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

Overview

Cover layout matches CSS object-fit: cover: preserve aspect ratio, fill a fixed box, clip overflow. This package does not ship a widget named CoverBox. In Flutter, use BoxFit.cover, or when you need exact sizes use Size calculation tools (fastCoverScanSize / fastCoverScanScale).

Approach	When
Image + BoxFit.cover	Photos / network images—simplest
ClipRect + Transform.scale	Known intrinsic size, custom child (video frame, canvas)
fastCoverScanSize / fastCoverScanScale	Numeric layout or animation

---

BoxFit.cover (recommended)

Fill a fixed box and clip:

import 'package:flutter/material.dart';

SizedBox(
  width: 200,
  height: 200,
  child: ClipRRect(
    borderRadius: BorderRadius.circular(8),
    child: Image.network(
      imageUrl,
      fit: BoxFit.cover,
    ),
  ),
);

DecorationImage also supports fit: BoxFit.cover:

Container(
  width: double.infinity,
  height: 180,
  decoration: BoxDecoration(
    borderRadius: BorderRadius.circular(12),
    image: DecorationImage(
      image: NetworkImage(imageUrl),
      fit: BoxFit.cover,
    ),
  ),
);

---

With scan utilities

When the child is not an Image, or you want the same rules as scan-size:

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

Widget coverChild({
  required Size boxSize,
  required Size contentSize,
  required Widget child,
}) {
  final scale = fastCoverScanScale(boxSize, contentSize);

  return SizedBox(
    width: boxSize.width,
    height: boxSize.height,
    child: ClipRect(
      child: Center(
        child: Transform.scale(
          scale: scale,
          child: SizedBox(
            width: contentSize.width,
            height: contentSize.height,
            child: child,
          ),
        ),
      ),
    ),
  );
}

You can also set SizedBox dimensions from fastCoverScanSize(boxSize, contentSize) and clip to the container.

---

Notes

• Constraints: Give the child a bounded size (SizedBox, Expanded, etc.) so cover has a clip region.
• Performance: Large images benefit from cacheWidth / cacheHeight or smaller URLs—orthogonal to cover math but important in lists.
• vs contain: BoxFit.contain shows the whole image with possible letterboxing; cover fills the box and may crop.