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Flutter-on-Fuchsia Velocity

  • Project lead:
  • Area(s): Flutter

Problem statement

Fuchsia's Flutter integration is currently languishing under technical debt and out-of-tree workflows with sharp edges. These two factors make it difficult for existing or new engineers to contribute to Flutter-on-Fuchsia in a meaningful way. They have also led the Flutter team to largely abandon maintenance of Flutter-on-Fuchsia and transfer ownership of that task back to the Fuchsia team.

The out-of-tree location of the Fuchsia-Flutter code (known as the flutter_runner) combined with the sequence of rollers between the upstream repository and GI means that changes to the Dart VM or Flutter engine often do not show up in GI for multiple weeks after they are introduced upstream.

Ultimately it is difficult to make changes to the flutter_runner right now and mistakes are very costly due to roll times. Since several products use flutter for the primary shell, any bugs have a high user-facing cost and sometimes can't be fixed for weeks.

Solution statement

Flutter on Fuchsia will migrate to become a Custom Flutter Engine Embedder using the well-defined Embedder API (and ABI) created by the Flutter team. As part of this we will remove the native VM hooks for dart:zircon and dart:fuchsia and reimplement these packages using dart:ffi (because the embedder API does not allow for custom native hooks).

This refactoring will allow the Fuchsia-specific code to be maintained independently of the core flutter engine code, with the 2 being separated by a well-defined ABI. In the process of disentangling the Fuchsia-specific code from the code flutter engineer code, it also removes a lot of technical debt (~8kLOC of legacy code scheduled for deletion).


Moving the flutter embedder code into //sdk will be a large workflow change for Fuchsia engineers working on Flutter. Those engineers will no longer submit changes to //flutter/shell/platform/fuchsia in the flutter engine tree, but instead will submit those changes into the fuchsia tree.

The flutter_runner_unittests are moved in-tree as well, so they run on fuchsia CI instead. These tests are small and self-contained so moving them is not expected to be problematic.

Finally, engineers working in the flutter embedder code will be limited to only the APIs exposed by the Flutter team in embedder.h

Risks and mitigations

  • Risk: dart:zircon and dart:fuchsia migration to FFI might encounter unexpected hurdles
  • Risk: lack of test coverage over the code that is being transformed for this task

  • Mitigation: Prototype an FFI-based dart:zircon API and verify it works

  • Mitigation: Improve test coverage

  • Mitigation: Can (and did) migrate the legacy render code (in flow/) without migrating dart:zircon and dart:fuchsia