Support Xcode 16.2 onPreferenceChange SDK change

[philippzagar]

Support Xcode 16.2 onPreferenceChange SDK change

♻️ Current situation & Problem

As shown in #47 and StanfordSpezi/SpeziLLM#76, Apple chose to change the signature of the onPreferenceChange view modifier in SwiftUI with the latest Xcode 16.2 RC (also the betas) and the iOS 18.2 SDKs.
In its current state, this prevents the SpeziViews package from compiling in these environments.

The onPreferenceChange view modfier now takes a @Sendable closure, therefore we cannot capture @MainActor isolated properties on the View directly anymore: https://developer.apple.com/documentation/swiftui/view/onpreferencechange(_:perform:)?changes=latest_minor
However, as the @Sendable closure is still run on the MainActor's serial executor (at least in my testing on 18.2 RC SDKs), we can use MainActor.assumeIsolated to avoid scheduling a MainActor Task, which could delay execution and cause unexpected UI behavior.

⚙️ Release Notes

• Support Xcode 16.2 onPreferenceChange SDK change
• Use Swift 6 language mode in the UITest app

📚 Documentation

Properly documented in-line why this change was necessary

✅ Testing

Tested the changes locally on Xcode 16.2 RC as well as iOS 18.2 betas.
We should lifting the CI builds / testing to these versions as well, as soon as they're out (should only be a couple of days).

📝 Code of Conduct & Contributing Guidelines

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• I agree to follow the Code of Conduct and Contributing Guidelines.

[philippzagar]

There's also a concurrency warning in ReceiveValidationModifier, as the ValidationState.Binding (a non-Sendable type) is used in the (now) @Sendable closure of .onPreferenceChange().
An easy fix would be to annotate ValidationState.Binding as @unchecked Sendable, but that's technically no correct as the nested ValidationContext cannot be easily made Sendable in its current form.
Looking forward to your opinion @Supereg!

[philippzagar]

@Supereg Please do a quick review of the PR, would be great to get this code in soon as the Xcode 16.2 RC was released nearly a week ago (Dec 5), meaning that the public release will probably come in a couple of days.
As lots of Spezi packages and apps depend on SpeziViews, it would be great to do this before the public release 🚀

Just noticed: I didn't take a look into why the snapshot tests fail yet, will probably be better if you take a quick glance into that, would be way quicker!

[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.

Project coverage is 74.91%. Comparing base (f875144) to head (2cf85ba).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...iViews/Views/Layout/HorizontalGeometryReader.swift	69.24%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
- Coverage   74.97%   74.91%   -0.06%     
==========================================
  Files          52       52              
  Lines        1574     1586      +12     
==========================================
+ Hits         1180     1188       +8     
- Misses        394      398       +4     

Files with missing lines	Coverage Δ
Sources/SpeziValidation/ValidationRule.swift	53.71% <ø> (ø)
Sources/SpeziViews/Views/Button/AsyncButton.swift	66.95% <ø> (ø)
...iViews/Views/Layout/HorizontalGeometryReader.swift	86.67% <69.24%> (-13.33%)	⬇️

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[philippzagar]

@PSchmiedmayer Please merge the PR with bypassing merge requirements. Minor decrease in test coverage as we added a "safety" case in case SwiftUI ever moves the onPreferenceChange() closure off the MainActor.

[PSchmiedmayer]

Amazing, thank you for the fix @philippzagar!