单例

#import "ServiceManager.h"

 static ServiceManager *defaultManager;

        

@implementation ServiceManager

        

 +(ServiceManager *)defaultManager{

    if(!defaultManager)

       defaultManager=[[self allocWithZone:NULL] init];

        return  defaultManager;
}

@end

使用GCD的写法

#import "ServiceManager.h"

@implementation ServiceManager

+(ServiceManager *)sharedManager{

    static dispatch_once_t predicate;

    static ServiceManager * sharedManager;

    dispatch_once(&predicate, ^{

        sharedManager=[[ServiceManager alloc] init];

    });

    return sharedManager;

}

@end

dispatch_once这个函数， 它可以保证整个应用程序生命周期中某段代码只被执行一次！