[IOS 下重温设计模式] Singleton

@interface Singleton : NSObject 
{

}

+ (Singleton *) sharedInstance;

- (void) operation;


@end

#import "Singleton.h"


@implementation Singleton


static Singleton *sharedSingleton_ = nil;

- (void) operation
{
  // do something
  NSLog(@"Singleton");
}

+ (Singleton *) sharedInstance
{
  if (sharedSingleton_ == nil)
  {
    sharedSingleton_ = [NSAllocateObject([self class], 0, NULL) init];
  }
  
  return sharedSingleton_;
}


+ (id) allocWithZone:(NSZone *)zone
{
  return [[self sharedInstance] 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;
}

@end

=============================

@interface MySingleton : Singleton
{

}

@end

@implementation MySingleton

- (id) init
{
  
  return self; 
}

- (void) operation
{
  // do something
  NSLog(@"MySingleton");
}

@end

=============================

client:

  Singleton *s = [MySingleton sharedInstance];
  
  [s operation];