Speed up first asset load by encoding asset manifest in binary rather than JSON (flutter#113637)

Author: andrewkolos

dev/benchmarks/microbenchmarks/lib/foundation/decode_and_parse_asset_manifest.dart

@@ -3,9 +3,9 @@
 // found in the LICENSE file.
 
 import 'dart:convert';
+import 'dart:typed_data';
 
-import 'package:flutter/foundation.dart';
-import 'package:flutter/services.dart' show PlatformAssetBundle;
+import 'package:flutter/services.dart' show PlatformAssetBundle, StandardMessageCodec;
 import 'package:flutter/widgets.dart';
 
 import '../common.dart';
@@ -18,16 +18,14 @@ void main() async {
   final BenchmarkResultPrinter printer = BenchmarkResultPrinter();
   WidgetsFlutterBinding.ensureInitialized();
   final Stopwatch watch = Stopwatch();
-  final PlatformAssetBundle bundle = PlatformAssetBundle();
 
-  final ByteData assetManifestBytes = await bundle.load('money_asset_manifest.json');
+  final ByteData assetManifest = await loadAssetManifest();
+
   watch.start();
   for (int i = 0; i < _kNumIterations; i++) {
-    bundle.clear();
-    final String json = utf8.decode(assetManifestBytes.buffer.asUint8List());
-    // This is a test, so we don't need to worry about this rule.
+    // This is effectively a test.
     // ignore: invalid_use_of_visible_for_testing_member
-    await AssetImage.manifestParser(json);
+    AssetImage.parseAssetManifest(assetManifest);
   }
   watch.stop();
 
@@ -40,3 +38,49 @@ void main() async {
 
   printer.printToStdout();
 }
+
+final RegExp _extractRatioRegExp = RegExp(r'/?(\d+(\.\d*)?)x$');
+
+Future<ByteData> loadAssetManifest() async {
+  double parseScale(String key) {
+    final Uri assetUri = Uri.parse(key);
+    String directoryPath = '';
+    if (assetUri.pathSegments.length > 1) {
+      directoryPath = assetUri.pathSegments[assetUri.pathSegments.length - 2];
+    }
+    final Match? match = _extractRatioRegExp.firstMatch(directoryPath);
+    if (match != null && match.groupCount > 0) {
+      return double.parse(match.group(1)!);
+    }
+    return 1.0;
+  }
+
+  final Map<String, dynamic> result = <String, dynamic>{};
+  final PlatformAssetBundle bundle = PlatformAssetBundle();
+
+  // For the benchmark, we use the older JSON format and then convert it to the modern binary format.
+  final ByteData jsonAssetManifestBytes = await bundle.load('money_asset_manifest.json');
+  final String jsonAssetManifest = utf8.decode(jsonAssetManifestBytes.buffer.asUint8List());
+
+  final Map<String, dynamic> assetManifest = json.decode(jsonAssetManifest) as Map<String, dynamic>;
+
+  for (final MapEntry<String, dynamic> manifestEntry in assetManifest.entries) {
+    final List<dynamic> resultVariants = <dynamic>[];
+    final List<String> entries = (manifestEntry.value as List<dynamic>).cast<String>();
+    for (final String variant in entries) {
+      if (variant == manifestEntry.key) {
+        // With the newer binary format, don't include the main asset in it's
+        // list of variants. This reduces parsing time at runtime.
+        continue;
+      }
+      final Map<String, dynamic> resultVariant = <String, dynamic>{};
+      final double variantDevicePixelRatio = parseScale(variant);
+      resultVariant['asset'] = variant;
+      resultVariant['dpr'] = variantDevicePixelRatio;
+      resultVariants.add(resultVariant);
+    }
+    result[manifestEntry.key] = resultVariants;
+  }
+
+  return const StandardMessageCodec().encodeMessage(result)!;
+}

dev/integration_tests/flutter_gallery/test/example_code_parser_test.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:async';
+
 import 'package:flutter/services.dart';
 import 'package:flutter_gallery/gallery/example_code_parser.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -58,4 +60,9 @@ class TestAssetBundle extends AssetBundle {
 
   @override
   String toString() => '$runtimeType@$hashCode()';
+
+  @override
+  Future<T> loadStructuredBinaryData<T>(String key, FutureOr<T> Function(ByteData data) parser) async {
+    return parser(await load(key));
+  }
 }