Objective-C 單例模式

//Creating a Singleton Instance
static MyGizmoClass *sharedGizmoManager = nil; + (MyGizmoClass*)sharedManager{    if (sharedGizmoManager == nil) {        sharedGizmoManager = [[super allocWithZone:NULL] init];    }    return sharedGizmoManager;} + (id)allocWithZone:(NSZone *)zone{    return [[self sharedManager] retain];} - (id)copyWithZone:(NSZone *)zone{    return self;} - (id)retain{    return self;} - (NSUInteger)retainCount{    return NSUIntegerMax;  //denotes an object that cannot be released} - (void)release{    //do nothing} - (id)autorelease{    return self;}

 

再來個現代版本的代碼 ，這種實現利用了GCD（Grand Central Dispatch）和ARC（Automatic Reference Counting）

//ARC開啟
+ (id)sharedInstance{  static dispatch_once_t pred = 0;  __strong static id _sharedObject = nil;  dispatch_once(&pred, ^{    _sharedObject = [[self alloc] init]; // or some other init method  });  return _sharedObject;}