NSKVOSupport: Implement legacy KVO API

Implements the setKeys:triggerChangeNotificationsForDependentKey: class
method. Please do not use it. It is fundamentally broken, and requires
the object's meta class to hold additional state.

Keys from this class method are the last resort when retrieving
dependencies via keyPathsForValuesAffectingValueForKey:.
This aligns with the implementation in Foundation.

Source/NSKVOSupport.m

@@ -569,9 +569,29 @@ - (bool) isEmpty
 
 #pragma region KVO Core Implementation - NSObject category
 
+static const char *const KVO_MAP = "_NSKVOMap";
+
 @implementation
 NSObject (NSKeyValueObserving)
 
++ (void) setKeys: (NSArray *) triggerKeys 
+triggerChangeNotificationsForDependentKey: (NSString *) dependentKey
+{
+  NSMutableDictionary<NSString *, NSSet *> *affectingKeys;
+  NSSet *triggerKeySet;
+
+  affectingKeys = objc_getAssociatedObject(self, KVO_MAP);
+  if (nil == affectingKeys)
+    {
+      affectingKeys = [NSMutableDictionary dictionaryWithCapacity: 10];
+      objc_setAssociatedObject(self, KVO_MAP, affectingKeys,
+                               OBJC_ASSOCIATION_RETAIN);
+    }
+
+  triggerKeySet = [NSSet setWithArray: triggerKeys];
+  [affectingKeys setValue: triggerKeySet forKey: dependentKey];
+}
+
 - (void) observeValueForKeyPath: (NSString *)keyPath
                        ofObject: (id)object
                          change: (NSDictionary<NSString *, id> *)change
@@ -631,6 +651,7 @@ + (NSSet *) keyPathsForValuesAffectingValueForKey: (NSString *)key
   static NSSet		*emptySet = nil;
   static gs_mutex_t	lock = GS_MUTEX_INIT_STATIC;
   NSUInteger 		keyLength;
+  NSDictionary *affectingKeys;
 
   if (nil == emptySet)
     {
@@ -701,6 +722,20 @@ + (NSSet *) keyPathsForValuesAffectingValueForKey: (NSString *)key
         {
           return [self performSelector:sel];
         }
+
+      // We compute an NSSet from information provided by previous invocations
+      // of the now-deprecated setKeys:triggerChangeNotificationsForDependentKey:
+      // if the original imp returns an empty set.
+      // This aligns with Apple's backwards compatibility.
+      affectingKeys = (NSDictionary *)objc_getAssociatedObject(self, KVO_MAP);
+      if (unlikely(nil != affectingKeys))
+        {
+          NSSet *set = [affectingKeys objectForKey:key];
+          if (set != nil)
+            {
+            return set;
+            }
+        }
     }
   return emptySet;
 }