Support passing TTS callback in Swift API (#1218)

@@ -757,6 +757,14 @@ class SherpaOnnxGeneratedAudioWrapper {
   }
 }
 
+typealias TtsCallbackWithArg = (
+  @convention(c) (
+    UnsafePointer<Float>?,  // const float* samples
+    Int32,  // int32_t n
+    UnsafeMutableRawPointer?  // void *arg
+  ) -> Int32
+)?
+
 class SherpaOnnxOfflineTtsWrapper {
   /// A pointer to the underlying counterpart in C
   let tts: OpaquePointer!
@@ -780,6 +788,17 @@ class SherpaOnnxOfflineTtsWrapper {
 
     return SherpaOnnxGeneratedAudioWrapper(audio: audio)
   }
+
+  func generateWithCallbackWithArg(
+    text: String, callback: TtsCallbackWithArg, arg: UnsafeMutableRawPointer, sid: Int = 0,
+    speed: Float = 1.0
+  ) -> SherpaOnnxGeneratedAudioWrapper {
+    let audio: UnsafePointer<SherpaOnnxGeneratedAudio>? =
+      SherpaOnnxOfflineTtsGenerateWithCallbackWithArg(
+        tts, toCPointer(text), Int32(sid), speed, callback, arg)
+
+    return SherpaOnnxGeneratedAudioWrapper(audio: audio)
+  }
 }
 
 // spoken language identification

+class MyClass {
+  func playSamples(samples: [Float]) {
+    print("Play \(samples.count) samples")
+  }
+}
+
 func run() {
   let model = "./vits-piper-en_US-amy-low/en_US-amy-low.onnx"
   let tokens = "./vits-piper-en_US-amy-low/tokens.txt"
@@ -11,6 +17,27 @@ func run() {
   let modelConfig = sherpaOnnxOfflineTtsModelConfig(vits: vits)
   var ttsConfig = sherpaOnnxOfflineTtsConfig(model: modelConfig)
 
+  let myClass = MyClass()
+
+  // We use Unretained here so myClass must be kept alive as the callback is invoked
+  //
+  // See also
+  // https://medium.com/codex/swift-c-callback-interoperability-6d57da6c8ee6
+  let arg = Unmanaged<MyClass>.passUnretained(myClass).toOpaque()
+
+  let callback: TtsCallbackWithArg = { samples, n, arg in
+    let o = Unmanaged<MyClass>.fromOpaque(arg!).takeUnretainedValue()
+    var savedSamples: [Float] = []
+    for index in 0..<n {
+      savedSamples.append(samples![Int(index)])
+    }
+
+    o.playSamples(samples: savedSamples)
+
+    // return 1 so that it continues generating
+    return 1
+  }
+
   let tts = SherpaOnnxOfflineTtsWrapper(config: &ttsConfig)
 
   let text =
@@ -18,11 +45,15 @@ func run() {
   let sid = 99
   let speed: Float = 1.0
 
-  let audio = tts.generate(text: text, sid: sid, speed: speed)
+  let audio = tts.generateWithCallbackWithArg(
+    text: text, callback: callback, arg: arg, sid: sid, speed: speed)
   let filename = "test.wav"
-  audio.save(filename: filename)
-
-  print("\nSaved to:\n\(filename)")
+  let ok = audio.save(filename: filename)
+  if ok == 1 {
+    print("\nSaved to:\(filename)")
+  } else {
+    print("Failed to save to \(filename)")
+  }
 }
 
 @main