[AVFoundation] Fix a few issues with AudioUnit. (#12954)

* Add an (IntPtr, bool) constructor so that AudioUnit works with Runtime.GetINativeObject.
* Keep track of ownership, so that AudioUnit doesn't free the native resources
  when it doesn't own them.
* Update a test to verify that calling 'AVAudioIONode.AudioUnit' multiple
  times and disposing the result between them works (this fails if AudioUnit
  doesn't keep track of ownership).

src/AudioUnit/AudioUnit.cs

@@ -298,7 +298,7 @@ namespace AudioUnit
 	public class AudioUnit : IDisposable, ObjCRuntime.INativeObject
 	{
 #pragma warning disable 649 // Field 'AudioUnit.handle' is never assigned to, and will always have its default value
-		internal IntPtr handle;
+		IntPtr handle;
 #pragma warning restore 649
 		public IntPtr Handle {
 			get {
@@ -314,14 +314,21 @@ namespace AudioUnit
 
 		GCHandle gcHandle;
 		bool _isPlaying;
+		bool owns;
 
 		Dictionary<uint, RenderDelegate> renderer;
 		Dictionary<uint, InputDelegate> inputs;
 
 		internal AudioUnit (IntPtr ptr)
+			: this (ptr, false)
+		{
+		}
+
+		internal AudioUnit (IntPtr ptr, bool owns)
 		{
 			handle = ptr;
 			gcHandle = GCHandle.Alloc(this);
+			this.owns = owns;
 		}
 		
 		public AudioUnit (AudioComponent component)
@@ -336,6 +343,7 @@ namespace AudioUnit
 				throw new AudioUnitException (err);
 			
 			gcHandle = GCHandle.Alloc(this);
+			owns = true;
 		}
 
 		public AudioComponent Component {
@@ -795,9 +803,11 @@ namespace AudioUnit
 		protected virtual void Dispose (bool disposing)
 		{
 			if (handle != IntPtr.Zero){
-				Stop ();
-				AudioUnitUninitialize (handle);
-				AudioComponentInstanceDispose (handle);
+				if (owns) {
+					Stop ();
+					AudioUnitUninitialize (handle);
+					AudioComponentInstanceDispose (handle);
+				}
 				gcHandle.Free();
 				handle = IntPtr.Zero;
 			}

tests/monotouch-test/AVFoundation/AVAudioIONode.cs

@@ -21,10 +21,16 @@ namespace Xamarin.Mac.Tests
 
 			Asserts.EnsureYosemite ();
 
-			AVAudioEngine eng = new AVAudioEngine();
-			AVAudioIONode node = eng.OutputNode;
-			AUUnit unit = node.AudioUnit;
-			unit.GetElementCount (AudioUnitScopeType.Global);
+			using (AVAudioEngine eng = new AVAudioEngine ()) {
+				using (AVAudioIONode node = eng.OutputNode) {
+					using (AUUnit unit = node.AudioUnit)
+						unit.GetElementCount (AudioUnitScopeType.Global);
+					using (AUUnit unit = node.AudioUnit)
+						unit.GetElementCount (AudioUnitScopeType.Global);
+					using (AUUnit unit = node.AudioUnit)
+						unit.GetElementCount (AudioUnitScopeType.Global);
+				}
+			}
 			// Make sure this doens't crash.
 		}
 	}