NSKeyValueObserving: Implement setKeys:triggerChangeNotificationsForDependentKey:

ChangeLog

@@ -1,3 +1,14 @@
+2025-02-04 Hugo Melder <[email protected]>
+
+	* Source/NSKVOSupport.m:
+	* Tests/base/NSKVOSupport/legacy.m:
+	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.
+
 2025-01-04 Richard Frith-Macdonald <[email protected]>
 
 	* Headers/Foundation/NSRegularExpression.h:

Headers/Foundation/NSKeyValueObserving.h

@@ -229,7 +229,7 @@ typedef NSString *NSKeyValueChangeKey;
  * they should also be sent for dependentKey.
  */
 + (void) setKeys: (NSArray*)triggerKeys
-triggerChangeNotificationsForDependentKey: (NSString*)dependentKey;
+triggerChangeNotificationsForDependentKey: (NSString*)dependentKey __attribute__((deprecated));
 
 
 #if OS_API_VERSION(MAC_OS_X_VERSION_10_5,GS_API_LATEST)

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;
 }