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！