Allow providing pre-compiled main.jsbundle (CI optimization)

[andreialecu]

Introduction

Since MacOS compute time is generally the most expensive on CI platforms, it seems that it would be possible to reduce the amount of time taken during xcode compilation by skipping the react-native bundle compilation step.

For context, I wanted to create a workflow to run react-native bundle on Linux VMs and then feed the pre-compiled assets to a MacOS build step further down the pipeline, but ran into issues.

CircleCI specifically mentions this possibility on their RN landing page: https://circleci.com/circleci-react/

Run Devtools in Linux
Build macOS and run devtools in Linux jobs to increase overall build throughput.

Additionally, depending on the complexity of the pipeline, the bundle could be reused or even cached in a CI workflow, greatly reducing overall processing time.

Details

There is a SKIP_BUNDLING env variable that is checked here: https://github.com/facebook/react-native/blob/master/scripts/react-native-xcode.sh#L30

Setting SKIP_BUNDLING=true as an environment variable does work to skip the bundling step.

However, I found it hard to figure out where to actually place the assets, because the destination folder seems to be DEST=$CONFIGURATION_BUILD_DIR/$UNLOCALIZED_RESOURCES_FOLDER_PATH, which is largely unknown before running XCode.

Discussion points

Seems like it might be a good idea to add a feature of reusing pre-built bundles.

For example, an env variable like export RN_BUNDLE_PREBUILD_DIR=./dist/ios could be checked and if present, the build step would simply copy the contents of that directory over to its expected $DEST directory. It would be great if this worked for both Android and iOS. For iOS it should be a very simple change to the above .sh script. I haven't checked Android yet.

Any thoughts?

[retyui]

@andreialecu
Usually, compiling js bundle it a small piece of the build process a lot of time xcode takes to build a native code

You can try the next way to reduce financial expenses: https://dev.to/retyui/react-native-how-speed-up-ios-build-4x-using-cache-pods-597c

[andreialecu]

@retyui You may want to also take a look at Subito-it/PodBuilder#10 for a less complicated solution for prebuilding pods that doesn't necessarily require fastlane.

I haven't been able to use other solutions for prebuilding in real-life projects (such as the ones mentioned in your article) due to various issues which are detailed in the thread above.

[retyui]

I read that Wix engineer use "expo" way to run js code on once precompile app