Flutter Engine notes

Saturday, 25 November 2023

This is a quick-start guide of setting up the Flutter engine development environment, making a simple change, and using it in a Flutter app.

My setup is fairly standard – I’m on M1 Mac, Android emulator is arm64, iOS simulator is arm64 as well. No Rosetta2 involved.


This step is well explained by the official guide.

First step is to clone depot_tools into $HOME:

git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git ~/depot_tools

Since I don’t do Engine development often, I don’t want depot_tools in my $PATH by default. Instead, I export it only when I need it:

export PATH="$HOME/depot_tools:$PATH"


Build host-side executables

Generate build files:

./flutter/tools/gn --unoptimized --mac-cpu arm64

Run build:

ninja -C out/host_debug_unopt_arm64

Build for Android emulator

Generate build files:

./flutter/tools/gn --android --android-cpu arm64 --unoptimized

Run build:

ninja -C out/android_debug_unopt_arm64

To build only the JAR (skipping things like generating javadoc, linting):

ninja -C out/android_debug_unopt_arm64 android_jar

Build for iOS simulator

Generate build files:

./flutter/tools/gn --ios --unoptimized --simulator --simulator-cpu arm64

Run build:

ninja -C out/ios_debug_sim_unopt_arm64


Let’s assume that:

export FLUTTER_ENGINE=~/dev/bartekpacia/engine/src/

Android emulator ARM64:

flutter \
    --local-engine android_debug_unopt_arm64 \
    --local-engine-host host_debug_unopt_arm64 \
    --local-engine-src-path "$FLUTTER_ENGINE" \

iOS simulator ARM64 (only the --local-engine argument differs):

flutter \
    --local-engine ios_debug_sim_unopt_arm64 \
    --local-engine-host host_debug_unopt_arm64 \
    --local-engine-src-path "$FLUTTER_ENGINE" \

Theoretically, passing --local-engine-src-path $FLUTTER_ENGINE is redundant - the default value is $FLUTTER_ENGINE. For example you could set:

export FLUTTER_ENGINE=~/dev/bartekpacia/engine/src

Unfortunately I’ve found it to be a bit flaky, and prefer to pass --local-engine-src-path explicitly and not set $FLUTTER_ENGINE envvar.


Run Android tests of AccessibilityBridge with Roboelectric:

./testing/run_tests.py \
    --type java \
    --variant host_debug_unopt_arm64 \
    --android android_debug_unopt_arm64 \
    --java-filter io.flutter.view.AccessibilityBridge

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