fix: guard FlutterTaskQueue for backward compatibility

ios/optimizely_flutter_sdk/Sources/optimizely_flutter_sdk/SwiftOptimizelyFlutterSdkPlugin.swift

@@ -49,11 +49,8 @@ public class SwiftOptimizelyFlutterSdkPlugin: NSObject, FlutterPlugin {
         registrar.addMethodCallDelegate(instance, channel: channel)
 
         // Separate logger channel for outgoing log calls
-        let taskQueue = registrar.messenger().makeBackgroundTaskQueue?()
-        let loggerChannel = FlutterMethodChannel(name: OptimizelyFlutterLogger.LOGGER_CHANNEL, 
-                                                binaryMessenger: registrar.messenger(), 
-                                                codec: FlutterStandardMethodCodec.sharedInstance(),
-                                                taskQueue: taskQueue)
+        let loggerChannel = FlutterMethodChannel(name: OptimizelyFlutterLogger.LOGGER_CHANNEL,
+                                                binaryMessenger: messenger)
         OptimizelyFlutterLogger.setChannel(loggerChannel)
     }
 

specs/002-flutter-task-queue-compat/spec.md

@@ -0,0 +1,101 @@
+# Spec: FlutterTaskQueue Backward Compatibility
+
+**Date**: 2026-08-22
+**Status**: Implemented
+**Severity**: Build-breaking on older Flutter + modern Xcode
+
+## Problem
+
+`SwiftOptimizelyFlutterSdkPlugin.register(with:)` calls
+`registrar.messenger().makeBackgroundTaskQueue?()` and passes the result to
+`FlutterMethodChannel(name:binaryMessenger:codec:taskQueue:)` when creating the
+logger channel (line 52-56 of `SwiftOptimizelyFlutterSdkPlugin.swift`).
+
+The `FlutterTaskQueue` protocol and the `makeBackgroundTaskQueue` method were
+introduced in Flutter 3.3. However, on older Flutter versions (e.g. 3.16.0) the
+`FlutterTaskQueue` protocol is declared in the ObjC header as a forward
+reference without a full definition. When compiled with a modern Xcode (26.6+)
+and iOS SDK (26.5+), the Swift compiler cannot import the incomplete protocol
+and emits two hard errors:
+
+```
+error: value of type 'any FlutterBinaryMessenger' has no member 'makeBackgroundTaskQueue'
+note: protocol 'FlutterTaskQueue' is incomplete
+
+error: extra argument 'taskQueue' in call
+```
+
+This breaks the CocoaPods build path in the `optimizely-flutter-testapp` CI,
+which uses Flutter 3.16.0 with the latest macOS runner (Xcode 26.6).
+
+## Root Cause
+
+Flutter 3.16.0's `Flutter.xcframework` ships an ObjC header where
+`FlutterTaskQueue` is forward-declared but not fully defined. Swift's strict
+ObjC interop (tightened in recent Xcode toolchains) rejects methods whose
+return type references an incomplete protocol. Newer Flutter versions (3.44+)
+ship a complete protocol definition, so the same code compiles cleanly.
+
+## Impact
+
+- **Testapp iOS CI**: Build failure on every run against the SDK's master
+  branch or any feature branch.
+- **Consumer apps**: Any app using an older Flutter version (< 3.22) with a
+  modern Xcode will hit the same compilation error.
+- **Functionality**: The `taskQueue` parameter is an optimization that routes
+  logger channel callbacks to a background serial queue. Passing `nil` falls
+  back to the platform's default (main thread), which is how the logger already
+  operated before this code was added. There is no functional regression from
+  removing it.
+
+## Solution
+
+Remove `makeBackgroundTaskQueue` and `taskQueue` entirely from the logger
+channel creation. The logger already dispatches all callbacks to the main
+thread via `DispatchQueue.main.async` in `OptimizelyFlutterLogger.swift`, so
+the background task queue provided no additional value. Dropping it eliminates
+the compile-time dependency on the incomplete `FlutterTaskQueue` protocol.
+
+### Before
+
+```swift
+let taskQueue = registrar.messenger().makeBackgroundTaskQueue?()
+let loggerChannel = FlutterMethodChannel(name: OptimizelyFlutterLogger.LOGGER_CHANNEL,
+                                        binaryMessenger: registrar.messenger(),
+                                        codec: FlutterStandardMethodCodec.sharedInstance(),
+                                        taskQueue: taskQueue)
+```
+
+### After
+
+```swift
+let loggerChannel = FlutterMethodChannel(name: OptimizelyFlutterLogger.LOGGER_CHANNEL,
+                                        binaryMessenger: messenger)
+```
+
+### Why drop `taskQueue` instead of guarding it?
+
+A runtime guard (`responds(to:)`) is insufficient because the Swift compiler
+rejects the code at **compile time** — `FlutterTaskQueue` is incomplete, so any
+code path that references `makeBackgroundTaskQueue` or casts to `FlutterTaskQueue`
+fails to compile regardless of whether it would execute at runtime.
+
+`performSelector`-based workarounds are fragile and hard to maintain for a minor
+optimization. The logger channel already dispatches callbacks to the main thread
+via `DispatchQueue.main.async` in `OptimizelyFlutterLogger.swift`, so the
+background task queue adds no real value here.
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `ios/Classes/SwiftOptimizelyFlutterSdkPlugin.swift` | Remove `makeBackgroundTaskQueue` / `taskQueue` usage from logger channel creation |
+
+## Testing
+
+- **CI**: Re-run testapp iOS integration tests with Flutter 3.16.0 — build
+  should succeed.
+- **Local**: Build with Flutter 3.44.0 + Xcode 26.6 — `taskQueue` path should
+  still be taken.
+- **Existing tests**: No Dart-layer changes, so `flutter test` should pass
+  unchanged.