Add Observable conformance to ThrowingStub

Author: Prachi Gauriar

.github/workflows/VerifyChanges.yaml

@@ -6,16 +6,18 @@ on:
   push:
     branches: ["main"]
 
+env:
+  XCODE_VERSION: 26.0.1
+
 jobs:
   lint:
     name: Lint
     runs-on: macos-26
     steps:
       - name: Checkout
         uses: actions/checkout@v4
-      - name: Select Xcode 26.0.0
-        run: |
-          sudo xcode-select -s /Applications/Xcode_26.0.0.app
+      - name: Select Xcode ${{ env.XCODE_VERSION }}
+        run: sudo xcode-select -s /Applications/Xcode_${{ env.XCODE_VERSION }}.app
       - name: Lint
         run: |
           Scripts/lint
@@ -29,7 +31,7 @@ jobs:
       matrix:
         include:
 #           - platform: iOS
-#             xcode_destination: "platform=iOS Simulator,name=iPhone 16 Pro"
+#             xcode_destination: "platform=iOS Simulator,name=iPhone 17 Pro"
           - platform: macOS
             xcode_destination: "platform=macOS,arch=arm64"
 #           - platform: tvOS
@@ -47,9 +49,8 @@ jobs:
       XCODE_TEST_PRODUCTS_PATH: .build/DevTesting.xctestproducts
 
     steps:
-      - name: Select Xcode 26.0.0
-        run: |
-          sudo xcode-select -s /Applications/Xcode_26.0.0.app
+      - name: Select Xcode ${{ env.XCODE_VERSION }}
+        run: sudo xcode-select -s /Applications/Xcode_${{ env.XCODE_VERSION }}.app
 
       - name: Checkout
         uses: actions/checkout@v4

CHANGELOG.md

@@ -1,5 +1,12 @@
 # DevTesting Changelog
 
+## 1.4.0: October 12, 2025
+
+`Stub` and `ThrowingStub` now conform to `Observable`. The only property that is tracked is
+``calls``. Changes to dependent properties like ``callArguments`` and ``callResults`` can also be
+tracked, but changes to ``resultQueue`` and ``defaultResult`` are not.
+
+
 ## 1.3.0: October 2, 2025
 
 Adds functions for randomly generating dates within a specified range.

Scripts/test-all-platforms

@@ -23,7 +23,7 @@ print_error() {
 
 # Platforms to test
 PLATFORMS=(
-    "iOS Simulator,name=iPhone 16 Pro"
+    "iOS Simulator,name=iPhone 17 Pro"
     "macOS"
     "tvOS Simulator,name=Apple TV 4K"
     "watchOS Simulator,name=Apple Watch Series 10"

Sources/DevTesting/Stubbing/Stub.swift

@@ -9,14 +9,22 @@ import Foundation
 import os
 
 /// A stub for a function.
+///
+/// - Note: ``Stub`` and `ThrowingStub` are `Observable`, but the only property that is tracked is ``calls``. Changes to
+///   dependent properties like ``callArguments`` and ``callResults`` can also be tracked, but changes to
+///   ``resultQueue`` and ``defaultResult`` are not.
+@Observable
 public final class ThrowingStub<Arguments, ReturnType, ErrorType> where ErrorType: Error {
     /// A recorded call to the stub.
-    public struct Call {
+    ///
+    /// This type conforms to Sendable, but both properties are `nonisolated(unsafe)`. It is the consumer’s
+    /// responsibility to ensure that the type is used in a safe way.
+    public struct Call: Sendable {
         /// The call’s arguments.
-        public let arguments: Arguments
+        public nonisolated(unsafe) let arguments: Arguments
 
         /// The result of the call.
-        public let result: Result<ReturnType, ErrorType>
+        public nonisolated(unsafe) let result: Result<ReturnType, ErrorType>
     }
 
 
@@ -88,13 +96,16 @@ public final class ThrowingStub<Arguments, ReturnType, ErrorType> where ErrorTyp
     ///
     /// If you just need the call’s arguments, you can use ``callArguments`` instead.
     public var calls: [Call] {
+        access(keyPath: \.calls)
         return mutableProperties.withLockUnchecked { $0.calls }
     }
 
 
     /// Clears the stub’s recorded calls.
     public func clearCalls() {
-        mutableProperties.withLockUnchecked { $0.calls = [] }
+        withMutation(keyPath: \.calls) {
+            mutableProperties.withLockUnchecked { $0.calls = [] }
+        }
     }
 
 
@@ -105,19 +116,21 @@ public final class ThrowingStub<Arguments, ReturnType, ErrorType> where ErrorTyp
     ///
     /// - Parameter arguments: The arguments with which to call the stub.
     public func callAsFunction(_ arguments: Arguments) throws(ErrorType) -> ReturnType {
-        let result = mutableProperties.withLockUnchecked { (properties) in
-            let result =
-                if properties.resultQueue.isEmpty {
-                    properties.defaultResult
-                } else {
-                    properties.resultQueue.removeFirst()
-                }
-
-            properties.calls.append(
-                .init(arguments: arguments, result: result)
-            )
-
-            return result
+        let result = withMutation(keyPath: \.calls) {
+            mutableProperties.withLockUnchecked { (properties) in
+                let result =
+                    if properties.resultQueue.isEmpty {
+                        properties.defaultResult
+                    } else {
+                        properties.resultQueue.removeFirst()
+                    }
+
+                properties.calls.append(
+                    .init(arguments: arguments, result: result)
+                )
+
+                return result
+            }
         }
 
         return try result.get()

Tests/DevTestingTests/Stubbing/StubTests.swift

@@ -195,4 +195,90 @@ struct StubTests {
         // queue just for posterity
         #expect(stub.returnValueQueue.isEmpty)
     }
+
+
+    // MARK: - Observable
+
+    @Test
+    func callAsFunctionTriggersObservation() async throws {
+        // set up stub
+        let stub = Stub<String, Int>(defaultReturnValue: 42)
+
+        // set up observations stream
+        let observations = Observations { stub.calls }
+        var iterator = observations.makeAsyncIterator()
+
+        // expect initial empty calls
+        let initialCalls = await iterator.next()
+        #expect(initialCalls?.isEmpty == true)
+
+        // exercise by calling stub
+        _ = stub("first")
+
+        // expect calls updated with first call
+        let callsAfterFirst = await iterator.next()
+        #expect(callsAfterFirst?.count == 1)
+        #expect(callsAfterFirst?.first?.arguments == "first")
+
+        // exercise by calling stub again
+        _ = stub("second")
+
+        // expect calls updated with second call
+        let callsAfterSecond = await iterator.next()
+        #expect(callsAfterSecond?.count == 2)
+        #expect(callsAfterSecond?.last?.arguments == "second")
+    }
+
+
+    @Test
+    func clearCallsTriggersObservation() async throws {
+        // set up stub with some calls
+        let stub = Stub<String, Int>(defaultReturnValue: 42)
+        _ = stub("first")
+        _ = stub("second")
+
+        // set up observations stream
+        let observations = Observations { stub.calls }
+        var iterator = observations.makeAsyncIterator()
+
+        // expect initial calls
+        let initialCalls = await iterator.next()
+        #expect(initialCalls?.count == 2)
+
+        // exercise by clearing calls
+        stub.clearCalls()
+
+        // expect calls cleared
+        let clearedCalls = await iterator.next()
+        #expect(clearedCalls?.isEmpty == true)
+    }
+
+
+    @Test
+    mutating func observability() async {
+        let arguments = [1, 2, 3, 4, 5]
+        let stub = Stub<Int, Void>()
+
+        Task {
+            for value in arguments {
+                try? await Task.sleep(for: .milliseconds(100))
+                stub(value)
+            }
+        }
+
+        let observationTask = Task {
+            var i = 0
+            for await argument in Observations({ stub.callArguments.last }).dropFirst() {
+                #expect(argument == arguments[i])
+                i += 1
+
+                if argument == arguments.count {
+                    break
+                }
+            }
+        }
+
+        await observationTask.value
+        #expect(stub.callArguments == arguments)
+    }
 }