Add support for WASM deferred loading. by biggs0125 · Pull Request #189308 · flutter/flutter

biggs0125 · 2026-07-11T02:03:44Z

The dart2wasm compiler supports deferred loading now producing multiple wasm modules when deferred imports are present.

This change exposes wasm deferred loading for Flutter apps as well. It adds a few things:

Add a --enable-wasm-deferred-loading flag that gets plumbed through to the compiler.
Add a loader config entry that allows the user to specify custom module loading logic. Provide a default implementation that works for standard Flutter builds.
Add tests for all of this code.

gemini-code-assist

Code Review

This pull request adds support for multi-module deferred loading in Dart2Wasm. It introduces a wasmDeferredModulesLoader configuration option, adds the --enable-wasm-deferred-loading flag to the web build command, and updates the build system to include deferred Wasm modules and their source maps. Unit tests are also added to verify these changes. Feedback recommends optimizing the default JavaScript loader to enable streaming compilation by passing the fetch promise directly, and hoisting regular expression instantiations out of the file-filtering loop in the build target to prevent performance overhead.

Apply suggestions Co-authored-by: Harry Terkelsen <[email protected]>

harryterkelsen · 2026-07-13T21:35:34Z

+  static final RegExp _partWasmRegex = RegExp(r'main\.dart_module[0-9].*\.wasm');
+  static final RegExp _partWasmMapRegex = RegExp(r'main\.dart_module[0-9].*\.wasm\.map');


Since these regular expressions are not anchored, _partWasmRegex.hasMatch('main.dart_module1.wasm.map') will return true because the filename starts with main.dart_module1.wasm.

This means that when compilerConfig.sourceMaps is false, the .wasm.map files will still be incorrectly matched and included in buildFiles by the _partWasmRegex check.

We should anchor them with ^ and $ to prevent substring matches.

Suggested change

static final RegExp _partWasmRegex = RegExp(r'main\.dart_module[0-9].*\.wasm');

static final RegExp _partWasmMapRegex = RegExp(r'main\.dart_module[0-9].*\.wasm\.map');

static final RegExp _partWasmRegex = RegExp(r'^main\.dart_module[0-9].*\.wasm$');

static final RegExp _partWasmMapRegex = RegExp(r'^main\.dart_module[0-9].*\.wasm\.map$');

Good catch, I updated the regexes, thanks!

harryterkelsen

Thanks for the contribution! I've left a few inline comments on the JS loader options and test structure.

Additionally, I noticed a bug in packages/flutter_tools/lib/src/build_system/targets/web.dart where the regular expressions for matching dynamic modules were unanchored, causing .wasm.map files to be matched as .wasm files. This causes source map files to be included in the build output even when source maps are disabled. I left an inline suggestion to anchor them with ^ and $.

We should also add a test to packages/flutter_tools/test/general.shard/build_system/targets/web_test.dart to verify this behavior and prevent regression:

  test('Dart2WasmTarget.buildFiles respects compilerConfig.sourceMaps and matches modules', () {
    final File wasmFile = environment.buildDir.childFile('main.dart.wasm')..createSync();
    final File mjsFile = environment.buildDir.childFile('main.dart.mjs')..createSync();
    final File mapFile = environment.buildDir.childFile('main.dart.wasm.map')..createSync();
    
    final File partWasmFile = environment.buildDir.childFile('main.dart_module1.wasm')..createSync();
    final File partMapFile = environment.buildDir.childFile('main.dart_module1.wasm.map')..createSync();

    final targetWithMaps = Dart2WasmTarget(const WasmCompilerConfig(sourceMaps: true), const NoOpAnalytics());
    expect(targetWithMaps.buildFiles(environment), containsAll(<File>[wasmFile, mjsFile, mapFile, partWasmFile, partMapFile]));

    final targetWithoutMaps = Dart2WasmTarget(const WasmCompilerConfig(sourceMaps: false), const NoOpAnalytics());
    expect(targetWithoutMaps.buildFiles(environment), containsAll(<File>[wasmFile, mjsFile, partWasmFile]));
    expect(targetWithoutMaps.buildFiles(environment), isNot(contains(mapFile)));
    expect(targetWithoutMaps.buildFiles(environment), isNot(contains(partMapFile)));
  });

harryterkelsen

LGTM

flutter-dashboard · 2026-07-14T00:50:21Z

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Stack trace:

#0      GitHub.handleStatusCode (package:github/src/common/github.dart:486:5)
#1      GitHub.request (package:github/src/common/github.dart:422:7)
<asynchronous suspension>
#2      GitHub.requestJson (package:github/src/common/github.dart:323:22)
<asynchronous suspension>
#3      RetryOptions.retry (package:retry/retry.dart:131:16)
<asynchronous suspension>
#4      LuciBuildService._scheduleTryBuilds (package:cocoon_service/src/service/luci_build_service.dart:322:26)
<asynchronous suspension>
#5      Scheduler._runCiTestingStage (package:cocoon_service/src/service/scheduler.dart:1466:7)
<asynchronous suspension>
#6      Scheduler.proceedToCiTestingStage (package:cocoon_service/src/service/scheduler.dart:1516:7)
<asynchronous suspension>
#7      Scheduler._closeSuccessfulEngineBuildStage (package:cocoon_service/src/service/scheduler.dart:1341:5)
<asynchronous suspension>
#8      Scheduler.processCheckRunCompleted (package:cocoon_service/src/service/scheduler.dart:1273:11)
<asynchronous suspension>
#9      PresubmitLuciSubscription.post (package:cocoon_service/src/request_handlers/presubmit_luci_subscription.dart:173:7)
<asynchronous suspension>
#10     RequestHandler.service (package:cocoon_service/src/request_handling/request_handler.dart:42:20)
<asynchronous suspension>
#11     SubscriptionHandler.service (package:cocoon_service/src/request_handling/subscription_handler.dart:134:5)
<asynchronous suspension>
#12     createServer.<anonymous closure> (package:cocoon_service/server.dart:448:7)
<asynchronous suspension>
#13     main.<anonymous closure>.<anonymous closure> (file:///app/app_dart/bin/gae_server.dart:187:9)
<asynchronous suspension>

harryterkelsen

LGTM

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Add hook for deferred loading to wasm embedder.

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flutter-dashboard Bot added the CICD Run CI/CD label Jul 11, 2026

biggs0125 requested a review from harryterkelsen July 11, 2026 02:03

github-actions Bot added tool Affects the "flutter" command-line tool. See also t: labels. engine flutter/engine related. See also e: labels. platform-web Web applications specifically team-web Owned by Web platform team labels Jul 11, 2026

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Comment thread engine/src/flutter/lib/web_ui/flutter_js/src/entrypoint_loader.js Outdated

Comment thread packages/flutter_tools/lib/src/build_system/targets/web.dart

natebiggs added 2 commits July 10, 2026 22:36

Apply fixes

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Merge remote-tracking branch 'origin/master' into deferred-loading

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harryterkelsen reviewed Jul 13, 2026

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Comment thread engine/src/flutter/lib/web_ui/test/engine/deferred_loader_test.dart

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Comment thread engine/src/flutter/lib/web_ui/test/engine/deferred_loader_test.dart

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Apply suggestions Co-authored-by: Harry Terkelsen <[email protected]>

biggs0125 requested a review from harryterkelsen July 13, 2026 21:35

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natebiggs added 2 commits July 13, 2026 19:01

PR review updates.

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Merge branch 'master' into deferred-loading

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biggs0125 requested a review from harryterkelsen July 13, 2026 23:03

harryterkelsen previously approved these changes Jul 13, 2026

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Fix analysis error.

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biggs0125 dismissed harryterkelsen’s stale review via b4e7870 July 14, 2026 00:26

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harryterkelsen added the autosubmit Merge PR when tree becomes green via auto submit App label Jul 14, 2026

biggs0125 enabled auto-merge July 14, 2026 20:03

Merge branch 'master' into deferred-loading

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biggs0125 added this pull request to the merge queue Jul 14, 2026

Merged via the queue into flutter:master with commit 80157ee Jul 15, 2026
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Roll Flutter from 846664bf191a to fc1ad955f164 (18 revisions) flutter/packages#12203

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Add support for WASM deferred loading.#189308

Add support for WASM deferred loading.#189308
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		static final RegExp _partWasmRegex = RegExp(r'main\.dart_module[0-9].*\.wasm');
		static final RegExp _partWasmMapRegex = RegExp(r'main\.dart_module[0-9].*\.wasm\.map');

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