Created
September 26, 2012 12:51
-
-
Save advantis/3787824 to your computer and use it in GitHub Desktop.
Retrospective of Singleton pattern in Cocoa
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
// Simple implementation (not thread-safe) | |
+ (Singleton *) sharedSingleton | |
{ | |
static Singleton *instance; | |
if (nil == instance) | |
{ | |
instance = [self new]; | |
} | |
return instance; | |
} | |
// Using @synchronized directive (overhead on every call) | |
+ (Singleton *) sharedSingleton | |
{ | |
static Singleton *instance; | |
@synchronized (self) | |
{ | |
if (nil == instance) | |
{ | |
instance = [self new]; | |
} | |
} | |
return instance; | |
} | |
// Using Double-Checked Locking pattern (not really safe, depends on compiler) | |
// See http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf | |
+ (Singleton *) sharedSingleton | |
{ | |
static Singleton * volatile instance; | |
if (nil == instance) | |
{ | |
@synchronized (self) | |
{ | |
if (nil == instance) | |
{ | |
instance = [self new]; | |
} | |
} | |
} | |
return instance; | |
} | |
// Using OS-specific atomic operations (possible temporary creation of two instances) | |
+ (Singleton *) sharedSingleton | |
{ | |
static Singleton * volatile instance; | |
while (nil == instance) | |
{ | |
Singleton *temp = [self new]; | |
if (!OSAtomicCompareAndSwapPtrBarrier(0x0, temp, &instance)) | |
{ | |
[temp release]; | |
} | |
} | |
return instance; | |
} | |
// Using the “Method Swizzling” technique (impact on superclases implementation) | |
static Singleton *instance; | |
+ (Singleton *) sharedSingleton | |
{ | |
@synchronized(self) | |
{ | |
if (nil == instance) | |
{ | |
instance = [self new]; | |
Method origMethod = class_getClassMethod(self, _cmd); | |
Method newMethod = class_getClassMethod(self, @selector(someMethod)); | |
method_exchangeImplementations(origMethod, newMethod); | |
} | |
} | |
return instance; | |
} | |
+ (Singleton *) someMethod | |
{ | |
return instance; | |
} | |
// Using Grand Central Dispatch (only for iOS >= 4.0) | |
+ (Singleton *) sharedSingleton | |
{ | |
static Singleton *instance; | |
static dispatch_once_t predicate; | |
dispatch_once(&predicate, ^{ instance = [self new]; }); | |
return instance; | |
} | |
// Using Cocoa architecture (premature instantiation, impact on superclases implementation) | |
static Singleton *instance; | |
+ (void) initialize | |
{ | |
if ([Singleton class] == self) | |
{ | |
instance = [self new]; | |
} | |
} | |
+ (Singleton *) sharedSingleton | |
{ | |
return instance; | |
} | |
// Using pthread | |
static Singleton *instance; | |
static void SingletonInit() { instance = [Singleton new]; } | |
+ (Singleton*) sharedSingleton | |
{ | |
static pthread_once_t once_control = PTHREAD_ONCE_INIT; | |
pthread_once(&once_control, SingletonInit); | |
return instance; | |
} |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment