Optimize NSActionDispatcher usage (#4162)

* [NSActionDispatcher] Remove unused class

* [NSActionDispatcher] Move selectorname and selector variable into each class

* [NSActionDispatcher] Add specialized versions of NSActionDispatcher that use SynchronizationContext parameters

* [NSObject] Add specialized overloads for *InvokeOnMainThread which use SynchronizationContext parameters

* [AppKit,UIKit] Use the synchronization context specialized versions of *InvokeOnMainThread

This finishes the PR adding the following value:

1. There is no Action wrapper being constructed on async continuations,
thus on every await call we gain: 1 less allocation (lambda capture),
1 less indirected call to the actual continuation, and possibly
Action being removed by the linker, as its no longer used.

2. NSActionDispatcher* classes might now be linked out, due to
the static selector no longer being used everywhere

3. One unused class removed

* PR feedback

* Fix build

* PR feedback 2

* Seal this class

* Fix some renames not followed because they were undef ifdef'd code

src/AppKit/AppKitSynchronizationContext.cs

@@ -14,12 +14,12 @@ namespace AppKit {
 
 		public override void Post (SendOrPostCallback d, object state)
 		{
-			NSRunLoop.Main.BeginInvokeOnMainThread (() => d (state));
+			NSRunLoop.Main.BeginInvokeOnMainThread (d, state);
 		}
 
 		public override void Send (SendOrPostCallback d, object state)
 		{
-			NSRunLoop.Main.InvokeOnMainThread (() => d (state));
+			NSRunLoop.Main.InvokeOnMainThread (d, state);
 		}
 	}
 }

src/Foundation/NSAction.cs

@@ -24,17 +24,31 @@
 
 using System;
 using System.Runtime.InteropServices;
+using System.Threading;
 using ObjCRuntime;
 
 namespace Foundation {
 
 #if !COREBUILD
 	// Use this for synchronous operations
-	[Register ("__MonoMac_NSActionDispatcher")]
-	internal sealed class NSActionDispatcher : NSObject {
+	internal abstract class NSDispatcher : NSObject
+	{
 		public const string SelectorName = "xamarinApplySelector";
 		public static readonly Selector Selector = new Selector (SelectorName);
 
+		protected NSDispatcher ()
+		{
+			IsDirectBinding = false;
+		}
+
+		[Export (SelectorName)]
+		[Preserve (Conditional = true)]
+		public abstract void Apply ();
+	}
+
+	// Use this for synchronous operations
+	[Register ("__MonoMac_NSActionDispatcher")]
+	internal sealed class NSActionDispatcher : NSDispatcher {
 		readonly Action action;
 
 		public NSActionDispatcher (Action action)
@@ -43,40 +57,28 @@ namespace Foundation {
 				throw new ArgumentNullException ("action");
 
 			this.action = action;
-			IsDirectBinding = false;
 		}
 
-		[Export (SelectorName)]
-		[Preserve (Conditional = true)]
-		public void Apply ()
-		{
-			action ();
-		}
+		public override void Apply () => action ();
 	}
 
 	// Use this for synchronous operations
-	[Register ("__MonoMac_ActionDispatcher")]
-	internal sealed class ActionDispatcher : NSObject {
-		public const string SelectorName = "xamarinApplySelector";
-		public static readonly Selector Selector = new Selector (SelectorName);
+	[Register ("__MonoMac_NSSynchronizationContextDispatcher")]
+	internal sealed class NSSynchronizationContextDispatcher : NSDispatcher
+	{
+		readonly SendOrPostCallback d;
+		readonly object state;
 
-		readonly Action action;
-
-		public ActionDispatcher (Action action)
+		public NSSynchronizationContextDispatcher (SendOrPostCallback d, object state)
 		{
-			if (action == null)
-				throw new ArgumentNullException ("action");
+			if (d == null)
+				throw new ArgumentNullException (nameof (d));
 
-			this.action = action;
-			IsDirectBinding = false;
+			this.d = d;
+			this.state = state;
 		}
 
-		[Export (SelectorName)]
-		[Preserve (Conditional = true)]
-		public void Apply ()
-		{
-			action ();
-		}
+		public override void Apply () => d (state);
 	}
 
 	// Used this for NSTimer support
@@ -104,41 +106,80 @@ namespace Foundation {
 		}
 	}
 
+	abstract class NSAsyncDispatcher : NSDispatcher {
+		readonly GCHandle gch;
+
+		protected NSAsyncDispatcher ()
+		{
+			gch = GCHandle.Alloc (this);
+		}
+
+		public override void Apply ()
+		{
+			gch.Free ();
+
+			//
+			// Although I would like to call Dispose here, to
+			// reduce the load on the GC, we have some useful diagnostic
+			// code in our runtime that is useful to track down
+			// problems, so we are removing the Dispose and letting
+			// the GC and our pipeline do their job.
+			// 
+#if MONOTOUCH
+			// MonoTouch has fixed the above problems, and we can call
+			// Dispose here.
+			Dispose ();
+#endif
+		}
+	}
+
 	// Use this for asynchronous operations
 	[Register ("__MonoMac_NSAsyncActionDispatcher")]
-	internal class NSAsyncActionDispatcher : NSObject {
-		GCHandle gch;
+	internal sealed class NSAsyncActionDispatcher : NSAsyncDispatcher {
 		Action action;
 
 		public NSAsyncActionDispatcher (Action action)
 		{
+			if (action == null)
+				throw new ArgumentNullException (nameof (action));
+
 			this.action = action;
-			gch = GCHandle.Alloc (this);
-			IsDirectBinding = false;
 		}
 
-		[Export (NSActionDispatcher.SelectorName)]
-		[Preserve (Conditional = true)]
-		public void Apply ()
+		public override void Apply ()
 		{
 			try {
 				action ();
 			} finally {
-				action = null; // this is a one-shot dispatcher
-				gch.Free ();
+				action = null;
+				base.Apply ();
+			}
+		}
+	}
 
-				//
-				// Although I would like to call Dispose here, to
-				// reduce the load on the GC, we have some useful diagnostic
-				// code in our runtime that is useful to track down
-				// problems, so we are removing the Dispose and letting
-				// the GC and our pipeline do their job.
-				// 
-#if MONOTOUCH
-				// MonoTouch has fixed the above problems, and we can call
-				// Dispose here.
-				Dispose ();
-#endif
+	// Use this for asynchronous operations
+	[Register ("__MonoMac_NSAsyncSynchronizationContextDispatcher")]
+	internal sealed class NSAsyncSynchronizationContextDispatcher : NSAsyncDispatcher {
+		SendOrPostCallback d;
+		object state;
+
+		public NSAsyncSynchronizationContextDispatcher (SendOrPostCallback d, object state)
+		{
+			if (d == null)
+				throw new ArgumentNullException (nameof (d));
+
+			this.d = d;
+			this.state = state;
+		}
+
+		public override void Apply ()
+		{
+			try {
+				d (state);
+			} finally {
+				d = null; // this is a one-shot dispatcher
+				state = null;
+				base.Apply ();
 			}
 		}
 	}

src/Foundation/NSObject2.cs

@@ -576,10 +576,22 @@ namespace Foundation {
 			var d = new NSAsyncActionDispatcher (action);
 #if MONOMAC
 			Messaging.void_objc_msgSend_IntPtr_IntPtr_bool (d.Handle, Selector.PerformSelectorOnMainThreadWithObjectWaitUntilDoneHandle, 
-			                                                NSActionDispatcher.Selector.Handle, d.Handle, false);
+		                                                        NSDispatcher.Selector.Handle, d.Handle, false);
 #else
 			Messaging.void_objc_msgSend_IntPtr_IntPtr_bool (d.Handle, Selector.GetHandle (Selector.PerformSelectorOnMainThreadWithObjectWaitUntilDone), 
-			                                                Selector.GetHandle (NSActionDispatcher.SelectorName), d.Handle, false);
+			                                                Selector.GetHandle (NSDispatcher.SelectorName), d.Handle, false);
+#endif
+		}
+
+		internal void BeginInvokeOnMainThread (System.Threading.SendOrPostCallback cb, object state)
+		{
+			var d = new NSAsyncSynchronizationContextDispatcher (cb, state);
+#if MONOMAC
+			Messaging.void_objc_msgSend_IntPtr_IntPtr_bool (d.Handle, Selector.PerformSelectorOnMainThreadWithObjectWaitUntilDoneHandle,
+		                                                        NSDispatcher.Selector.Handle, d.Handle, false);
+#else
+			Messaging.void_objc_msgSend_IntPtr_IntPtr_bool (d.Handle, Selector.GetHandle (Selector.PerformSelectorOnMainThreadWithObjectWaitUntilDone),
+			                                                Selector.GetHandle (NSDispatcher.SelectorName), d.Handle, false);
 #endif
 		}
 		
@@ -588,14 +600,27 @@ namespace Foundation {
 			using (var d = new NSActionDispatcher (action)) {
 #if MONOMAC
 				Messaging.void_objc_msgSend_IntPtr_IntPtr_bool (d.Handle, Selector.PerformSelectorOnMainThreadWithObjectWaitUntilDoneHandle, 
-				                                                NSActionDispatcher.Selector.Handle, d.Handle, true);
+		                                                                NSDispatcher.Selector.Handle, d.Handle, true);
 #else
 				Messaging.void_objc_msgSend_IntPtr_IntPtr_bool (d.Handle, Selector.GetHandle (Selector.PerformSelectorOnMainThreadWithObjectWaitUntilDone), 
-				                                                Selector.GetHandle (NSActionDispatcher.SelectorName), d.Handle, true);
+				                                                Selector.GetHandle (NSDispatcher.SelectorName), d.Handle, true);
 #endif
 			}
 		}		
 
+		internal void InvokeOnMainThread (System.Threading.SendOrPostCallback cb, object state)
+		{
+			using (var d = new NSSynchronizationContextDispatcher (cb, state)) {
+#if MONOMAC
+				Messaging.void_objc_msgSend_IntPtr_IntPtr_bool (d.Handle, Selector.PerformSelectorOnMainThreadWithObjectWaitUntilDoneHandle,
+			                                                        NSDispatcher.Selector.Handle, d.Handle, true);
+#else
+				Messaging.void_objc_msgSend_IntPtr_IntPtr_bool (d.Handle, Selector.GetHandle (Selector.PerformSelectorOnMainThreadWithObjectWaitUntilDone),
+				                                                Selector.GetHandle (NSDispatcher.SelectorName), d.Handle, true);
+#endif
+			}
+		}
+
 		public static NSObject FromObject (object obj)
 		{
 			if (obj == null)
@@ -739,13 +764,13 @@ namespace Foundation {
 		public virtual void Invoke (Action action, double delay)
 		{
 			var d = new NSAsyncActionDispatcher (action);
-			d.PerformSelector (NSActionDispatcher.Selector, null, delay);
+			d.PerformSelector (NSDispatcher.Selector, null, delay);
 		}
 
 		public virtual void Invoke (Action action, TimeSpan delay)
 		{
 			var d = new NSAsyncActionDispatcher (action);
-			d.PerformSelector (NSActionDispatcher.Selector, null, delay.TotalSeconds);
+			d.PerformSelector (NSDispatcher.Selector, null, delay.TotalSeconds);
 		}
 
 		internal void ClearHandle ()

src/UIKit/UIKitSynchronizationContext.cs

@@ -21,12 +21,12 @@ namespace UIKit {
 
 		public override void Post (SendOrPostCallback d, object state)
 		{
-			NSRunLoop.Main.BeginInvokeOnMainThread ( () => d (state) );
+			NSRunLoop.Main.BeginInvokeOnMainThread (d, state);
 		}
 
 		public override void Send (SendOrPostCallback d, object state)
 		{
-			NSRunLoop.Main.InvokeOnMainThread ( () => d (state) );
+			NSRunLoop.Main.InvokeOnMainThread (d, state);
 		}
 	}