Usage

SimpleMeasurer — decoration that adapts to a text's size

import 'package:smart_measurer/smart_measurer.dart';

SimpleMeasurer(
  child: const Text('Hello world'),
  builder: (context, size, isPrecise, constraints) {
    return Container(
      width: size.width + 24,
      height: size.height + 24,
      decoration: BoxDecoration(
        color: isPrecise ? Colors.blue.shade100 : Colors.grey.shade200,
        borderRadius: BorderRadius.circular(8),
      ),
    );
  },
)

isPrecise is false during the (brief) window where the displayed size is an estimate — for example, right after an external constraint change, before the next measurement arrives. Use this flag to, for instance, avoid animating a transition until the size is certain.

SmartMeasurer — full control

SmartMeasurer(
  child: const Icon(Icons.star, size: 48),
  builder: (context, measuredChild, size, isPrecise, constraints) {
    return Stack(
      children: [
        CustomPaint(
          size: size,
          painter: HaloPainter(),
        ),
        Center(child: measuredChild), // measuredChild inserted exactly once
      ],
    );
  },
)

⚠️ measuredChild must appear exactly once in the tree returned by builder. Inserting it multiple times triggers a Flutter duplicate-key error (this is intended behavior: an immediate signal rather than a silent bug).

MeasuredDecoration — drawing with no delay

When you only need to paint a background/halo/border that depends on the size (not build a widget), avoid the one-frame delay:

MeasuredDecoration(
  painter: (canvas, size) {
    final paint = Paint()..color = Colors.deepPurple.shade100;
    canvas.drawRRect(
      RRect.fromRectAndRadius(
        Rect.fromLTWH(-12, -12, size.width + 24, size.height + 24),
        const Radius.circular(12),
      ),
      paint,
    );
  },
  child: const Text('No delay, never a flicker'),
)

Custom estimation

By default, the estimate used during the measurement delay reuses the last known precise size (if compatible with the new constraints), otherwise it falls back to Size.zero. Customize this behavior with estimateBuilder:

SimpleMeasurer(
  estimateBuilder: (constraints, previousSize) {
    // E.g.: estimate based on the character count of known text,
    // an average font width, etc.
    return previousSize ?? const Size(100, 40);
  },
  child: const Text('...'),
  builder: (context, size, isPrecise, constraints) => /* ... */ Container(),
)

See also

Check out the example/ folder for a full demo of all three widgets.

Known limitations

• SmartMeasurer / SimpleMeasurer have a one-frame delay after any change to external constraints or to child — this is a constraint of the Flutter pipeline, not a bug (see the "Why this package" section).
• MeasuredDecoration can only draw (Canvas), it cannot insert new interactive widgets.

Contributing

Issues and pull requests are welcome on the project's GitHub repository.