Compiling Flutter engine

In this part, we will try building Web engine and also handle some errors that may happen during the build. We will use felt tool for this. Before using it, let's add it to your PATH environment variables. The path would be: ENGINE_ROOT/src/flutter/lib/web_ui/dev. For e.g:

export PATH=/Users/huynq/Documents/GitHub/engine/src/flutter/lib/web_ui/dev:$PATH

Then you can call felt command from anywhere.

Next, as common steps stated in previous tutorials, we need to check out engine commit we want to build with and also update dependencies. In this tutorial, I assume we use the latest engine revision from main branch.

Update the Flutter Engine repo:

➜  cd /Users/huynq/Documents/GitHub/engine/src/flutter
➜  git pull upstream main

Update dependencies

➜  engine gclient sync
Syncing projects: 100% (137/137), done.                                                                            
Hook 'python3 src/flutter/tools/pub_get_offline.py' took 14.53 secs
Running hooks: 100% (14/14), done.

Felt build

This command will build Web engine target. Some targets could be used with: sdk, canvaskit, canvaskit_chromium and skwasm. You can check them all here.

➜  src git:(44ca359) felt build
Running on MacOS. Will check the file and user limits.
File limits too low increasing the file limits
User limits too low increasing the user limits
Running `dart pub get` in 'engine/src/flutter/lib/web_ui'
Running gn...
Generating GN files in: out/wasm_release
Generating compile_commands took 39ms
Done. Made 185 targets from 94 files in 3169ms
Running autoninja...
ninja: Entering directory `/Users/huynq/Documents/GitHub/engine/src/out/wasm_release'
[139/2539] LINK canvaskit/canvaskit.js
cache:INFO: generating system asset: symbol_lists/266569fa9c69a946597f128c95d4d54feb22c5ff.json... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/symbol_lists/266569fa9c69a946597f128c95d4d54feb22c5ff.json" for subsequent builds)
cache:INFO:  - ok
[2530/2539] LINK skwasm/skwasm.js
cache:INFO: generating system asset: symbol_lists/8332cd38f4026705eee5a4cdaee53cba8dad116b.json... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/symbol_lists/8332cd38f4026705eee5a4cdaee53cba8dad116b.json" for subsequent builds)
cache:INFO:  - ok
cache:INFO: generating system asset: sysroot/lib/wasm32-emscripten/lto/crtbegin.o... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/crtbegin.o" for subsequent builds)
system_libs:INFO: compiled 1 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libGL-mt-webgl2.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libGL-mt-webgl2.a" for subsequent builds)
system_libs:INFO: compiled 4 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libc_optz-mt.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libc_optz-mt.a" for subsequent builds)
system_libs:INFO: compiled 7 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libbulkmemory.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libbulkmemory.a" for subsequent builds)
system_libs:INFO: compiled 4 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libnoexit.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libnoexit.a" for subsequent builds)
system_libs:INFO: compiled 1 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libc-mt.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libc-mt.a" for subsequent builds)
system_libs:INFO: compiled 1108 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libdlmalloc-mt.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libdlmalloc-mt.a" for subsequent builds)
system_libs:INFO: compiled 1 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libcompiler_rt-mt.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libcompiler_rt-mt.a" for subsequent builds)
system_libs:INFO: compiled 175 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libc++-mt-noexcept.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libc++-mt-noexcept.a" for subsequent builds)
system_libs:INFO: compiled 49 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libc++abi-mt-noexcept.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libc++abi-mt-noexcept.a" for subsequent builds)
system_libs:INFO: compiled 16 inputs
cache:INFO:  - ok
cache:INFO: generating system library: sysroot/lib/wasm32-emscripten/lto/libsockets-mt.a... (this will be cached in "/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto/libsockets-mt.a" for subsequent builds)
system_libs:INFO: compiled 21 inputs
cache:INFO:  - ok
[2539/2539] STAMP obj/default.stamp
➜  src git:(44ca359)

Run your Flutter project

➜  flutterm create demo_web_engine
➜  cd demo_web_engine
➜  demo_web_engine flutterm --local-web-sdk=wasm_release run -d chrome --local-engine-src-path=/Users/huynq/Documents/GitHub/engine/src
Launching lib/main.dart on Chrome in debug mode...
Warning: In index.html:37: Local variable for "serviceWorkerVersion" is deprecated. Use "{{flutter_service_worker_version}}" template token instead.
Warning: In index.html:46: "FlutterLoader.loadEntrypoint" is deprecated. Use "FlutterLoader.load" instead.
Waiting for connection from debug service on Chrome...             22.8s
This app is linked to the debug service: ws://127.0.0.1:58589/P1bktcnKBgw=/ws
Debug service listening on ws://127.0.0.1:58589/P1bktcnKBgw=/ws

🔥  To hot restart changes while running, press "r" or "R".
For a more detailed help message, press "h". To quit, press "q".

A Dart VM Service on Chrome is available at: http://127.0.0.1:58589/P1bktcnKBgw=
The Flutter DevTools debugger and profiler on Chrome is available at: http://127.0.0.1:9100?uri=http://127.0.0.1:58589/P1bktcnKBgw=

Troubleshooting

felt build failed: buildtools/emsdk/upstream/emscripten/em++: No such file or directory

➜  src git:(44ca359) felt build
Running on MacOS. Will check the file and user limits.
File limits too low increasing the file limits
Password:
User limits too low increasing the user limits
Running `dart pub get` in 'engine/src/flutter/lib/web_ui'
Running gn...
Generating GN files in: out/wasm_release
Generating compile_commands took 44ms
Done. Made 185 targets from 94 files in 3670ms
Running autoninja...
ninja: Entering directory `/Users/huynq/Documents/GitHub/engine/src/out/wasm_release'
[7/4197] CXX canvaskit/obj/flutter/third_party/skia/src/ports/fontmgr_custom.SkFontMgr_custom.o
FAILED: canvaskit/obj/flutter/third_party/skia/src/ports/fontmgr_custom.SkFontMgr_custom.o 
/Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/em++ --em-config /Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/.emscripten -MD -MF canvaskit/obj/flutter/third_party/skia/src/ports/fontmgr_custom.SkFontMgr_custom.o.d -DUSE_OPENSSL=1 -D_LIBCPP_ENABLE_THREAD_SAFETY_ANNOTATIONS -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DSK_ENABLE_DUMP_GPU -DSK_FORCE_AAA -DSK_LEGACY_IGNORE_DRAW_VERTICES_BLEND_WITH_NO_SHADER -DSK_DISABLE_LEGACY_GRDIRECTCONTEXT_BOOLS -DSK_DISABLE_LEGACY_GRDIRECTCONTEXT_FLUSH -DSK_RESOLVE_FILTERS_BEFORE_RESTORE -DSK_DISABLE_LEGACY_SHADERCONTEXT -DSK_DISABLE_LOWP_RASTER_PIPELINE -DSK_FORCE_RASTER_PIPELINE_BLITTER -DSK_METAL_WAIT_UNTIL_SCHEDULED -DSK_DISABLE_EFFECT_DESERIALIZATION -DSK_ENABLE_PRECOMPILE -DSKNX_NO_SIMD -DSK_FORCE_8_BYTE_ALIGNMENT -DSK_DISABLE_TRACING -DSK_ASSUME_WEBGL=1 -DSK_USE_WEBGL -DSK_GANESH -DSK_GAMMA_APPLY_TO_A8 -DSKIA_IMPLEMENTATION=1 -DSK_TYPEFACE_FACTORY_FREETYPE -I../.. -Icanvaskit/gen -I../../flutter/third_party/skia -fno-strict-aliasing -fcolor-diagnostics -Wendif-labels -Werror -Wno-missing-field-initializers -Wno-unused-parameter -Wno-shift-negative-value -Wno-unused-function -Wno-unused-variable -Wno-unused-but-set-parameter -Wno-unused-but-set-variable -Wno-implicit-int-float-conversion -Wno-deprecated-copy -Wno-psabi -Wno-deprecated-literal-operator -Wno-non-c-typedef-for-linkage -Wno-range-loop-construct -Wstring-conversion -Wnewline-eof -O3 -g0 -fvisibility-inlines-hidden -std=c++17 -fno-rtti   -c ../../flutter/third_party/skia/src/ports/SkFontMgr_custom.cpp -o canvaskit/obj/flutter/third_party/skia/src/ports/fontmgr_custom.SkFontMgr_custom.o
/bin/sh: /Users/huynq/Documents/GitHub/engine/src/buildtools/emsdk/upstream/emscripten/em++: No such file or directory
[16/4197] ACTION //flutter/web_sdk:web_engine_library(//build/toolchain/wasm:wasm)
ninja: build stopped: subcommand failed.
Pipeline experienced the following failures:
  "ninja": Sub-process failed.
Command: autoninja -C /Users/huynq/Documents/GitHub/engine/src/out/wasm_release
Working directory: /Users/huynq/Documents/GitHub/engine/src/flutter/lib/web_ui
Exit code: 1

Test pipeline failed.

It's telling emsdk is missing from buildtools dir. You need to add it manually:

Step 1: Add download_emsdk to engine/.gclient file as below:

solutions = [
  {
    "managed": False,
    "name": "src/flutter",
    "url": "git@github.com:huycozy/engine.git",
    "custom_deps": {},
    "deps_file": "DEPS",
    "safesync_url": "",
    "custom_vars": {"download_emsdk": True},
  },
]

Step 2: Update dependencies again

➜  engine gclient sync

Then you will see emsdk is downloaded in buildtools.

web dependency error

You may hit errors like:

               ^
../../.pub-cache/hosted/pub.dev/web-0.5.1/lib/src/dom/webrtc_encoded_transform.dart:148:34: Error: Type 'invalid-type' is not a valid type for external `dart:js_interop` APIs. The only valid types are: @staticInterop types, JS types from `dart:js_interop`, void, bool, num, double, int, String, and any extension type that erases to one of these types.
Use one of the valid types instead.
  external JSArray<JSNumber> get contributingSources;
                                 ^
../../.pub-cache/hosted/pub.dev/web-0.5.1/lib/src/dom/webrtc_encoded_transform.dart:216:32: Error: Type 'invalid-type' is not a valid type for external `dart:js_interop` APIs. The only valid types are: @staticInterop types, JS types from `dart:js_interop`, void, bool, num, double, int, String, and any extension type that erases to one of these types.
Use one of the valid types instead.
  external JSPromise<JSNumber> generateKeyFrame([String rid]);
                               ^
../../.pub-cache/hosted/pub.dev/web-0.5.1/lib/src/dom/webrtc_encoded_transform.dart:237:30: Error: Type 'invalid-type' is not a valid type for external `dart:js_interop` APIs. The only valid types are: @staticInterop types, JS types from `dart:js_interop`, void, bool, num, double, int, String, and any extension type that erases to one of these types.
Use one of the valid types instead.
  external JSPromise<JSAny?> sendKeyFrameRequest();
                             ^
../../.pub-cache/hosted/pub.dev/web-0.5.1/lib/src/dom/webrtc_encoded_transform.dart:252:20: Error: Type 'invalid-type' is not a valid type for external `dart:js_interop` APIs. The only valid types are: @staticInterop types, JS types from `dart:js_interop`, void, bool, num, double, int, String, and any extension type that erases to one of these types.
Use one of the valid types instead.
  external factory RTCRtpScriptTransform(
                   ^
../../.pub-cache/hosted/pub.dev/web-0.5.1/lib/src/dom/xhr.dart:263:40: Error: Type 'invalid-type' is not a valid type for external `dart:js_interop` APIs. The only valid types are: @staticInterop types, JS types from `dart:js_interop`, void, bool, num, double, int, String, and any extension type that erases to one of these types.
Use one of the valid types instead.
  external JSArray<FormDataEntryValue> getAll(String name);
                                       ^
Failed to compile application.

It could be web package version depreciation issue. Just try downgrading package version then retry.

dart pub downgrade web

Or use a specific package version in project:

dependencies:
  flutter:
    sdk: flutter
  web: 0.4.2

Hope this will be useful for you!

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Compiling Flutter Engine Locally - Web engine (Part III)

Troubleshooting

References

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