Dependencies: dynamically link XCTest

Use delayed dynamic linking for registering the test observer. This is
particularly important for Windows which does not support weak linking
and weak symbols. Even in the case that `Dependencies` is imported due
to use, we will not pull in the test observer until runtime, as a best
effort. If the DependenciesTestObserver library is found in the search
path, it will be loaded and the observer initialised. If we can't find
the library, we will simply continue without the test observer.

Package@swift-5.9.swift

@@ -19,7 +19,7 @@ let package = Package(
     .library(
       name: "DependenciesMacros",
       targets: ["DependenciesMacros"]
-    )
+    ),
   ],
   dependencies: [
     .package(url: "https://github.com/apple/swift-syntax", from: "509.0.0"),
@@ -31,6 +31,12 @@ let package = Package(
     .package(url: "https://github.com/pointfreeco/xctest-dynamic-overlay", from: "1.0.0"),
   ],
   targets: [
+    .target(
+      name: "DependenciesTestObserver",
+      dependencies: [
+        .product(name: "XCTestDynamicOverlay", package: "xctest-dynamic-overlay"),
+      ]
+    ),
     .target(
       name: "Dependencies",
       dependencies: [
@@ -85,6 +91,16 @@ let package = Package(
   )
 #endif
 
+#if !os(macOS)
+package.products.append(
+  .library(
+    name: "DependenciesTestObserver",
+    type: .dynamic,
+    targets: ["DependenciesTestObserver"]
+  )
+)
+#endif
+
 //for target in package.targets {
 //  target.swiftSettings = target.swiftSettings ?? []
 //  target.swiftSettings?.append(

Sources/Dependencies/DependencyValues.swift

@@ -1,4 +1,23 @@
 import Foundation
+#if os(Windows)
+  import WinSDK
+#elseif os(Linux)
+  import Glibc
+#endif
+
+#if _runtime(_ObjC)
+extension DispatchQueue {
+  fileprivate static func mainSync<R>(execute block: @Sendable () -> R) -> R {
+    if Thread.isMainThread {
+      return block()
+    } else {
+      return Self.main.sync(execute: block)
+    }
+  }
+}
+
+final class TestObserver: NSObject {}
+#endif
 
 /// A collection of dependencies that is globally available.
 ///
@@ -95,8 +114,46 @@ public struct DependencyValues: Sendable {
   /// provide access only to default values. Instead, you rely on the dependency values' instance
   /// that the library manages for you when you use the ``Dependency`` property wrapper.
   public init() {
-    #if canImport(XCTest)
-      _ = setUpTestObservers
+    #if _runtime(_ObjC)
+      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
+      typealias RegisterTestObserver = @convention(thin) (@convention(c) () -> Void) -> Void
+      var pRegisterTestObserver: RegisterTestObserver? = nil
+
+      #if os(Windows)
+        let hModule = LoadLibraryA("DependenciesTestObserver.dll")
+        if let hModule, let pAddress = GetProcAddress(hModule, "$s24DependenciesTestObserver08registerbC0yyyyXCF") {
+          pRegisterTestObserver = unsafeBitCast(pAddress, to: RegisterTestObserver.self)
+        }
+      #else
+        let hModule: UnsafeMutableRawPointer? = dlopen("libDependenciesTestObserver.so", RTLD_NOW)
+        if let hModule, let pAddress = dlsym(hModule, "$s24DependenciesTestObserver08registerbC0yyyyXCF") {
+          pRegisterTestObserver = unsafeBitCast(pAddress, to: RegisterTestObserver.self)
+        }
+      #endif
+      pRegisterTestObserver?({
+        DependencyValues._current.cachedValues.cached = [:]
+      })
     #endif
   }
 
@@ -348,59 +405,3 @@ private final class CachedValues: @unchecked Sendable {
     return value
   }
 }
-
-// NB: We cannot statically link/load XCTest on Apple platforms, so we dynamically load things
-//     instead on platforms where XCTest is available.
-#if canImport(XCTest)
-  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 {}
-
-    extension DispatchQueue {
-      fileprivate static func mainSync<R>(execute block: @Sendable () -> R) -> R {
-        if Thread.isMainThread {
-          return block()
-        } else {
-          return Self.main.sync(execute: block)
-        }
-      }
-    }
-  #else
-    import XCTest
-
-    private final class TestObserver: NSObject, XCTestObservation {
-      func testCaseWillStart(_ testCase: XCTestCase) {
-        DependencyValues._current.cachedValues.cached = [:]
-      }
-    }
-  #endif
-#endif

Sources/DependenciesTestObserver/TestObserver.swift

@@ -0,0 +1,22 @@
+
+import XCTest
+import XCTestDynamicOverlay
+
+#if !_runtime(_ObjC)
+  final class TestObserver: NSObject, XCTestObservation {
+    private let resetCache: @convention(c) () -> Void
+    internal init(_ resetCache: @convention(c) () -> Void) {
+      self.resetCache = resetCache
+    }
+    public func testCaseWillStart(_ testCase: XCTestCase) {
+      self.resetCache()
+    }
+  }
+#endif
+
+public func registerTestObserver(_ resetCache: @convention(c) () -> Void) {
+  guard _XCTIsTesting else { return }
+  #if !_runtime(_ObjC)
+    XCTestObservationCenter.shared.addTestObserver(TestObserver(resetCache))
+  #endif
+}