[AVFoundation] Obsolete AVSpeechSynthesizer.WriteUtteranceAsync. Fixes …

…xamarin#20338.

The callback may be called multiple times, which means the async version won't
work correctly.

So obsolete and hide the Async binding for now, and remove in XAMCORE_5_0.

References:
* https://discord.com/channels/732297728826277939/732297808148824115/1219903715939582022
* https://developer.apple.com/documentation/avfaudio/avspeechsynthesizer/3141659-writeutterance?language=objc

Fixes xamarin#20338.

src/AVFoundation/AVSpeechSynthesizer.cs

@@ -0,0 +1,34 @@
+using System;
+using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.Versioning;
+using System.Threading.Tasks;
+
+using ObjCRuntime;
+
+#nullable enable
+
+namespace AVFoundation {
+
+	public partial class AVSpeechSynthesizer {
+#if !XAMCORE_5_0
+#if NET
+		[SupportedOSPlatform ("tvos13.0")]
+		[SupportedOSPlatform ("ios13.0")]
+		[SupportedOSPlatform ("maccatalyst")]
+		[SupportedOSPlatform ("macos")]
+#endif
+		[BindingImpl (BindingImplOptions.Optimizable)]
+		[Obsolete ("Do not use this API, it doesn't work correctly. Use the non-Async version (WriteUtterance) instead.")]
+		[EditorBrowsable (EditorBrowsableState.Never)]
+		public unsafe virtual Task<AVAudioBuffer> WriteUtteranceAsync (AVSpeechUtterance utterance)
+		{
+			var tcs = new TaskCompletionSource<AVAudioBuffer> ();
+			WriteUtterance(utterance, (obj_) => {
+				tcs.SetResult (obj_!);
+			});
+			return tcs.Task;
+		}
+#endif
+	}
+}

src/avfoundation.cs

@@ -14530,7 +14530,6 @@ interface AVSpeechSynthesizer {
 		[Export ("speakUtterance:")]
 		void SpeakUtterance (AVSpeechUtterance utterance);
 
-		[Async]
 		[Watch (6, 0), TV (13, 0), iOS (13, 0)]
 		[MacCatalyst (13, 1)]
 		[Export ("writeUtterance:toBufferCallback:")]

src/frameworks.sources

@@ -291,6 +291,7 @@ AVFOUNDATION_SOURCES = \
 	AVFoundation/AVPlayerLayer.cs \
 	AVFoundation/AVPlayerViewController.cs \
 	AVFoundation/AVSampleBufferExtensions.cs \
+	AVFoundation/AVSpeechSynthesizer.cs \
 	AVFoundation/AVTextStyleRule.cs \
 	AVFoundation/Events.cs \
 	AVFoundation/AVPlayerLooper.cs \