Removal of AssetManifest.json

Summary

Flutter apps currently include an asset file named AssetManifest.json. This file effectively contains a list of assets. Application code can read it using the AssetBundle API to determine what assets are available at runtime.

The AssetManifest.json file is an undocumented implementation detail. It’s no longer used by the framework, so it will no longer be generated in a future release. If your application code needs to get a list of available assets, use the AssetManifest API instead.

Migration guide

Reading asset manifest from Flutter application code

Before:

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

final String assetManifestContent = await rootBundle.loadString('AssetManifest.json');
final Map<Object?, Object?> decodedAssetManifest = 
    json.decode(assetManifestContent) as Map<String, Object?>;
final List<String> assets = decodedAssetManifest.keys.toList().cast<String>();

After:

import 'package:flutter/services.dart';

final AssetManifest assetManifest = await AssetManifest.loadFromAssetBundle(rootBundle);
final List<String> assets = assetManifest.listAssets();

Reading asset manifest information from Dart code outside of a Flutter app

The flutter CLI tool generates a new file, AssetManifest.bin. This replaces AssetManifest.json. This file contains the same information of AssetManifest.json, but in a different format. If you need to be able to read this file from code that is not part of a Flutter app(and therefore cannot use the AssetManifest API), you can still parse the file yourself.

The standard_message_codec package can be used to parse the contents.

import 'dart:io';
import 'dart:typed_data';

import 'package:standard_message_codec/standard_message_codec.dart';

void main() {
  // The path to AssetManifest.bin depends on the target platform.
  final String pathToAssetManifest = './build/web/assets/AssetManifest.bin';
  final Uint8List manifest = File(pathToAssetManifest).readAsBytesSync();
  final Map<Object?, Object?> decoded = const StandardMessageCodec()
      .decodeMessage(ByteData.sublistView(manifest));
  final List<String> assets = decoded.keys.cast<String>().toList();
}

Keep in mind that AssetManifest.bin is an implementation detail of Flutter. Reading this file is not an officially supported workflow. The contents or format of the file may change in a future release without announcement.

Timeline

AssetManifest.json will no longer be generated starting with the fourth stable release after 3.19 or one year after the release of 3.19, whichever comes last.

References

Relevant issues:

• When building a Flutter app, the flutter tool generates an AssetManifest.json file that is unused by the framework (Issue #143577)