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[flutter_tools] Track asset transformer dependencies for hot reload (Reland #187947) by bkonyi · Pull Request #188808 · flutter/flutter · GitHub
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[flutter_tools] Track asset transformer dependencies for hot reload (Reland #187947) - #188808

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bkonyi:reland-187947
Jul 1, 2026
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[flutter_tools] Track asset transformer dependencies for hot reload (Reland #187947)#188808
auto-submit[bot] merged 1 commit into
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bkonyi:reland-187947

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@bkonyi

@bkonyi bkonyi commented Jun 30, 2026

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Reland of #187947 (reverted in #188751)

This is a reland of the previously reverted PR #187947. The failure in Mac web_tool_tests that triggered the revert was determined to be an unrelated flake. Specifically, the test failed in build 29841 (which also failed in 29846 without the PR).

Original description:
[flutter_tools] Track asset transformer dependencies for hot reload

Allow asset transformers to register additional input dependencies for hot reload by passing a --depfile flag to the transformer subprocess and parsing the resulting Ninja-format depfile.

Key changes:

  • AssetTransformer now passes --depfile and parses it using DepfileService, returning dependencies in a new AssetTransformationResult.
  • DevelopmentAssetTransformer tracks these dependencies and provides them to DevFS.
  • DevFS.updateBundle does not skip transformed assets on startup, running the transformer to collect dependencies but optimizing by not uploading them to the device unless requested (e.g., for Web).
  • DevFS.updateBundle checks if any tracked dependencies are invalidated and triggers re-transformation.
  • DevFS.update and WebDevFS.update append collected dependencies to the watched sources set.
  • copyAssets build target collects and adds these dependencies to its inputs to ensure correct rebuilds.
  • Added unit and integration tests to verify the behavior.

Fixes #187051

@flutter-dashboard flutter-dashboard Bot added the CICD Run CI/CD label Jun 30, 2026
@bkonyi
bkonyi requested a review from nshahan June 30, 2026 19:05
@github-actions github-actions Bot added the tool Affects the "flutter" command-line tool. See also t: labels. label Jun 30, 2026
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Code Review

This pull request updates the Flutter build system to support tracking and resolving dependencies generated by asset transformers. It enables parsing relative paths in depfiles relative to a base directory, tracks these dependencies in AssetTransformer and DevelopmentAssetTransformer, and monitors them in DevFS to trigger hot reloads when they are invalidated. Feedback on these changes suggests adding the --depfile argument to the transformer command, using absolute paths when filtering dependencies, and filtering out empty paths in DepfileService to prevent tracking the base directory as a dependency.

Comment thread packages/flutter_tools/lib/src/build_system/tools/asset_transformer.dart
Comment on lines +122 to 129
final command = <String>[
_dartBinaryPath,
'run',
transformer.package,
'--input=${asset.path}',
'--output=${output.path}',
...transformer.args,
];

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high

The PR description states that the asset transformer subprocess is passed a --depfile flag to register additional input dependencies. However, the constructed command list does not include the --depfile argument. If the transformer expects this flag to know where to write the depfile, it will not generate one, and dependency tracking will fail.\n\nConsider adding '--depfile=${output.path}.d' to the command arguments to match the described behavior.

Suggested change
final command = <String>[
_dartBinaryPath,
'run',
transformer.package,
'--input=${asset.path}',
'--output=${output.path}',
...transformer.args,
];
final command = <String>[
_dartBinaryPath,
'run',
transformer.package,
'--input=${asset.path}',
'--output=${output.path}',
'--depfile=${output.path}.d',
...transformer.args,
];

Comment thread packages/flutter_tools/lib/src/build_system/tools/asset_transformer.dart
Comment on lines +107 to +110
final String tempDirPath = tempDirectory.path;
final List<File> filteredDependencies = allDependencies
.where((File file) => !_fileSystem.path.isWithin(tempDirPath, file.path))
.toList();

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medium

If file.path is a relative path (which can happen if workingDirectory is relative or baseDirectory is null during parsing), comparing it directly with the absolute tempDirPath using isWithin may fail to correctly identify files inside the temporary directory.\n\nUsing file.absolute.path ensures that both paths are absolute, making the isWithin check robust and reliable.

Suggested change
final String tempDirPath = tempDirectory.path;
final List<File> filteredDependencies = allDependencies
.where((File file) => !_fileSystem.path.isWithin(tempDirPath, file.path))
.toList();
final String tempDirPath = tempDirectory.path;
final List<File> filteredDependencies = allDependencies
.where((File file) => !_fileSystem.path.isWithin(tempDirPath, file.absolute.path))
.toList();

Comment thread packages/flutter_tools/lib/src/build_system/depfile.dart
Comment on lines 118 to +121
// be resilient to the outputs of other tools which write or user edits to depfiles.
.toSet()
// Normalize the path before creating a file object.
.map((String path) => _fileSystem.file(_fileSystem.path.normalize(path)))
.map((String path) {

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medium

If the depfile contains empty lines or trailing spaces, split('\n') and trim() can produce empty strings. If an empty string is passed to the path resolver, _fileSystem.path.isRelative(path) will be true, and it will resolve to baseDirectory.path. This causes the base directory itself to be tracked as a dependency, which can trigger unnecessary hot reloads whenever any file in the project changes.\n\nFiltering out empty paths before mapping them to File objects prevents this issue.

        // be resilient to the outputs of other tools which write or user edits to depfiles.
        .toSet()
        .where((String path) => path.isNotEmpty)
        // Normalize the path before creating a file object.
        .map((String path) {

nshahan
nshahan approved these changes Jun 30, 2026
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LGTM

@bkonyi bkonyi added the autosubmit Merge PR when tree becomes green via auto submit App label Jun 30, 2026
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auto-submit Bot added this pull request to the merge queue Jul 1, 2026
Merged via the queue into flutter:master with commit 02bb9dd Jul 1, 2026
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@flutter-dashboard flutter-dashboard Bot removed the autosubmit Merge PR when tree becomes green via auto submit App label Jul 1, 2026
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auto-submit Bot pushed a commit to flutter/packages that referenced this pull request Jul 8, 2026
@engine-flutter-autoroll
Manual roll Flutter from ca9f874f5284 to 6995038d96ef (44 revisions) (#…
effd3a0 
…12135)

Manual roll Flutter from ca9f874f5284 to 6995038d96ef (44 revisions)

Manual roll requested by stuartmorgan@google.com

flutter/flutter@ca9f874...6995038

2026-07-03 engine-flutter-autoroll@skia.org Roll Fuchsia Linux SDK from sx_eN0J_f2BV6jqjW... to Jr08vyeibMSv3Oxst... (flutter/flutter#188946)
2026-07-03 engine-flutter-autoroll@skia.org Roll Packages from 420e135 to 2fbe873 (1 revision) (flutter/flutter#188945)
2026-07-03 engine-flutter-autoroll@skia.org Roll Dart SDK from 786212a2ce0d to 1f9a08ce0638 (4 revisions) (flutter/flutter#188944)
2026-07-03 engine-flutter-autoroll@skia.org Roll Skia from 5358ab75b840 to 919956953af6 (7 revisions) (flutter/flutter#188943)
2026-07-02 j.ihlas@betclicgroup.com [Windows] Keep regular windows in place when another is activated (flutter/flutter#188016)
2026-07-02 okorohelijah@google.com [flutter_tools] Provide guided message when iOS/macOS build fails due to low minimum version (flutter/flutter#188812)
2026-07-02 bdero@google.com [Flutter GPU] Load a ShaderLibrary from shader bundle bytes (flutter/flutter#188596)
2026-07-02 dacoharkes@google.com [tool] Enable record_use experiment by default on all channels (flutter/flutter#188887)
2026-07-02 engine-flutter-autoroll@skia.org Roll Packages from e742106 to 420e135 (13 revisions) (flutter/flutter#188916)
2026-07-02 engine-flutter-autoroll@skia.org Roll Fuchsia Linux SDK from I2h2eXk06RrA3pIG2... to sx_eN0J_f2BV6jqjW... (flutter/flutter#188915)
2026-07-02 256906086+mvincentong@users.noreply.github.com Clarify layout callback debug flag docs (flutter/flutter#186879)
2026-07-02 engine-flutter-autoroll@skia.org Roll Skia from 0c4faca350cc to 5358ab75b840 (2 revisions) (flutter/flutter#188899)
2026-07-02 engine-flutter-autoroll@skia.org Roll Dart SDK from e47361c7fe9a to 786212a2ce0d (2 revisions) (flutter/flutter#188898)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from 0fc8ba72e802 to 0c4faca350cc (2 revisions) (flutter/flutter#188886)
2026-07-01 robert.ancell@canonical.com Hide draft PRs from the triage list (flutter/flutter#188885)
2026-07-01 47866232+chunhtai@users.noreply.github.com Stricten isSemantics and matchesSemantics children mismatch check (flutter/flutter#188827)
2026-07-01 jesswon@google.com [AGP 9] Support Enabling Built-in Kotlin (flutter/flutter#188543)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from d19e557ac317 to 0fc8ba72e802 (4 revisions) (flutter/flutter#188879)
2026-07-01 bkonyi@google.com [ci] Increase test timeout for Mac_x64 build_tests shards (flutter/flutter#188804)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from bd4ae38ca3bb to d19e557ac317 (1 revision) (flutter/flutter#188865)
2026-07-01 engine-flutter-autoroll@skia.org Roll Dart SDK from 26d723eb89af to e47361c7fe9a (5 revisions) (flutter/flutter#188864)
2026-07-01 katelovett@google.com Update triage links for material_ui and cupertino_ui --> Design triage (flutter/flutter#188567)
2026-07-01 flar@google.com [Impeller] Share a single ContextGLES among all PlaygroundImplGLES (flutter/flutter#188080)
2026-07-01 homobonus-luca@hotmail.it Use null-aware elements in dev/devicelab/lib/integration_tests.dart (flutter/flutter#187852)
2026-07-01 engine-flutter-autoroll@skia.org Roll Packages from 274ed3e to e742106 (18 revisions) (flutter/flutter#188863)
2026-07-01 116356835+AbdeMohlbi@users.noreply.github.com Add android 17 to embedding (flutter/flutter#187965)
2026-07-01 47866232+chunhtai@users.noreply.github.com Adds semantics role check to isSemantics and matchesSemantics (flutter/flutter#188825)
2026-07-01 rmacnak@google.com Roll Dart SDK from e1bdb9ce3327 to 26d723eb89af (3 revisions) (flutter/flutter#188795)
2026-07-01 zhongliu88889@gmail.com [web] Apply autocapitalize to text editing elements (flutter/flutter#188351)
2026-07-01 engine-flutter-autoroll@skia.org Roll Fuchsia Linux SDK from RymJjIj7dd5vQ3Cnh... to I2h2eXk06RrA3pIG2... (flutter/flutter#188852)
2026-07-01 bdero@google.com [Impeller] Compute dispatch takes 3D workgroup counts and honors the shader local_size (flutter/flutter#188601)
2026-07-01 codedoctor@linwood.dev Improve stylus support on linux (flutter/flutter#186831)
2026-07-01 matt.boetger@gmail.com Resolve issue  Catch am start failures with 'Error type' and prevent hang (flutter/flutter#187196)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from ef178c9898af to bd4ae38ca3bb (3 revisions) (flutter/flutter#188834)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from 3ac99be47229 to ef178c9898af (3 revisions) (flutter/flutter#188831)
2026-07-01 31859944+LongCatIsLooong@users.noreply.github.com Add a macosArm64Only feature flag (flutter/flutter#188598)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from 15302f1625b2 to 3ac99be47229 (1 revision) (flutter/flutter#188819)
2026-07-01 bkonyi@google.com [flutter_tools] Track asset transformer dependencies for hot reload (Reland #187947) (flutter/flutter#188808)
2026-06-30 737941+loic-sharma@users.noreply.github.com Add TapRegion samples (flutter/flutter#188685)
2026-06-30 31859944+LongCatIsLooong@users.noreply.github.com Print a warning in `flutter doctor` when running on Intel Macs (flutter/flutter#188760)
2026-06-30 zhongliu88889@gmail.com [framework] Keep scrollable semantics role stable (flutter/flutter#187963)
2026-06-30 1961493+harryterkelsen@users.noreply.github.com feat(skills): Add shepherd-prs skill for managing approved external contributor PRs (flutter/flutter#188534)
2026-06-30 engine-flutter-autoroll@skia.org Roll Skia from 71947c4110b0 to 15302f1625b2 (17 revisions) (flutter/flutter#188815)
2026-06-30 bkonyi@google.com [Tool] Run re-entrant upgrade in original CWD (flutter/flutter#188794)
...
kalyujniy pushed a commit to brickit-app/camera that referenced this pull request Jul 8, 2026
@engine-flutter-autoroll @kalyujniy
Manual roll Flutter from ca9f874f5284 to 6995038d96ef (44 revisions) (f…
9cf24cd 
…lutter#12135)

Manual roll Flutter from ca9f874f5284 to 6995038d96ef (44 revisions)

Manual roll requested by stuartmorgan@google.com

flutter/flutter@ca9f874...6995038

2026-07-03 engine-flutter-autoroll@skia.org Roll Fuchsia Linux SDK from sx_eN0J_f2BV6jqjW... to Jr08vyeibMSv3Oxst... (flutter/flutter#188946)
2026-07-03 engine-flutter-autoroll@skia.org Roll Packages from 420e135 to 2fbe873 (1 revision) (flutter/flutter#188945)
2026-07-03 engine-flutter-autoroll@skia.org Roll Dart SDK from 786212a2ce0d to 1f9a08ce0638 (4 revisions) (flutter/flutter#188944)
2026-07-03 engine-flutter-autoroll@skia.org Roll Skia from 5358ab75b840 to 919956953af6 (7 revisions) (flutter/flutter#188943)
2026-07-02 j.ihlas@betclicgroup.com [Windows] Keep regular windows in place when another is activated (flutter/flutter#188016)
2026-07-02 okorohelijah@google.com [flutter_tools] Provide guided message when iOS/macOS build fails due to low minimum version (flutter/flutter#188812)
2026-07-02 bdero@google.com [Flutter GPU] Load a ShaderLibrary from shader bundle bytes (flutter/flutter#188596)
2026-07-02 dacoharkes@google.com [tool] Enable record_use experiment by default on all channels (flutter/flutter#188887)
2026-07-02 engine-flutter-autoroll@skia.org Roll Packages from e742106 to 420e135 (13 revisions) (flutter/flutter#188916)
2026-07-02 engine-flutter-autoroll@skia.org Roll Fuchsia Linux SDK from I2h2eXk06RrA3pIG2... to sx_eN0J_f2BV6jqjW... (flutter/flutter#188915)
2026-07-02 256906086+mvincentong@users.noreply.github.com Clarify layout callback debug flag docs (flutter/flutter#186879)
2026-07-02 engine-flutter-autoroll@skia.org Roll Skia from 0c4faca350cc to 5358ab75b840 (2 revisions) (flutter/flutter#188899)
2026-07-02 engine-flutter-autoroll@skia.org Roll Dart SDK from e47361c7fe9a to 786212a2ce0d (2 revisions) (flutter/flutter#188898)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from 0fc8ba72e802 to 0c4faca350cc (2 revisions) (flutter/flutter#188886)
2026-07-01 robert.ancell@canonical.com Hide draft PRs from the triage list (flutter/flutter#188885)
2026-07-01 47866232+chunhtai@users.noreply.github.com Stricten isSemantics and matchesSemantics children mismatch check (flutter/flutter#188827)
2026-07-01 jesswon@google.com [AGP 9] Support Enabling Built-in Kotlin (flutter/flutter#188543)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from d19e557ac317 to 0fc8ba72e802 (4 revisions) (flutter/flutter#188879)
2026-07-01 bkonyi@google.com [ci] Increase test timeout for Mac_x64 build_tests shards (flutter/flutter#188804)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from bd4ae38ca3bb to d19e557ac317 (1 revision) (flutter/flutter#188865)
2026-07-01 engine-flutter-autoroll@skia.org Roll Dart SDK from 26d723eb89af to e47361c7fe9a (5 revisions) (flutter/flutter#188864)
2026-07-01 katelovett@google.com Update triage links for material_ui and cupertino_ui --> Design triage (flutter/flutter#188567)
2026-07-01 flar@google.com [Impeller] Share a single ContextGLES among all PlaygroundImplGLES (flutter/flutter#188080)
2026-07-01 homobonus-luca@hotmail.it Use null-aware elements in dev/devicelab/lib/integration_tests.dart (flutter/flutter#187852)
2026-07-01 engine-flutter-autoroll@skia.org Roll Packages from 274ed3e to e742106 (18 revisions) (flutter/flutter#188863)
2026-07-01 116356835+AbdeMohlbi@users.noreply.github.com Add android 17 to embedding (flutter/flutter#187965)
2026-07-01 47866232+chunhtai@users.noreply.github.com Adds semantics role check to isSemantics and matchesSemantics (flutter/flutter#188825)
2026-07-01 rmacnak@google.com Roll Dart SDK from e1bdb9ce3327 to 26d723eb89af (3 revisions) (flutter/flutter#188795)
2026-07-01 zhongliu88889@gmail.com [web] Apply autocapitalize to text editing elements (flutter/flutter#188351)
2026-07-01 engine-flutter-autoroll@skia.org Roll Fuchsia Linux SDK from RymJjIj7dd5vQ3Cnh... to I2h2eXk06RrA3pIG2... (flutter/flutter#188852)
2026-07-01 bdero@google.com [Impeller] Compute dispatch takes 3D workgroup counts and honors the shader local_size (flutter/flutter#188601)
2026-07-01 codedoctor@linwood.dev Improve stylus support on linux (flutter/flutter#186831)
2026-07-01 matt.boetger@gmail.com Resolve issue  Catch am start failures with 'Error type' and prevent hang (flutter/flutter#187196)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from ef178c9898af to bd4ae38ca3bb (3 revisions) (flutter/flutter#188834)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from 3ac99be47229 to ef178c9898af (3 revisions) (flutter/flutter#188831)
2026-07-01 31859944+LongCatIsLooong@users.noreply.github.com Add a macosArm64Only feature flag (flutter/flutter#188598)
2026-07-01 engine-flutter-autoroll@skia.org Roll Skia from 15302f1625b2 to 3ac99be47229 (1 revision) (flutter/flutter#188819)
2026-07-01 bkonyi@google.com [flutter_tools] Track asset transformer dependencies for hot reload (Reland #187947) (flutter/flutter#188808)
2026-06-30 737941+loic-sharma@users.noreply.github.com Add TapRegion samples (flutter/flutter#188685)
2026-06-30 31859944+LongCatIsLooong@users.noreply.github.com Print a warning in `flutter doctor` when running on Intel Macs (flutter/flutter#188760)
2026-06-30 zhongliu88889@gmail.com [framework] Keep scrollable semantics role stable (flutter/flutter#187963)
2026-06-30 1961493+harryterkelsen@users.noreply.github.com feat(skills): Add shepherd-prs skill for managing approved external contributor PRs (flutter/flutter#188534)
2026-06-30 engine-flutter-autoroll@skia.org Roll Skia from 71947c4110b0 to 15302f1625b2 (17 revisions) (flutter/flutter#188815)
2026-06-30 bkonyi@google.com [Tool] Run re-entrant upgrade in original CWD (flutter/flutter#188794)
...
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[flutter_tools] Allow asset transformers to register additional input dependencies for hot reload

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