Removal of AssetManifest.json

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 of Flutter. If your application code needs to get a list of available assets, use the AssetManifest API instead.

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();

The flutter CLI tool generates a new file, AssetManifest.bin. This replaces AssetManifest.json. This file contains the same information as AssetManifest.json, but in a different format. If you need to read this file from code that is not part of a Flutter app, and therefore can't 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.

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 later.

Relevant issues:

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