Why Flutter copies the src of standard libraries to sky_engine?

Question

Since dart:ffi is available from Dart 2.2.0-dev.2.0, I've been trying to use that library. The sample app works fine for me and I also tried to use it for my Flutter app but I couldn't call it with import "dart:ffi", although Flutter on my machine was HEAD of master that used Dart 2.3.0-dev.0.0.

I checked what happened, then I found that sky_engine didn't contain ffi.dart while it contained other libraries (e.g. "dart:core"). Also I noticed that the source files of those libraries were copied from $FLUTTER_ROOT/bin/cache/dart-sdk/lib to $FLUTTER_ROOT/bin/cache/pkg/sky_engine using BUILD.gn or _embedder.yaml and that seemed to be why I couldn't use the dart:ffi in my Flutter app.

However, in the first place, why does Flutter need sky_engine, which is "the interface between Dart and the Flutter Engine"? Why not calling them directly without this glue code?

Sky was the original name for Flutter - just imagine it was called `flutter_engine` :) — creativecreatorormaybenot, Jan 30 '21 at 00:26
Would you consider editing the question to include a [repro]. Also I think you want to clarify the title as I imagine the answer to "why flutter copies..." can be opinion based, but if you instead were asking something like "how to import dart:ffi" that would increase the chances for an answer. — Alex.F, Jul 16 '21 at 13:37

score 1 · Answer 1 · answered Sep 17 '21 at 13:48

Flutter has a good documentation for FFI to be able to call native C APIs. Adding import 'dart:ffi' as ffi; on the Flutter app works without issues as of my testing with Flutter 2.5. If you're able to provide a minimal repro of your issue, this will help folks to understand the question better.

As for the question on why sky_engine is used by Flutter, that's just simply because it's the "flutter_engine" - similar to what has been already mentioned in the comments.

Why Flutter copies the src of standard libraries to sky_engine?

1 Answers1