#import 
     
      @interface Singleton : NSObject + (Singleton*)sharedManager;
     

@end

#import "Singleton.h"static Singleton* sharedGizmoManager = nil;//static 静态实例是关键@implementation Singleton+ (Singleton*)sharedManager{    @synchronized(self){        if(sharedGizmoManager == nil) {            [[self alloc] init];// assignment not done here        }    }    return sharedGizmoManager;}//唯一一次alloc单例，之后均返回nil+ (id)allocWithZone:(NSZone*)zone{    @synchronized(self){        if(sharedGizmoManager == nil) {            sharedGizmoManager = [super allocWithZone:zone];            return sharedGizmoManager;  // assignment and return on first allocation        }    }    return nil; //on subsequent allocation attempts return nil}//copy返回单例本身- (id)copyWithZone:(NSZone*)zone{    return self;}//retain返回单例本身- (id)retain{    return self;}//引用计数总是为1- (unsigned)retainCount{    return UINT_MAX;  //denotes an object that cannot be released}//release不做任何处理- (oneway void)release{    }//autorelease返回单例本身- (id)autorelease{    return self;}@end