Optimize JSONMessageCodec UTF-8 conversion by FelixMittermeier · Pull Request #190529 · flutter/flutter

FelixMittermeier · 2026-08-04T14:00:10Z

JSONMessageCodec currently serializes values to a JSON String and then passes that string through StringCodec for UTF-8 conversion. Decoding performs the reverse sequence.

This change uses cached JSON/UTF-8 converters directly, reducing conversion work and temporary allocations while preserving the existing wire format. JSONMethodCodec also benefits because it delegates its encoding and decoding to JSONMessageCodec.

This affects framework channels such as text input (TextField/EditableText), clipboard and haptics, system UI, navigation, localization, and keyboard/system events. It does not affect StandardMessageCodec or standalone jsonEncode/jsonDecode calls.

Median of nine samples comparing the old and new implementations in the same process:

Environment and payload	Encode	Decode
macOS arm64 AOT, 186 bytes	1.03 → 0.75 µs (-27%)	0.58 → 0.42 µs (-29%)
macOS arm64 AOT, 4.15 KB	39.4 → 25.6 µs (-35%)	29.4 → 15.3 µs (-48%)
Physical iPad, AOT profile, 186 bytes	1.35 → 0.98 µs (-28%)	0.79 → 0.57 µs (-28%)
Physical iPad, AOT profile, 4.15 KB	48.2 → 30.7 µs (-36%)	36.4 → 20.1 µs (-45%)

Also added a regression test that verifies the exact UTF-8 wire format, a four-byte Unicode character crossing the encoder’s default 256-byte chunk boundary, and decoding from a bounded ByteData view. The codec tests passed on both the Dart VM and Chrome, along with the related text-input/platform-channel tests and analysis.

Pre-launch Checklist

gemini-code-assist

Code Review

This pull request refactors JSONMessageCodec to use JsonUtf8Encoder and a fused UTF-8/JSON decoder directly instead of routing through StringCodec, and adds a test to verify UTF-8 encoding and chunk boundaries. Feedback suggests optimizing encodeMessage by checking if the result of _encoder.convert is already a Uint8List to avoid redundant memory allocations and copies.

elliette · 2026-08-05T16:59:39Z

Hi @FelixMittermeier! Thanks for the contribution! Do you mind listing an issue in the PR description (see "I listed at least one issue that this PR fixes in the description above" in checklist above). If one does not already exist, please file one. Thanks!

FelixMittermeier · 2026-08-05T19:24:49Z

@elliette Ah sorry, I clicked too fast yesterday. I unchecked this point in the PR description now. There is no existing issue for exactly this topic and IMO it also does not really make sense here to create a new one because there is no actual problem/bug. The current PR just optimises performance but does not fix any broken behavior etc.

Just found this quick win while benchmarking my app and searching for possible improvements.

elliette

Thanks, LGTM!

justinmc

Looks good besides a comment about the test below. Thanks for the performance improvement and for all of the profiling you did.

justinmc · 2026-08-14T22:46:01Z

+    test('should preserve UTF-8 encoding and ByteData bounds across encoder chunks', () {
+      // Place a four-byte character across JsonUtf8Encoder's default 256-byte chunk boundary.
+      final message = '${'a' * 254}\u{1F602}';
+      final Uint8List expected = utf8.encode(jsonEncode(message));
+
+      final ByteData encoded = json.encodeMessage(message)!;
+      expect(Uint8List.sublistView(encoded), orderedEquals(expected));
+
+      final padded = Uint8List(expected.length + 2)
+        ..[0] = 0x5b
+        ..setRange(1, expected.length + 1, expected)
+        ..[expected.length + 1] = 0x5d;
+      final view = ByteData.sublistView(padded, 1, expected.length + 1);
+
+      expect(json.decodeMessage(view), message);
+    });


This test also passes on master. Is there anything else you can test that will fail if your change gets reverted?

Not sure how to make a unit test fail when this change is reverted, since the optimization preserves the same result before and after. Do you think of anything specifically?

justinmc · 2026-08-14T23:01:57Z

+  static final JsonUtf8Encoder _encoder = JsonUtf8Encoder();
+  static final Converter<List<int>, Object?> _decoder = utf8.decoder.fuse(json.decoder);


I guess it is best practice to keep these around. The memory impact should be negligible.

auto-submit · 2026-08-14T23:32:58Z

autosubmit label was removed for flutter/flutter/190529, because The base commit of the PR is older than 7 days and can not be merged. Please merge the latest changes from the main into this branch and resubmit the PR.

justinmc · 2026-08-18T22:23:01Z

There's a failure that looks like an infra problem, but I can't rerun it for some reason. @FelixMittermeier could you push a merge commit to cause it to run again?

FelixMittermeier · 2026-08-19T08:03:18Z

@justinmc Alright done 👍

Roll Flutter from c2437523d308 to 65c9a8dc60bc (195 revisions) flutter/flutter@c243752...65c9a8d 2026-08-22 [email protected] Roll ICU from d578f2e8b7bd to 8cc91d9b6ab9 (1 revision) (flutter/flutter#191542) 2026-08-22 [email protected] Roll Skia from 666a9b3d5cf0 to ad2106c0bb64 (3 revisions) (flutter/flutter#191527) 2026-08-22 [email protected] Roll Fuchsia Test Scripts from KaOq3EE4qJ9fnaaaK... to 0iCv10IlKfiilEBOU... (flutter/flutter#191524) 2026-08-22 [email protected] [flutter_tools] Add tests for negative lookahead regex in test runner and batch entrypoints (flutter/flutter#191438) 2026-08-22 [email protected] Roll Skia from 0c37868737fa to 666a9b3d5cf0 (2 revisions) (flutter/flutter#191518) 2026-08-21 [email protected] Revert "[input] Migrate fuchsia.ui.pointerinjector to TouchSource (#190855) (flutter/flutter#191509) 2026-08-21 [email protected] Fixes windows gallery benchmarks by forcing mobile layout (flutter/flutter#191507) 2026-08-21 [email protected] [flutter_tools] Restrict WebAssetServer source resolution to source map extensions (flutter/flutter#191501) 2026-08-21 [email protected] Roll Skia from f6900c5b8439 to 0c37868737fa (2 revisions) (flutter/flutter#191504) 2026-08-21 [email protected] Refactor `FlutterDevice.connect` and VM service discovery (flutter/flutter#191221) 2026-08-21 [email protected] [flutter_tools] Fix crash when migrating flow-style exclude lists in analysis_options.yaml (flutter/flutter#191269) 2026-08-21 [email protected] [rules] Add packages/flutter_tools/gradle/AGENTS.md rules (flutter/flutter#191486) 2026-08-21 [email protected] [flutter_tools] refactor CLI argument architecture with typed option descriptors and bundles (PoC) (flutter/flutter#191018) 2026-08-21 [email protected] tools: Extract Dart SDK to temp directory before moving to final location (flutter/flutter#191263) 2026-08-21 [email protected] [web] Move CanvasKit fragment shader classes to canvaskit/fragment_shader.dart (flutter/flutter#191451) 2026-08-21 [email protected] Roll Fuchsia Linux SDK from GCQlmt6h-esJsNubS... to ic6GjOSn-KN508XyK... (flutter/flutter#191485) 2026-08-21 [email protected] [Widget Preview] Isolate PageStorage scope in widget preview group expansion tile (flutter/flutter#191378) 2026-08-21 [email protected] [flutter_tools] Deprecate --build and --no-build flags on flutter run (flutter/flutter#191358) 2026-08-21 [email protected] Roll Skia from 70988bed1b3b to f6900c5b8439 (2 revisions) (flutter/flutter#191481) 2026-08-21 [email protected] Roll Packages from 1785501 to 252bb33 (6 revisions) (flutter/flutter#191480) 2026-08-21 [email protected] Roll Fuchsia Linux SDK from 20IJas24bZiNmCZTK... to GCQlmt6h-esJsNubS... (flutter/flutter#191415) 2026-08-21 [email protected] Roll Skia from 2ba6971bd0d1 to 70988bed1b3b (2 revisions) (flutter/flutter#191476) 2026-08-21 [email protected] Roll Skia from 1d5f72537ba6 to 2ba6971bd0d1 (2 revisions) (flutter/flutter#191473) 2026-08-21 [email protected] Roll Skia from 2c25efd2e369 to 1d5f72537ba6 (1 revision) (flutter/flutter#191472) 2026-08-21 [email protected] Roll Skia from 09b1b810850a to 2c25efd2e369 (9 revisions) (flutter/flutter#191470) 2026-08-21 [email protected] [Impeller] fix position of cached single glyph text shadows (flutter/flutter#191325) 2026-08-21 [email protected] Roll Skia from abdf8821f313 to 09b1b810850a (15 revisions) (flutter/flutter#191458) 2026-08-21 [email protected] Document that programmatic TextEditingController changes do not run input formatters (flutter/flutter#190166) 2026-08-21 [email protected] Adds new gallery benchmarks to windows (skia and impeller) (flutter/flutter#191454) 2026-08-21 [email protected] iOS: Deprecate FlutterEngine.isGpuDisabled (flutter/flutter#191393) 2026-08-21 [email protected] Revert: [web] Unskip decoration image lerp tests (flutter/flutter#191462) 2026-08-20 [email protected] android_hardware_smoke_test: Improve reliability (flutter/flutter#191374) 2026-08-20 [email protected] Don't size or offset the Android platform view before it is laid out (flutter/flutter#190895) 2026-08-20 [email protected] [iOS][add2app] Skip building SwiftPM plugins when generating CocoaPods artifacts (flutter/flutter#190736) 2026-08-20 [email protected] Optimize JSONMessageCodec UTF-8 conversion (flutter/flutter#190529) 2026-08-20 [email protected] [FML] Replace deprecated wstring_convert with Win32 APIs (flutter/flutter#191394) 2026-08-20 [email protected] SliverFillRemaining extends beyond viewport size when fillOverscroll is true (flutter/flutter#191236) 2026-08-20 [email protected] [flutter_tools] Update argParser usageLineLength when --wrap-column is passed (flutter/flutter#191264) 2026-08-20 [email protected] [flutter_tools] Fix UNC path resolution in depfile parsing on Windows (flutter/flutter#191265) 2026-08-20 [email protected] [web] Unskip decoration image lerp tests (flutter/flutter#191426) 2026-08-20 [email protected] Do not inject 'type' and 'method' into service extension responses (flutter/flutter#190946) 2026-08-20 [email protected] [flutter_tools] Fix crash in symbolize command on stream error (flutter/flutter#191273) 2026-08-20 [email protected] [flutter_tools] Prevent deletion of shared native asset hooks outputs as stale (flutter/flutter#191272) 2026-08-20 [email protected] [flutter_tools] Support package wildcard assets in app pubspec (flutter/flutter#191266) 2026-08-20 [email protected] Add SliverClipRect and SliverClipRRect (flutter/flutter#179003) 2026-08-20 [email protected] [flutter_tools] Implement Diagnostics extension slice and doctor integration (flutter/flutter#191162) ...

gaaclarke · 2026-08-24T23:08:54Z

hey @loic-sharma just a heads up that this PR changed the baseline on the app framework uncompressed size a bit (60KB). I wanted to let you know in case you cared.

https://flutter-flutter-perf.luci.app/e/?begin=1787250572&enable_chart_tooltip=true&end=1787607292&keys=X4462dd6f3027d8a2f76647ca355d496a&plotSummary=true&selected=commit%3D2809409%26name%3D%252Carch%253Dintel%252Cbranch%253Dmaster%252Cconfig%253Ddefault%252Cdevice_type%253Dnone%252Cdevice_version%253Dnone%252Chost_type%253Dmac%252Csub_result%253Dapp_framework_uncompressed_bytes%252Ctest%253Dhello_world_macos__compile%252C%26tableCol%3D2%26tableRow%3D1497&use_titles=true&xbaroffset=2809409

justinmc · 2026-08-24T23:15:45Z

Hmm I wouldn't have expected an app size change from this.

Optimize JSONMessageCodec UTF-8 conversion

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Merge branch 'master' into optimize-json-message-codec

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		static final JsonUtf8Encoder _encoder = JsonUtf8Encoder();
		static final Converter<List<int>, Object?> _decoder = utf8.decoder.fuse(json.decoder);

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