Flutter: AssetBundleFindHelpers extension class on AssetBundle

asset_bundle_find_helpers.dart

import 'package:flutter/services.dart';

/// Implements @matthew-carroll 's proposal in https://github.com/flutter/flutter/issues/137043
///
/// Example use/results
/// 
/// in main():
/// 
/// await rootBundle.initAssetBundleFindHelpers(); //strictly speaking this does not need to be awaited
/// 
/// 
/// somewhere in a build():
/// 
/// debugPrint('assets() list is ${DefaultAssetBundle.of(context).assets}');
/// debugPrint('Assets with PNG extension ${DefaultAssetBundle.of(context).findAssetsByExtension('png')}');
/// debugPrint('Assets with SVG extension ${DefaultAssetBundle.of(context).findAssetsByExtension('.svg')}');
/// debugPrint('Assets with "barcode" name ${DefaultAssetBundle.of(context).findAssetsByName('barcode')}');
/// debugPrint('Assets with "barcode.png" name ${DefaultAssetBundle.of(context).findAssetsByName('barcode.png')}');
/// debugPrint('Assets with "images/barcode" name ${DefaultAssetBundle.of(context).findAssetsByName('images/barcode')}');
/// debugPrint('Assets with "images/barcode.png" name ${DefaultAssetBundle.of(context).findAssetsByName('images/barcode.png')}');
/// debugPrint('Assets with "car_icon" name ${DefaultAssetBundle.of(context).findAssetsByName('car_icon')}');
/// debugPrint('Assets with "arrow-left" extension ${DefaultAssetBundle.of(context).findAssetsByName('arrow-left')}');
///   
/// 
/// Console displays:
/// 
/// assets() list is [assets/barcode.svg,assets/images/ae.png,assets/images/barcode-big.png,assets/images/barcode.png,
/// 				assets/images/other-barcode.png,assets/images/calendar.png,assets/images/car_icon.png,assets/images/car_icon_on_white.png,
/// 				assets/images/card.png,assets/barcode.svg,assets/images/24-support.svg,assets/images/alarm.svg,assets/images/arrow-down.svg,
/// 				assets/images/arrow-left.svg]
/// Assets with PNG extension [assets/images/ae.png,assets/images/barcode-big.png,assets/images/barcode.png,assets/images/other-barcode.png
///                 assets/images/calendar.png,assets/images/car_icon.png,assets/images/car_icon_on_white.png,assets/images/card.png]
/// Assets with SVG extension [assets/barcode.svg,assets/images/24-support.svg,assets/images/alarm.svg,
///                 assets/images/arrow-down.svg,assets/images/arrow-left.svg]
/// Assets with "barcode" name [assets/barcode.svg, assets/images/barcode.png]
/// Assets with "barcode.png" name [assets/images/barcode.png]
/// Assets with "images/barcode" name [assets/images/barcode.png]
/// Assets with "images/barcode.png" name [assets/images/barcode.png]
/// Assets with "car_icon" name [assets/images/car_icon.png]
/// Assets with "arrow-left" extension [assets/images/arrow-left.svg]

extension AssetBundleFindHelpers on AssetBundle {
    // Some cached info for extension (static maps because extensions can't add instance fields)
    static Map<AssetBundle,AssetManifest> bundleToManifestMap = {};
    static Map<AssetBundle,List<String>> bundleToAssetListMap = {};
    static RegExp parseAssetNameWithExtensionRegEx = RegExp(r'^(.+\/)*(.+)\.(.+)$'); // group 1 is path, group 2 is filename, group 3 is extension
    static RegExp parseAssetPathNameNoExtRegEx = RegExp(r'^(.+\/)*(.+)$'); // group 1 is path, group 2 is filename

    /// Retrieve and cache the AssetManifest for the bundle.
    /// It is required to call this before any other helper methods.
    /// Typically this would be something like `await rootBundle.initAssetBundleFindHelpers();`
    Future<AssetManifest> initAssetBundleFindHelpers() {
    if(bundleToManifestMap.containsKey(this)) {
        return Future.value(bundleToManifestMap[this]!);
    }
    final retAM = AssetManifest.loadFromAssetBundle(this);
    retAM.then((am) {
      // store AssetManifest in our cache map
      bundleToManifestMap[this] = am;
    });
    return retAM;
  }

  /// Return list of assets for which the filename matches the specified [regex].
  List<String> findAssetsByNameThatMatch(RegExp regex) {
    if(!bundleToManifestMap.containsKey(this)) {
      throw ('Call initAssetBundleFindHelpers() for this AssetBundle before calling this api extension.');
    }
    return assets.where((item) => regex.hasMatch(item)).toList();
  }

  /// Return list of assets for which match the specified [name].
  /// The assets whole filename must match the specified [name].
  /// Optionally a required prefix path can be added to [name].
  /// Optionally a required extension can be added to [name].
  List<String> findAssetsByName(String name) {
    if(!bundleToManifestMap.containsKey(this)) {
      throw ('Call initAssetBundleFindHelpers() for this AssetBundle before calling this api extension.');
    }

    String? targetPath, targetExt;
    String targetName = name;
    var match = parseAssetNameWithExtensionRegEx.firstMatch(name);
    if(match!=null) {
      // name has extension (and possibly path)
      targetPath = match.group(1);
      targetName = match.group(2) ?? name;
      targetExt = match.group(3);
    } else {
      // name has no extension (and possibly path)
      match = parseAssetPathNameNoExtRegEx.firstMatch(name);
      if(match!=null) {
        targetPath = match.group(1);
        targetName = match.group(2) ?? name;
      }
    }
    // escape any regex chars in name
    final escapedName = RegExp.escape(targetName);
    // make regexp to match this name with any prefix path or extension
    // (name can contain subpath)
    final regExForThisName = RegExp(r'^(.+\/)*' + escapedName + r'\.(.+)');
    final prelimMatches = findAssetsByNameThatMatch(regExForThisName);
    // now prune down prelim match list as required by possible path or extension on incoming name
    return prelimMatches.where((item) {
          final match = parseAssetNameWithExtensionRegEx.firstMatch(item);
          return match!=null &&
                    match.group(2)==targetName &&
                    (targetPath==null || (match.group(1)!=null && match.group(1)!.endsWith(targetPath))) &&
                    (targetExt==null || (match.group(3)!=null && match.group(3)==targetExt));
        }).toList(); 
  }

  /// Return list of all assets with specified [extension].
  /// The [extension] can optionally included the leading '.', ie. 'png' or '.png'.
  List<String> findAssetsByExtension(String extension) {
    if(!bundleToManifestMap.containsKey(this)) {
      throw ('Call initAssetBundleFindHelpers() for this AssetBundle before calling this api extension.');
    }
    if(!extension.startsWith('.')) extension = '.$extension';
    return assets.where((item) => item.endsWith(extension)).toList();
  }

  // Return a list of all assets in the manifest.
  List<String> get assets {
    if(!bundleToManifestMap.containsKey(this)) {
      throw ('Call initAssetBundleFindHelpers() for this AssetBundle before calling this api extension.');
    }
    // cache the asset list so we don't keep recreating it
    if(bundleToAssetListMap.containsKey(this)) {
      return bundleToAssetListMap[this]!;
    }
    return (bundleToAssetListMap[this] = bundleToManifestMap[this]!.listAssets());
  }
}