Objective-C 单例模式

//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;
}

 

再来个现代版本的代码 ，这种实现利用了GCD（Grand Central Dispatch）和ARC（Automatic Reference Counting）

//ARC开启
+ (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;
}