Move _keyPathsForValuesAffectingValueForKey body into class method an…

…d statically construct empty NSSet

Source/NSKVOSupport.m

@@ -307,73 +307,6 @@ - (bool)isEmpty
 static void
 _removeKeyObserver(_NSKVOKeyObserver *keyObserver);
 
-// Private helper that backs the default implementation of
-// keyPathsForValuesAffectingValueForKey: Returns nil instead of constructing an
-// empty set if no keyPaths are found Also used internally as a minor
-// optimization, to avoid constructing an empty set when it is not needed
-static NSSet *
-_keyPathsForValuesAffectingValueForKey(Class self, NSString *key)
-{
- // This function can be a KVO bottleneck, so it will prefer to use c string
- // manipulation when safe
- NSUInteger keyLength = [key length];
- if (keyLength > 0)
- {
- static const char *const sc_prefix = "keyPathsForValuesAffecting";
- static const size_t sc_prefixLength = 26; // strlen(sc_prefix)
- static const size_t sc_bufferSize = 128;
-
- // max length of a key that can guaranteed fit in the char buffer,
- // even if UTF16->UTF8 conversion causes length to double, or a null
- // terminator is needed
- static const size_t sc_safeKeyLength
- = (sc_bufferSize - sc_prefixLength) / 2 - 1; // 50
-
- const char *rawKey;
- size_t rawKeyLength;
- SEL sel;
-
- rawKey = [key UTF8String];
- rawKeyLength = strlen(rawKey);
-
- if (keyLength <= sc_safeKeyLength)
- {
- // fast path using c string manipulation, will cover most cases, as
- // most keyPaths are short
- char selectorName[sc_bufferSize]
- = "keyPathsForValuesAffecting"; // 26 chars
- selectorName[sc_prefixLength] = toupper(rawKey[0]);
- // Copy the rest of the key, including the null terminator
- memcpy(&selectorName[sc_prefixLength + 1], &rawKey[1], rawKeyLength);
- sel = sel_registerName(selectorName);
- }
- else // Guaranteed path for long keyPaths
- {
- size_t keyLength;
- size_t bufferSize;
- char *selectorName;
-
- keyLength = strlen(rawKey);
- bufferSize = sc_prefixLength + keyLength + 1;
- selectorName = (char *) malloc(bufferSize);
- memcpy(selectorName, sc_prefix, sc_prefixLength);
-
- selectorName[sc_prefixLength] = toupper(rawKey[0]);
- // Copy the rest of the key, including the null terminator
- memcpy(&selectorName[sc_prefixLength + 1], &rawKey[1], keyLength);
-
- sel = sel_registerName(selectorName);
- free(selectorName);
- }
-
- if ([self respondsToSelector:sel])
- {
- return [self performSelector:sel];
- }
- }
- return nil;
-}
-
 // Add all observers with declared dependencies on this one:
 // * All keypaths that could trigger a change (keypaths for values affecting
 // us).
@@ -672,7 +605,77 @@ + (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
 
 + (NSSet *)keyPathsForValuesAffectingValueForKey:(NSString *)key
 {
- return _keyPathsForValuesAffectingValueForKey(self, key) ?: [NSSet set]; // TODO(Hugo): Avoid constructing an empty set in every call
+ static NSSet *emptySet = nil;
+ static gs_mutex_t lock = GS_MUTEX_INIT_STATIC;
+ if (nil == emptySet)
+ {
+ GS_MUTEX_LOCK(lock);
+ if (nil == emptySet)
+ {
+ emptySet = [[NSSet alloc] init];
+ [NSObject leakAt: &emptySet];
+ }
+ GS_MUTEX_UNLOCK(lock);
+ }
+
+ // This function can be a KVO bottleneck, so it will prefer to use c string
+ // manipulation when safe
+ NSUInteger keyLength = [key length];
+ if (keyLength > 0)
+ {
+ static const char *const sc_prefix = "keyPathsForValuesAffecting";
+ static const size_t sc_prefixLength = 26; // strlen(sc_prefix)
+ static const size_t sc_bufferSize = 128;
+
+ // max length of a key that can guaranteed fit in the char buffer,
+ // even if UTF16->UTF8 conversion causes length to double, or a null
+ // terminator is needed
+ static const size_t sc_safeKeyLength
+ = (sc_bufferSize - sc_prefixLength) / 2 - 1; // 50
+
+ const char *rawKey;
+ size_t rawKeyLength;
+ SEL sel;
+
+ rawKey = [key UTF8String];
+ rawKeyLength = strlen(rawKey);
+
+ if (keyLength <= sc_safeKeyLength)
+ {
+ // fast path using c string manipulation, will cover most cases, as
+ // most keyPaths are short
+ char selectorName[sc_bufferSize]
+ = "keyPathsForValuesAffecting"; // 26 chars
+ selectorName[sc_prefixLength] = toupper(rawKey[0]);
+ // Copy the rest of the key, including the null terminator
+ memcpy(&selectorName[sc_prefixLength + 1], &rawKey[1], rawKeyLength);
+ sel = sel_registerName(selectorName);
+ }
+ else // Guaranteed path for long keyPaths
+ {
+ size_t keyLength;
+ size_t bufferSize;
+ char *selectorName;
+
+ keyLength = strlen(rawKey);
+ bufferSize = sc_prefixLength + keyLength + 1;
+ selectorName = (char *) malloc(bufferSize);
+ memcpy(selectorName, sc_prefix, sc_prefixLength);
+
+ selectorName[sc_prefixLength] = toupper(rawKey[0]);
+ // Copy the rest of the key, including the null terminator
+ memcpy(&selectorName[sc_prefixLength + 1], &rawKey[1], keyLength);
+
+ sel = sel_registerName(selectorName);
+ free(selectorName);
+ }
+
+ if ([self respondsToSelector:sel])
+ {
+ return [self performSelector:sel];
+ }
+ }
+ return emptySet;
 }
 
 - (void)addObserver:(id)observer