Dev Journal

import 'dart:convert';
import 'dart:io';

import 'package:flutter/services.dart';
import 'package:glob/glob.dart';
import 'package:glob/list_local_fs.dart';

/// A custom [AssetBundle] that reads files from a directory.
///
/// This is meant to be used in place of [rootBundle] for testing
class DiskAssetBundle extends CachingAssetBundle {
  static const _assetManifestDotJson = 'AssetManifest.json';

  /// Creates a [DiskAssetBundle] by loading files from [path].
  static Future<AssetBundle> loadFromPath(
    String path, {
    String? from,
  }) async {
    // Prepare the file search pattern
    path = _formatPath(path);
    String pattern = path;
    if (!pattern.endsWith('/')) {
      pattern += '/';
    }
    pattern += '**';

    // Load the assets
    final cache = <String, ByteData>{};
    await for (final entity in Glob(pattern).list(root: from)) {
      if (entity is File) {
        final bytes = await (entity as File).readAsBytes();

        // Keep only the asset name relative to the folder
        String name = _formatPath(entity.path);
        name = name.substring(name.indexOf(path) + path.length);
        cache[name] = ByteData.view(bytes.buffer);
      }
    }

    // Create the asset manifest
    final manifest = <String, List<String>>{};
    cache.forEach((key, _) {
      manifest[key] = [key];
    });
    cache[_assetManifestDotJson] = ByteData.view(
      Uint8List.fromList(jsonEncode(manifest).codeUnits).buffer,
    );

    return DiskAssetBundle._(cache);
  }

  /// Format a file path to only forward slashes
  static String _formatPath(String path) {
    return path.replaceAll(r'\', '/');
  }

  /// The cache of assets
  final Map<String, ByteData> _cache;

  /// Private constructor
  DiskAssetBundle._(this._cache);

  /// Load an asset from the cache
  @override
  Future<ByteData> load(String key) async {
    return _cache[key]!;
  }
}

import 'dart:io';
import 'dart:ui';

import 'package:flutter/services.dart';
import 'package:flutter_test/flutter_test.dart';

import 'disk_asset_bundle.dart';

void main() async {
  // Path to the goldens folder (i.e. project/test/_goldens)
  String goldens =
    '${Directory.current.path.replaceAll(r'\', '/')}/test/_goldens';

  // This is required before an asset bundle is available
  TestWidgetsFlutterBinding.ensureInitialized();

  // Create the custom assets bundle from the resources folder
  // (i.e. project/test/_resources)
  AssetBundle assets = await DiskAssetBundle.loadFromPath('test/_resources/');

  group('Example', () {

    /// This test will compare an image to a golden file
    test('Does match golden', () async {
      final image = await assets.load('image.png');

      await expectLater(
        image,
        matchesGoldenFile('$goldens/image-no-transform.png'),
      );
    });
  });
}