TestObserver implementation question

[john-flanagan]

I'm trying to better understand test dependency caching. If I'm understanding correctly this code clears out the dependency cache between each test case:

swift-dependencies/Sources/Dependencies/DependencyValues.swift

Lines 355 to 397 in 51721ee

	#if !canImport(ObjectiveC)
	import XCTest
	#endif
	private let setUpTestObservers: Void = {
	if _XCTIsTesting {
	#if canImport(ObjectiveC)
	DispatchQueue.mainSync {
	guard
	let XCTestObservation = objc_getProtocol("XCTestObservation"),
	let XCTestObservationCenter = NSClassFromString("XCTestObservationCenter"),
	let XCTestObservationCenter = XCTestObservationCenter as Any as? NSObjectProtocol,
	let XCTestObservationCenterShared =
	XCTestObservationCenter
	.perform(Selector(("sharedTestObservationCenter")))?
	.takeUnretainedValue()
	else { return }
	let testCaseWillStartBlock: @convention(block) (AnyObject) -> Void = { _ in
	DependencyValues._current.cachedValues.cached = [:]
	}
	let testCaseWillStartImp = imp_implementationWithBlock(testCaseWillStartBlock)
	class_addMethod(
	TestObserver.self, Selector(("testCaseWillStart:")), testCaseWillStartImp, nil)
	class_addProtocol(TestObserver.self, XCTestObservation)
	_ =
	XCTestObservationCenterShared
	.perform(Selector(("addTestObserver:")), with: TestObserver())
	}
	#else
	XCTestObservationCenter.shared.addTestObserver(TestObserver())
	#endif
	}
	}()
	#if canImport(ObjectiveC)
	private final class TestObserver: NSObject {}
	#else
	private final class TestObserver: NSObject, XCTestObservation {
	func testCaseWillStart(_ testCase: XCTestCase) {
	DependencyValues._current.cachedValues.cached = [:]
	}
	}
	#endif

I'm just curious why the implementation is using objc_getProtocol, NSClassFromString, etc when Objective-C is available rather than always using XCTestObservationCenter.shared.addTestObserver(TestObserver()).

[john-flanagan]

I don't know how I missed the comment directly above the code I highlighted 🤦

// NB: We cannot statically link/load XCTest on Apple platforms, so we dynamically load things
//     instead and we limit this to debug builds to avoid App Store binary rejection.