POC for faster instantiation of NSObject subclasses by using factory …

…classes instead of reflection.

Idea: create an instance of a factory class that is able to create an instance
of the desired type without using reflection.

This POC is hardcoded to only implement support for UIView (since that's what
the test case is creating). If productized, the factory class would have to be
created by the generator.

Numbers are impressive: ~40% faster.

Downsides: a lot of extra generated code (bloat) - i.e. bigger apps.

Numbers
=======

Test case: rolfbjarne/TestApp@004283d

Fix 1 refers to PR xamarin#5009.
Fix 2 refers to PR xamarin#5013.
Fix 3 refers to PR xamarin#5016.
Fix 4 refers to PR xamarin#5017.
Fix 5 is this fix.

iPad Air 2
----------

| Configuration       | Before | After fix 1 | After fix 2  | After fix 3  | After fix 4  | After fix 5  | Improvement from fix 4 to fix 5 | Cumulative improvement |
| ------------------- | ------ | ----------: | -----------: | -----------: | -----------: | -----------: | ------------------------------: | ---------------------: |
| Release (link all)  | 477 ms |      481 ms |       224 ms |       172 ms |       148 ms |        86 ms |                     62 ms (42%) |           391 ms (82%) |
| Release (dont link) | 738 ms |      656 ms |       377 ms |       201 ms |       146 ms |        88 ms |                     58 ms (40%) |           650 ms (88%) |

iPhone X
--------

| Configuration       | Before | After fix 1 | After fix 2  | After fix 3  | After fix 4  | After fix 5  | Improvement from fix 4 to fix 5 | Cumulative improvement |
| ------------------- | ------ | ----------: | -----------: | -----------: | -----------: | -----------: | ------------------------------: | ---------------------: |
| Release (link all)  |  98 ms |       99 ms |        42 ms |        31 ms |        29 ms |        18 ms |                     11 ms (38%) |            80 ms (82%) |
| Release (dont link) | 197 ms |      153 ms |        91 ms |        43 ms |        28 ms |        17 ms |                     11 ms (39%) |           180 ms (91%) |

When linking all assemblies, the type map has 24 entries, and when not linking
at all it has 2993 entries.

src/ObjCRuntime/Runtime.cs

@@ -28,6 +28,13 @@
 #endif
 #endif
 
+namespace ObjCRuntime {
+	public interface INSObjectInstantiator
+	{
+		NSObject Create (IntPtr handle);
+	}
+}
+
 namespace ObjCRuntime {
 
 	public partial class Runtime {
@@ -39,7 +46,7 @@ public partial class Runtime {
 #endif
 
 		static Dictionary<IntPtrTypeValueTuple,Delegate> block_to_delegate_cache;
-		static Dictionary<Type, ConstructorInfo> intptr_ctor_cache;
+		static Dictionary<Type, Tuple<ConstructorInfo,INSObjectInstantiator>> intptr_ctor_cache;
 		static Dictionary<Type, ConstructorInfo> intptr_bool_ctor_cache;
 
 		static List <object> delegates;
@@ -249,7 +256,7 @@ unsafe static void Initialize (InitializationOptions* options)
 			Runtime.options = options;
 			delegates = new List<object> ();
 			object_map = new Dictionary <IntPtr, WeakReference> (IntPtrEqualityComparer);
-			intptr_ctor_cache = new Dictionary<Type, ConstructorInfo> (TypeEqualityComparer);
+			intptr_ctor_cache = new Dictionary<Type, Tuple<ConstructorInfo,INSObjectInstantiator>> (TypeEqualityComparer);
 			intptr_bool_ctor_cache = new Dictionary<Type, ConstructorInfo> (TypeEqualityComparer);
 			lock_obj = new object ();
 
@@ -1135,13 +1142,16 @@ internal static T ConstructNSObject<T> (IntPtr ptr) where T: NSObject
 			if (type == null)
 				throw new ArgumentNullException ("type");
 
-			var ctor = GetIntPtrConstructor (type);
-
-			if (ctor == null) {
+			var tuple = GetIntPtrConstructor (type);
+			if (tuple == null || (tuple.Item1 == null && tuple.Item2 == null)) {
 				MissingCtor (ptr, IntPtr.Zero, type, missingCtorResolution, selector, method);
 				return null;
 			}
 
+			if (tuple.Item2 != null)
+				return (T) (object) tuple.Item2.Create (ptr);
+
+			var ctor = tuple?.Item1;
 			return (T) ctor.Invoke (new object[] { ptr });
 		}
 
@@ -1162,7 +1172,7 @@ static T ConstructINativeObject<T> (IntPtr ptr, bool owns, Type type, MissingCto
 			return (T) ctor.Invoke (new object[] { ptr, owns});
 		}
 
-		static ConstructorInfo GetIntPtrConstructor (Type type)
+		static Tuple<ConstructorInfo, INSObjectInstantiator> GetIntPtrConstructor (Type type)
 		{
 			lock (intptr_ctor_cache) {
 				if (intptr_ctor_cache.TryGetValue (type, out var rv))
@@ -1172,9 +1182,14 @@ static ConstructorInfo GetIntPtrConstructor (Type type)
 			for (int i = 0; i < ctors.Length; ++i) {
 				var param = ctors[i].GetParameters ();
 				if (param.Length == 1 && param [0].ParameterType == typeof (IntPtr)) {
+					var ctor = ctors [i];
+					var instantiator = (INSObjectInstantiator) type.GetMethod ("CreateInstantiator", BindingFlags.NonPublic | BindingFlags.Static)?.Invoke (null, null);
+					var tuple = new Tuple<ConstructorInfo, INSObjectInstantiator> (ctors [i], instantiator);
 					lock (intptr_ctor_cache)
-						intptr_ctor_cache [type] = ctors [i];
-					return ctors [i];
+						intptr_ctor_cache [type] = tuple;
+					if (instantiator != null)
+						Console.WriteLine ("Found instantiator for: {0}", type.FullName);
+					return tuple;
 				}
 			}
 			return null;

src/UIKit/UIView.cs

@@ -21,6 +21,19 @@
 namespace UIKit {
 	public partial class UIView : IEnumerable {
 
+		class Instantiator : INSObjectInstantiator
+		{
+			public NSObject Create (IntPtr handle)
+			{
+				return new UIView (handle);
+			}
+		}
+		[Preserve (Conditional = true)]
+		static INSObjectInstantiator CreateInstantiator ()
+		{
+			return new Instantiator ();
+		}
+
 		public void Add (UIView view)
 		{
 			AddSubview (view);