Source on GitHub·lib/src/utils/fast_scan/

Overview

Top-level helpers fastCoverScanSize and fastCoverScanScale implement cover sizing similar to CSS object-fit: cover: given a parent box and a child’s intrinsic size, they compute either the target size or a uniform scale factor while preserving the child’s aspect ratio. Both take Flutter Size (width × height) and share the same algorithm.

API	Returns	Typical use
fastCoverScanSize	Size	Layout (SizedBox, custom paint)
fastCoverScanScale	double	Transform.scale, animation, multiply original dimensions

Clip overflow with ClipRect / ClipRRect; common cover patterns are in Cover layout.

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Algorithm

1. Zero dimensions — If any side of parentSize or childSize is 0, fastCoverScanSize returns childSize unchanged; fastCoverScanScale returns 1.0 (avoids division by zero).
2. Aspect ratio — aspect = width / height for parent and child.
3. Pick an axis (cover without distortion):
   • Parent aspect ≥ child aspect: align by width, scale = parent.width / child.width
   • Parent aspect < child aspect: align by height, scale = parent.height / child.height
4. Relationship — With non-zero sizes, fastCoverScanSize(parent, child) equals
   Size(child.width * s, child.height * s) where s = fastCoverScanScale(parent, child).
   s may be less than 1 when the child is already larger but still needs cover alignment.

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fastCoverScanSize

Returns the smallest uniformly scaled Size that covers parentSize while keeping the child’s aspect ratio (one dimension matches the parent; the other may extend beyond and is usually clipped).

Examples

Scale by width

import 'package:fast_package/fast_package.dart';

// Parent 100×100, child 50×80 — parent is “squarer”, align by width
fastCoverScanSize(Size(100, 100), Size(50, 80));
// Size(100.0, 160.0)

Scale by height

// Parent 200×100, child 100×100 — child is “squarer”, align by height
fastCoverScanSize(Size(200, 100), Size(100, 100));
// Size(200.0, 200.0)

Zero dimension

fastCoverScanSize(Size(100, 0), Size(50, 80));
// Size(50.0, 80.0) — original childSize

API reference

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fastCoverScanSize

Size fastCoverScanSize(Size parentSize, Size childSize);

Parameter	Type	Required	Description
parentSize	Size	yes	Parent (viewport) size.
childSize	Size	yes	Child’s original size.

Return	Type	Description
Cover size	Size	Same aspect ratio as childSize; if any side is zero, returns childSize.

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fastCoverScanScale

Returns the uniform scale factor s from the same rules; multiplying child width and height by s matches fastCoverScanSize.

Examples

Scale up to cover

fastCoverScanScale(Size(100, 100), Size(50, 50));
// 2.0 — same aspect ratio, by width: 100 / 50

Wide child, height axis

// Parent 100×100, child 200×100 — height alignment is enough
fastCoverScanScale(Size(100, 100), Size(200, 100));
// 1.0

Flat child, height axis

// Parent 100×100, child 100×50
fastCoverScanScale(Size(100, 100), Size(100, 50));
// 2.0 — parent.height / child.height

API reference

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fastCoverScanScale

double fastCoverScanScale(Size parentSize, Size childSize);

Parameter	Type	Required	Description
parentSize	Size	yes	Parent size.
childSize	Size	yes	Child’s original size.

Return	Type	Description
Scale factor	double	1.0 if any side is zero; otherwise width- or height-based ratio (may be greater or less than 1).

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Flutter notes

When you know container and image pixel sizes, compute scale first and clip overflow:

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

Widget coverImage({
  required Size containerSize,
  required Size imageSize,
  required ImageProvider image,
}) {
  final scale = fastCoverScanScale(containerSize, imageSize);

  return SizedBox(
    width: containerSize.width,
    height: containerSize.height,
    child: ClipRect(
      child: Center(
        child: Transform.scale(
          scale: scale,
          child: Image(
            image: image,
            width: imageSize.width,
            height: imageSize.height,
          ),
        ),
      ),
    ),
  );
}

For simple network images, Image with BoxFit.cover (see Cover layout) is often enough. These functions shine when you need numeric sizes for custom layout, animation, or non-Image children (canvas, video frames).

Typical cases: full-bleed backgrounds, video cover fit, thumbnail grids, responsive banners with fixed height.