fix: address code review feedback from PR #13 and #15

- Add thread-safety doc comments to AudioEngineManager callbacks
  (onSamples/onAudioLevel must be set before start())
- Remove unused termNormalizer field from ASREngineFactory.Result
- Remove dead destDir assignment in ModelDownloader delegate method

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ZhaoChaoqun

Sources/ASREngineFactory.swift

@@ -14,10 +14,9 @@ private let logger = Logger(subsystem: "com.typeless.app", category: "ASREngineF
 ///   no other code touches them during initialization (FSM is in `.initializing` state).
 enum ASREngineFactory {
 
-    /// Result of engine creation, containing the engine and a TermNormalizer.
+    /// Result of engine creation.
     struct Result {
         let engine: (any ASREngine)?
-        let termNormalizer: TermNormalizer?
     }
 
     /// Creates an ASR engine for the given model type, also loading supporting models
@@ -27,7 +26,7 @@ enum ASREngineFactory {
     ///   - modelType: Which ASR model to load.
     ///   - pipeline: The post-processing pipeline to configure with supporting models.
     ///   - recognitionQueue: The serial queue used by the ASR engine for decoding.
-    /// - Returns: The created ASR engine (nil on failure) and TermNormalizer.
+    /// - Returns: The created ASR engine (nil on failure).
     static func createEngine(
         for modelType: ASRModelType,
         pipeline: PostProcessingPipeline,
@@ -58,7 +57,7 @@ enum ASREngineFactory {
         }
         pipeline.termNormalizer = termNormalizer
 
-        return Result(engine: engine, termNormalizer: termNormalizer)
+        return Result(engine: engine)
     }
 
     // MARK: - Streaming Paraformer

Sources/AudioEngineManager.swift

@@ -14,10 +14,16 @@ final class AudioEngineManager {
 
     /// Callback invoked with 16kHz Float32 mono samples from the microphone.
     /// Called on a Core Audio thread — callers must dispatch to appropriate queues.
+    ///
+    /// Must be set before `start()` is called. Do not mutate while the engine is running.
+    /// Read from the Core Audio tap thread; writes must happen before the engine starts.
     var onSamples: (([Float]) -> Void)?
 
     /// Callback invoked with normalized audio level (0.0–1.0).
     /// Called on the main queue.
+    ///
+    /// Must be set before `start()` is called. Do not mutate while the engine is running.
+    /// Read from the Core Audio tap thread; writes must happen before the engine starts.
     var onAudioLevel: ((Float) -> Void)?
 
     private var audioEngine: AVAudioEngine?

Sources/ModelDownloader.swift

@@ -465,11 +465,10 @@ extension ModelDownloader: URLSessionDownloadDelegate {
     func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
         progressCallback?("正在解压模型...")
 
-        let destDir = (extractionHandler != nil) ? location : location  // location is temp file
         // Use the extraction handler injected by SherpaOnnxManager
         if let handler = extractionHandler {
-            // We need the modelsDirectory — get it via the manager's pathResolver
-            // The handler closure captures the destination directory
+            // The second parameter is unused — the handler closure captures its
+            // destination directory, so we pass an empty URL for API compatibility.
             let result = handler(location, URL(fileURLWithPath: ""))
             if result {
                 completionCallback?(true, nil)