[RN][macos] Enable Hermes for RNTester

[shwanton]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

On RN Core, Hermes for iOS can be enabled by setting a flag in the Podfile & running pod install
https://reactnative.dev/docs/hermes#ios

Since React Native 0.64, Hermes also runs on iOS. To enable Hermes for iOS, edit your ios/Podfile file and make the change illustrated below:

   use_react_native!(
     :path => config[:reactNativePath],
     # to enable hermes on iOS, change `false` to `true` and then install pods
     # By default, Hermes is disabled on Old Architecture, and enabled on New Architecture.
     # You can enable/disable it manually by replacing `flags[:hermes_enabled]` with `true` or `false`.
     :hermes_enabled => true
   )

In Core RNTester for iOS, Hermes is enabled by default
Not until 0.70 - https://github.com/facebook/react-native/blob/0.70-stable/scripts/react_native_pods.rb#L41

def use_react_native! (
  path: "../node_modules/react-native",
  fabric_enabled: false,
  new_arch_enabled: ENV['RCT_NEW_ARCH_ENABLED'] == '1',
  production: ENV['PRODUCTION'] == '1',
  hermes_enabled: true,
  flipper_configuration: FlipperConfiguration.disabled,
  app_path: '..',
  config_file_dir: '')

This will build & install the Hermes runtime & enable the RCT_USE_HERMES macro.
https://github.com/microsoft/react-native-macos/blob/main/packages/rn-tester/RNTester/AppDelegate.mm#L10-L16

In 0.69 Hermes is disabled by default - https://github.com/facebook/react-native/blob/0.69-stable/scripts/react_native_pods.rb#L31

---

On RN Desktop, the Hermes flag was disabled #780 due to M1 build reasons which have since been resolved. #952 #781

Curiously, the RNTester-macOS target AppDelegate was never updated to use Hermes once the Pod was added.
Only the RNTester target for mobile had the correct Hermes usage.

RNTester-macOS: https://github.com/microsoft/react-native-macos/blob/main/packages/rn-tester/RNTester-macOS/AppDelegate.mm
RNTester: https://github.com/microsoft/react-native-macos/blob/main/packages/rn-tester/RNTester/AppDelegate.mm

This change will not enable Hermes by default on RNTester. Pass USE_HERMES=1to pod install to enable.

---

The documentation for enabling Hermes on RN Desktop macOS can be simplified now.
https://microsoft.github.io/react-native-windows/docs/hermes#hermes-on-macos

Install the npm package yarn add 'hermes-engine-darwin@^0.4.3'

• hermes-engine-darwin is no longer published/required rn-macos's Hermes support: RNTester on 0.64 & 0.66 #952

Add (or uncomment) the following pod dependencies to your macOS target in your Podfile:
pod 'React-Core/Hermes', :path => '../node_modules/react-native-macos/'
pod 'hermes', :path => '../node_modules/hermes-engine-darwin'
pod 'libevent', :podspec => '../node_modules/react-native-macos/third-party-podspecs/libevent.podspec'

• Setting :hermes_enabled => true in Podfile should do all of the above.

---

Changelog

[macOS] [pod] Copy Hermes support to RNTester-macOS AppDelegate
[macOS] [pod] Remove pods(:hermes_enabled => true) in favor of using envvar USE_HERMES=1 during pod install.

Test Plan

RNTester with Hermes enabled:

cd packages/rn-tester

# Ensure fresh pod install. I noticed `RCT_USE_HERMES` would sometimes not work w/o full clean/rebuild
rm -rf Pods 

# Install pods w/ Hermes flag
USE_HERMES=1 bundle exec pod install --verbose

# Note the installing (or downloading) of hermes-engine

-> Installing hermes-engine (0.11.0)
  > Copying hermes-engine from `/Users/shawndempsey/Library/Caches/CocoaPods/Pods/Release/hermes-engine/0.11.0-84e3a` to `Pods/hermes-engine`

# Open in xcode
open RNTesterPods.xcworkspace

hermesworkingrntestermed.mp4

[Saadnajmi]

Just to be clear, this change brings the RNTester-macOS target back into parity with RN Core and our own RNTester target where enabling and disabling Hermes is as simple as a flag in the pod file? And from the looks of it, keeps it off by default?

[shwanton]

Yes, we don't have plans on enabling by default (which is what happens now on RN). However, we would like to enable it during development so we can connect & debug using flipper.

[Saadnajmi]

It looks good from me. I'll have @amgleitman give it a second look if he likes since he was involved with merging the changes from RN Core that included Hermes stuff.

[christophpurrer]

Awesome change!

[Saadnajmi]

I'll go ahead and merge this.

[christophpurrer]

If you want to try this out on an M1/M2/... MacBook you need to run

USE_HERMES=1 bundle exec arch -x86_64 pod install --verbose

[Saadnajmi]

If you want to try this out on an M1/M2/... MacBook you need to run

USE_HERMES=1 bundle exec arch -x86_64 pod install --verbose

The latest version of cocoapods from homebrew should have m1 support? I don't think you need the "arch -x86_64" bit anymore

[christophpurrer]

Ooh!
I don't know. I need to do that locally ;-)
I am using this version

chpurrer@chpurrer-mbp rn-tester % pod --version
1.11.3

[Saadnajmi]

Ooh! I don't know. I need to do that locally ;-) I am using this version

chpurrer@chpurrer-mbp rn-tester % pod --version 1.11.3

Latest I found.. looks like you have to use homebrew? https://stackoverflow.com/a/65334677
I'm also using 1.11.3