Compiling the engine

If you've never built the engine before, first see Setting up the Engine development environment.

Contents

Depending on the platform you are making changes for, you may be interested in all or only some of the sections below:

• General Compilation Tips
• Compiling for Android
• Compiling for iOS (from macOS)
• Compiling for macOS or Linux
• Compiling for Windows
• Compiling for Fuchsia
• Compiling for the Web

General Compilation Tips

• For local development and testing, it's generally preferable to use --unopt builds. These builds will have additional logging and checks enabled, and generally use build and link flags that lead to faster compilation and better debugging symbols. If you are trying to do performance testing with a local build, do not use the --unopt flag.
• Link Time Optimization: Optimized builds also perform Link Time Optimization of all binaries. This makes the linker take a lot of time and memory to produce binaries. If you need optimized binaries but don't want to perform LTO, add the --no-lto flag.
• Android and iOS expect both a host and android (or ios) build. It is critical to recompile the host build after upgrading the Dart SDK (e.g. via a gclient sync after merging up to head), since artifacts from the host build need to be version matched to artifacts in the Android/iOS build.
• Web, Desktop, and Fuchsia builds have only one build target (i.e. host or fuchsia).
• Make sure to exclude the out directory from any backup scripts, as many large binary artifacts are generated. This is also generally true for all of the directories outside of the engine/src/flutter directory.
• For Googlers: goma is a distributed compiler service that can vastly speed up build times. The variables to use goma compilation are set by default if goma-related environment variables are detected, and can be explicitly set via the --goma/--no-goma flag to the flutter/tools/gn wrapper script.
  • Since December 2020, Flutter uses Fuchsia RBE. The following script can be used to install and log into a correctly configured GOMA client:

#!/bin/bash

# Customize this to where you would like GOMA to be installed.
GOMA_DIR="$HOME/flutter_goma"

# Download client. Assumes cipd from depot_tools is on path.
echo 'fuchsia/third_party/goma/client/${platform}  release' | cipd ensure -ensure-file - -root "$GOMA_DIR"

# Authenticate
"$GOMA_DIR/goma_auth" login --browser

# Start GOMA
GOMA_LOCAL_OUTPUT_CACHE_DIR="$GOMA_DIR/.goma_cache" "$GOMA_DIR/goma_ctl.py" ensure_start

• GOMA will fail remotely if it tries to access files that reside outside of the build root. When building configurations for macOS or iOS (i.e. configurations that require an Xcode-vended SDK or toolchain) locally, you can have the build create and use symlinks by adding the --xcode-symlinks argument to the flutter/tools/gn wrapper script or export FLUTTER_GOMA_CREATE_XCODE_SYMLINKS=1 to your bash/zsh/whatever rc.

Compiling for Android (from macOS or Linux)

These steps build the engine used by flutter run for Android devices.

Run the following steps, from the src directory created in Setting up the Engine development environment:

1. git pull upstream master in src/flutter to update the Flutter Engine repo.

2. gclient sync to update dependencies.

3. Prepare your build files

   • ./flutter/tools/gn --android --unoptimized for device-side executables.
   • ./flutter/tools/gn --android --unoptimized --android-cpu=arm64 for newer 64-bit Android devices.
   • ./flutter/tools/gn --android --android-cpu x86 --unoptimized for x86 emulators.
   • ./flutter/tools/gn --android --android-cpu x64 --unoptimized for x64 emulators.
   • ./flutter/tools/gn --unoptimized for host-side executables, needed to compile the code.
     • On macOS hosts, add the --xcode-symlinks argument when using goma.

4. Build your executables

   • ninja -C out/android_debug_unopt for device-side executables.
   • ninja -C out/android_debug_unopt_arm64 for newer 64-bit Android devices.
   • ninja -C out/android_debug_unopt_x86 for x86 emulators.
   • ninja -C out/android_debug_unopt_x64 for x64 emulators.
   • ninja -C out/host_debug_unopt for host-side executables.
   • These commands can be combined. Ex: ninja -C out/android_debug_unopt && ninja -C out/host_debug_unopt
   • For MacOS, you will need older version of XCode(9.4 or below) to compile android_debug_unopt and android_debug_unopt_x86. If you only care about x64, you can ignore this
   • For Googlers, consider also adding the flag --goma to your gn command, then using autoninja to parallelize the build using Goma. Before that, you may need to set up GOMA_DIR environmental variable, which, depending on where you install Goma, may be in ~/goma or depot_tools/.cipd_bin.

This builds a debug-enabled ("unoptimized") binary configured to run Dart in checked mode ("debug"). There are other versions, see Flutter's modes.

If you're going to be debugging crashes in the engine, make sure you add android:debuggable="true" to the <application> element in the android/AndroidManifest.xml file for the Flutter app you are using to test the engine.

See The flutter tool for instructions on how to use the flutter tool with a local engine. You will typically use the android_debug_unopt build to debug the engine on a device, and android_debug_unopt_x64 to debug in on a simulator. Modifying dart sources in the engine will require adding a dependency_override section in you app's pubspec.yaml as detailed here.

Note that if you use particular android or ios engine build, you will need to have corresponding host build available next to it: if you use android_debug_unopt, you should have built host_debug_unopt, android_profile -> host_profile, etc. One caveat concerns cpu-flavored builds like android_debug_unopt_x86: you won't be able to build host_debug_unopt_x86 as that configuration is not supported. What you are expected to do is to build host_debug_unopt and symlink host_debug_unopt_x86 to it.

Compiling everything that matters on Linux

The following script will update all the builds that matter if you're developing on Linux and testing on Android and created the .gclient file in ~/dev/engine:

set -ex

cd ~/dev/engine/src/flutter
git fetch upstream
git rebase upstream/master
gclient sync
cd ..

flutter/tools/gn --unoptimized --runtime-mode=debug
flutter/tools/gn --android --unoptimized --runtime-mode=debug
flutter/tools/gn --android --runtime-mode=profile
flutter/tools/gn --android --runtime-mode=release

cd out
find . -mindepth 1 -maxdepth 1 -type d | xargs -n 1 sh -c 'ninja -C $0 || exit 255'

For --runtime-mode=profile build, please also consider adding --no-lto option to the gn command. It will make linking much faster with a small sacrifice on the binary size and memory usage (which probably doesn't matter for debugging or performance benchmark purposes.)

Compiling for iOS (from macOS)

These steps build the engine used by flutter run for iOS devices.

Run the following steps, from the src directory created in the steps above:

1. git pull upstream master in src/flutter to update the Flutter Engine repo.

2. gclient sync to update dependencies.

3. ./flutter/tools/gn --ios --unoptimized to prepare build files for device-side executables (or --ios --simulator --unoptimized for simulator, and if working on iPhone 4s or older, --ios --ios-cpu=arm --unoptimized).

• This also produces an Xcode project for working with the engine source code at out/ios_debug_unopt
• For a discussion on the various flags and modes, see Flutter's modes.
• Add the --xcode-symlinks argument when using goma.

4. ./flutter/tools/gn --unoptimized to prepare the build files for host-side executables.

• Add the --xcode-symlinks argument when using goma.

5. ninja -C out/ios_debug_unopt && ninja -C out/host_debug_unopt to build all artifacts (use out/ios_debug_sim_unopt for Simulator, out/out/ios_debug_unopt_arm for iPhone 4s or older).
   • For Googlers, consider also using the --goma flag with gn, then building with autoninja to parallelize the build using Goma.

See The flutter tool for instructions on how to use the flutter tool with a local engine. You will typically use the ios_debug_unopt build to debug the engine on a device, and ios_debug_sim_unopt to debug in on a simulator. Modifying dart sources in the engine will require adding a dependency_override section in you app's pubspec.yaml as detailed here.

If you are debugging crashes in the engine, you can connect the LLDB debugger from Xcode by opening ios/Runner.xcworkspace and starting the application by clicking the Run button (CMD + R).

To debug non-crashing code, open Xcode with ios/Runner.xcworkspace, expand Flutter->Runner->Supporting Files->main.m in the Runner project. Put a breakpoint in main() then set your desired breakpoint in the engine in lldb via breakpoint set -....

Using Xcode in this way will set a $LOCAL_ENGINE variable in your Generated.xcconfig file for the Flutter application. This variable will be set until you run flutter run again with either a different --local-engine option, or with none at all (which will unset it). This is important to note if you plan to re-run your Flutter Runner.xcodeproj directly from Xcode.

Compiling for macOS or Linux

These steps build the desktop embedding, and the engine used by flutter test on a host workstation.

1. git pull upstream master in src/flutter to update the Flutter Engine repo.

2. gclient sync to update your dependencies.

3. ./flutter/tools/gn --unoptimized to prepare your build files.

   • --unoptimized disables C++ compiler optimizations. On macOS, binaries are emitted unstripped; on Linux, unstripped binaries are emitted to an exe.unstripped subdirectory of the build.

• Add the --xcode-symlinks argument when using goma on macOS.

4. ninja -C out/host_debug_unopt to build a desktop unoptimized binary.
   • If you skipped --unoptimized, use ninja -C out/host_debug instead.
   • For Googlers, consider also using the --goma flag with gn, then building with autoninja to parallelize the build using Goma.

See The flutter tool for instructions on how to use the flutter tool with a local engine. You will typically use the host_debug_unopt build in this setup. Modifying dart sources in the engine will require adding a dependency_override section in you app's pubspec.yaml as detailed here.

Compiling for Windows

You can only build selected binaries on Windows (mainly gen_snapshot and the desktop embedding).

1. Make sure you have Visual Studio installed (non-Googlers only).

2. git pull upstream master in src/flutter to update the Flutter Engine repo.

3. If you are not a Google employee, you must set the following environment variables to point the depot tools at Visual Studio:

DEPOT_TOOLS_WIN_TOOLCHAIN=0
GYP_MSVS_OVERRIDE_PATH="C:\Program Files (x86)/Microsoft Visual Studio/2019/Community" (or your location for Visual Studio)
WINDOWSSDKDIR="C:\Program Files (x86)\Windows Kits\10" (or your location for Windows Kits)

Also, be sure that Python27 is before any other python in your Path.

4. gclient sync to update your dependencies.

5. switch to src/ directory.

6. python .\flutter\tools\gn --unoptimized to prepare your build files.

   • If you are only building gen_snapshot: python .\flutter\tools\gn [--unoptimized] --runtime-mode=[debug|profile|release] [--android].

7. ninja -C .\out\<dir created by previous step> to build.

   • If you used a non-debug configuration, use ninja -C .\out\<dir created by previous step> gen_snapshot. Release and profile are not yet supported for the desktop shell.

Compiling for Fuchsia

These steps build the Fuchsia embedding (flutter_runner) and test FAR files that can be deployed to a Fuchsia device.

Note these instructions assume use of x64, if arm64 is needed then just substitute as appropriate.

Fuchsia Source Checkout

Testing the Fuchsia embedding requires a Fuchsia source checkout. To get one, go to https://fuchsia.dev/fuchsia-src/get-started and follow the instructions to sync and build a Fuchsia checkout. The workstation (e.x. fx set workstation.nuc) product uses Flutter as its primary shell and is the primary way of testing Flutter on Fuchsia changes.

IMPORTANT: Dart Version synchronization on Fuchsia

The Fuchsia tree consumes the flutter_runner and associated Dart SDK as a set of prebuilts. Flutter apps within the Fuchsia tree are built against the version of the Dart SDK in these prebuilts. Because of this fact, developers must be careful to avoid any skew between the version of Dart VM built into the flutter_runner binary and the version of the Dart SDK & VM used by the Flutter toolchain (to compile flutter apps from Dart code). If there is any mismatch at all between the runner and toolchain, a runtime error results and Flutter won't work at all.

In practice, this means one of two workflows, depending on the nature of the change in question:

A. The change is to the engine C++ code only, and it isn't inside of the Dart VM or SDK. In this case it's important to match the git hash of Dart in the Fuchsia checkout to the one in the Flutter checkout. The easiest way to do this is (from your Fuchsia root):

git -C integration log -n 1 -- fuchsia/topaz/flutter | sed -ne 's/^.*flutter\/fuchsia .*to git_revision:\(.*\)/\1/p' Then use that git hash in step 1 under "build the engine".

B. The change is to the engine Dart code, or it involves changing the Dart VM or SDK. In this case, you do not need to pin to a specific revision. Work normally, then follow the specific instructions under "deploying flutter_runner" below.

Build the engine

1. git pull upstream master in src/flutter to update the Flutter Engine repo. git checkout <hash> if you want to checkout a specific git revision.

2. gclient sync to update your dependencies.

3. ./flutter/tools/gn --fuchsia --unoptimized to prepare your build files.

• NOTE: --unoptimized is broken on Fuchsia at the moment, see: https://github.com/flutter/flutter/issues/74872
• --unoptimized disables C++ compiler optimizations. On macOS, binaries are emitted unstripped; on Linux, unstripped binaries are emitted to an exe.unstripped subdirectory of the build.
• Add --fuchsia-cpu=x64 or --fuchsia-cpu=arm64 to target a particular architecture (default is x64).
• Add --runtime-mode=debug or --runtime-mode=release to switch between JIT and AOT builds. These correspond to a vanilla Fuchsia build and a --release Fuchsia build respectively.
• Add --no-lto if you don't care about performance or binary size and want a much faster build.
• Add the --xcode-symlinks argument when using goma on macOS.

4. ninja -C out/fuchsia_debug_unopt to build a Fuchsia unoptimized binary.
   • If you skipped --unoptimized, use ninja -C out/fuchsia_debug instead.
   • For Googlers, consider also using the --goma flag with gn, then building with autoninja to parallelize the build using Goma.

Deploy to Fuchsia

To test changes, you will first want to make the prebuilts writable. From your $FUCHSIA_DIR chmod -R +w prebuilt/third_party/flutter will make all of the flutter prebuilts writable.

After deploying any wanted changes to the Fuchsia checkout, perform fx build && fx ota to update your Fuchsia device with any changes you made.

flutter_runner

First copy the flutter_runner binary itself to your Fuchsia checkout:

cp out/fuchsia_debug_unopt/flutter_jit_runner-0.far $FUCHSIA_DIR/prebuilt/third_party/flutter/x64/jit/debug/flutter_jit_runner.far for standard (debug) builds

cp out/fuchsia_release/flutter_aot_product_runner-0.far $FUCHSIA_DIR/prebuilt/third_party/flutter/x64/aot/release/flutter_aot_product_runner.far for --release builds (you must build flutter with --runtime-mode=release)

If you are changing the Dart SDK or VM, you'll also want to update the dart toolchain that is used in your fuchsia checkout (note the use of aot/release in the destination, that is intentional):

cp -ra out/fuchsia_debug_unopt/host_bundle/dart_binaries-debug-linux-x64/* $FUCHSIA_DIR/prebuilt/third_party/flutter/x64/aot/release/dart_binaries/

cp -ra out/fuchsia_debug_unopt/dart_runner_patched_sdk/* $FUCHSIA_DIR/prebuilt/third_party/flutter/x64/aot/release/dart_runner_patched_sdk/

cp -ra out/fuchsia_debug_unopt/flutter_runner_patched_sdk/* $FUCHSIA_DIR/prebuilt/third_party/flutter/x64/aot/release/flutter_runner_patched_sdk/

debug symbols

Now copy debug symbols for the flutter_runner binary to your Fuchsia checkout (note that if you have a custom out/ folder in your Fuchsia checkout you will need to adjust --destination-base to match):

./flutter/tools/fuchsia/copy_debug_symbols.py --executable-name flutter_jit_runner --executable-path out/fuchsia_debug_unopt/exe.unstripped/flutter_jit_runner --destination-base $FUCHSIA_DIR/out/default/.build-id --read-elf $FUCHSIA_DIR/prebuilt/third_party/gcc/linux-x64/x86_64-elf/bin/readelf --unstripped for standard (debug) builds

./flutter/tools/fuchsia/copy_debug_symbols.py --executable-name flutter_aot_product_runner --executable-path out/fuchsia_release/exe.unstripped/flutter_aot_product_runner --destination-base $FUCHSIA_DIR/out/default/.build-id --read-elf $FUCHSIA_DIR/prebuilt/third_party/gcc/linux-x64/x86_64-elf/bin/readelf --unstripped for --release builds (you must build flutter with --runtime-mode=release)

tests

For any test FAR files, you may publish them to your device using pm publish (flow_tests.far used as an example; same note as above about the custom out/ folder applies):

./fuchsia/sdk/linux/tools/pm publish -a -r $FUCHSIA_DIR/out/default/amber-files -f out/fuchsia_debug_unopt/flow_tests.far fx test flow_tests

You can also copy test debug symbols by using the copy_debug_symbols.py script and substituting the test binary (such as flow_unittests) for the runner binary.

Compiling for the Web

For building the engine for the Web we use the felt tool.

To test Flutter with a local build of the Web engine, add --local-engine=host_debug_unopt to your flutter command, e.g.:

flutter run --local-engine=host_debug_unopt -d chrome
flutter test --local-engine=host_debug_unopt test/path/to/your_test.dart

Compiling for the Web on Windows

Compiling the web engine might take a few extra steps on Windows. Use cmd.exe and "run as administrator".

1. Make sure you have Visual Studio installed. Set the following environment variables. For Visual Studio use the path of the version you installed.
   • GYP_MSVS_OVERRIDE_PATH = "C:\Program Files (x86)\Microsoft Visual Studio\2019\Community"
   • GYP_MSVS_VERSION = 2017
2. Make sure, depot_tools, ninja and python are installed and added to the path. Also set the following environment variable for depot tools:
   • DEPOT_TOOLS_WIN_TOOLCHAIN = 0
   • Tip: if you get a python error try to use Python 2 instead of 3
3. git pull upstream master in src/flutter to update the Flutter Engine repo.
4. gclient sync to update your dependencies.
   • Tip: If you get a git authentication errors on this step try Git Bash instead
5. python .\flutter\tools\gn --unoptimized --full-dart-sdk to prepare your build files.
6. ninja -C .\out\<dir created by previous step> to build.

To test Flutter with a local build of the Web engine, add --local-engine=host_debug_unopt to your flutter command, e.g.:

flutter run --local-engine=host_debug_unopt -d chrome
flutter test --local-engine=host_debug_unopt test/path/to/your_test.dart

For testing the engine again use felt tool this time with felt_windows.bat.

felt_windows.bat test

Troubleshooting Compile Errors

Version Solving Failed

From time to time, as the Dart versions increase, you might see dependency errors such as:

The current Dart SDK version is 2.7.0-dev.0.0.flutter-1ef444139c.

Because ui depends on <a pub package> 1.0.0 which requires SDK version >=2.7.0 <3.0.0, version solving failed.

Running gclient sync does not update the tags, there are two solutions:

1. under engine/src/third_party/dart run git fetch --tags origin
2. or run gclient sync with with tags parameter: gclient sync --with_tags

See also: Debugging the engine, which includes instructions on running a Flutter app with a local engine.