⚠️ TODO: Verify test fails: #154589
⚠️ TODO: Update SPM umbrella issue to remove the pre action test
⚠️ TODO: Update this description to explain the new test.

Problem

Enabling the Swift Package Manager feature caused post-submit tests to fail on Mac x64 hosts:

♦ ... flutter --verbose assemble ... -dIosArchs=x86_64 ... profile_unpack_ios

Target profile_unpack_ios failed:
Exception: Binary ... build/ios/Profile-iphoneos/Flutter.framework/Flutter does not contain x86_64.

Running lipo -info:
Non-fat file: ... build/ios/Profile-iphoneos/Flutter.framework/Flutter is architecture: arm64

#0      UnpackIOS._thinFramework (package:flutter_tools/src/build_system/targets/ios.dart:351:7)
<asynchronous suspension>
#1      UnpackIOS.build (package:flutter_tools/src/build_system/targets/ios.dart:298:5)
<asynchronous suspension>
...

Background

As part of the Swift Package Manager project, an Xcode Scheme Pre-Action was added to unpack the Flutter framework. This ensures the Flutter framework is available before Swift Package Manager builds packages.

#150052 improved the performance of builds using Swift Package Manager:

When using SwiftPM, we use flutter assemble in an Xcode Pre-action to run the debug_unpack_macos (or profile/release) target. This target is also later used in a Run Script build phase. Depending on ARCHS build setting, the Flutter/FlutterMacOS binary is thinned. In the Run Script build phase, ARCHS is filtered to the active arch. However, in the Pre-action it doesn't always filter to the active arch. As a workaround, assume arm64 if the NATIVE_ARCH is arm, otherwise assume x86_64.

However, NATIVE_ARCH is the architecture of the host machine, not the target machine. As a result, Mac x86_64 hosts attempted to unpack the Flutter iOS framework using the x86_64 architecture when it is Arm64 only.

Solution

This updates the Xcode Pre-Action's logic: instead of using the host machine's architecture, it uses the build's SDKROOT. If the SDK root isn't a simulator, the pre-action assumes an Arm64 target build.

This change also adds a new ios_prepare_pre_action_test test project. This allows us to test the Xcode Scheme Pre-Action even if the Swift Package Manager feature is disabled.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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