[.NET/AudioUnit] Use [UnmanagedCallersOnly] instead of [MonoPInvokeCa…

…llback] for the ScheduledAudioFileRegionCallback function.

Ref xamarin#10470.

src/AudioUnit/AUScheduledAudioFileRegion.cs

@@ -24,7 +24,11 @@ public class AUScheduledAudioFileRegion : IDisposable {
  internal struct ScheduledAudioFileRegion
  {
  public AudioTimeStamp TimeStamp;
+#if NET
+ public unsafe delegate* unmanaged<IntPtr, ref ScheduledAudioFileRegion, AudioUnitStatus, void> CompletionHandler;
+#else
  public ScheduledAudioFileRegionCompletionHandler CompletionHandler;
+#endif
  public /* void * */ IntPtr CompletionHandlerUserData;
  public IntPtr AudioFile;
  public uint LoopCount;
@@ -51,6 +55,7 @@ public AUScheduledAudioFileRegion (AudioFile audioFile, AUScheduledAudioFileRegi
  this.completionHandler = completionHandler;
  }
 
+#if !NET
  internal delegate void ScheduledAudioFileRegionCompletionHandler (
  /* void * */IntPtr userData, 
  /* ScheduledAudioFileRegion * */ref ScheduledAudioFileRegion fileRegion, 
@@ -60,6 +65,9 @@ internal delegate void ScheduledAudioFileRegionCompletionHandler (
 
 #if !MONOMAC
  [MonoPInvokeCallback (typeof (ScheduledAudioFileRegionCompletionHandler))]
+#endif
+#else
+ [UnmanagedCallersOnly]
 #endif
  static void ScheduledAudioFileRegionCallback (IntPtr userData, ref ScheduledAudioFileRegion fileRegion, AudioUnitStatus status)
  {
@@ -85,13 +93,22 @@ internal ScheduledAudioFileRegion GetAudioFileRegion ()
  var ret = new ScheduledAudioFileRegion {
  TimeStamp = TimeStamp,
  CompletionHandlerUserData = ptr,
- CompletionHandler = ptr != IntPtr.Zero ? static_ScheduledAudioFileRegionCompletionHandler : null,
  AudioFile = AudioFile.Handle,
  LoopCount = LoopCount,
  StartFrame = StartFrame,
  FramesToPlay = FramesToPlay
  };
 
+ if (ptr != IntPtr.Zero) {
+ unsafe {
+#if NET
+ ret.CompletionHandler = &ScheduledAudioFileRegionCallback;
+#else
+ ret.CompletionHandler = static_ScheduledAudioFileRegionCompletionHandler;
+#endif
+ }
+ }
+
  alreadyUsed = true;
  return ret;
  }