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// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'package:flutter/foundation.dart';
import 'asset_bundle.dart';
import 'message_codecs.dart';
const String _kAssetManifestFilename = 'AssetManifest.bin';
/// Contains details about available assets and their variants.
/// See [Asset variants](
/// to learn about asset variants and how to declare them.
abstract class AssetManifest {
/// Loads asset manifest data from an [AssetBundle] object and creates an
/// [AssetManifest] object from that data.
static Future<AssetManifest> loadFromAssetBundle(AssetBundle bundle) {
return bundle.loadStructuredBinaryData(_kAssetManifestFilename, _AssetManifestBin.fromStandardMessageCodecMessage);
/// Lists the keys of all main assets. This does not include assets
/// that are variants of other assets.
/// The logical key maps to the path of an asset specified in the pubspec.yaml
/// file at build time.
/// See [Specifying assets](
/// and [Loading assets]( for more
/// information.
List<String> listAssets();
/// Retrieves metadata about an asset and its variants.
/// This method considers a main asset to be a variant of itself and
/// includes it in the returned list.
/// Throws an [ArgumentError] if [key] cannot be found within the manifest. To
/// avoid this, use a key obtained from the [listAssets] method.
List<AssetMetadata> getAssetVariants(String key);
// Lazily parses the binary asset manifest into a data structure that's easier to work
// with.
// The binary asset manifest is a map of asset keys to a list of objects
// representing the asset's variants.
// The entries with each variant object are:
// - "asset": the location of this variant to load it from.
// - "dpr": The device-pixel-ratio that the asset is best-suited for.
// New fields could be added to this object schema to support new asset variation
// features, such as themes, locale/region support, reading directions, and so on.
class _AssetManifestBin implements AssetManifest {
_AssetManifestBin(Map<Object?, Object?> standardMessageData): _data = standardMessageData;
factory _AssetManifestBin.fromStandardMessageCodecMessage(ByteData message) {
final dynamic data = const StandardMessageCodec().decodeMessage(message);
return _AssetManifestBin(data as Map<Object?, Object?>);
final Map<Object?, Object?> _data;
final Map<String, List<AssetMetadata>> _typeCastedData = <String, List<AssetMetadata>>{};
List<AssetMetadata> getAssetVariants(String key) {
// We lazily delay typecasting to prevent a performance hiccup when parsing
// large asset manifests. This is important to keep an app's first asset
// load fast.
if (!_typeCastedData.containsKey(key)) {
final Object? variantData = _data[key];
if (variantData == null) {
throw ArgumentError('Asset key "$key" was not found.');
_typeCastedData[key] = ((_data[key] ?? <Object?>[]) as Iterable<Object?>)
.cast<Map<Object?, Object?>>()
.map((Map<Object?, Object?> data) => AssetMetadata(
key: data['asset']! as String,
targetDevicePixelRatio: data['dpr']! as double,
main: false,
final AssetMetadata mainAsset = AssetMetadata(key: key,
targetDevicePixelRatio: null,
main: true
return <AssetMetadata>[mainAsset, ..._typeCastedData[key]!];
List<String> listAssets() {
return <String>[..._data.keys.cast<String>(), ..._typeCastedData.keys];
/// Contains information about an asset.
class AssetMetadata {
/// Creates an object containing information about an asset.
const AssetMetadata({
required this.key,
required this.targetDevicePixelRatio,
required this.main,
/// The device pixel ratio that this asset is most ideal for. This is determined
/// by the name of the parent folder of the asset file. For example, if the
/// parent folder is named "3.0x", the target device pixel ratio of that
/// asset will be interpreted as 3.
/// This will be null if the parent folder name is not a ratio value followed
/// by an "x".
/// See [Declaring resolution-aware image assets](
/// for more information.
final double? targetDevicePixelRatio;
/// The asset's key, which is the path to the asset specified in the pubspec.yaml
/// file at build time.
final String key;
/// Whether or not this is a main asset. In other words, this is true if
/// this asset is not a variant of another asset.
/// See [Asset variants](
/// for more about asset variants.
final bool main;