单例模式的创建

1.正常创建

#import "ServiceManager.h"

static ServiceManager *defaultManager;

@implementation ServiceManager

+(ServiceManager *)defaultManager{
    if(!defaultManager)
        defaultManager=[[self allocWithZone:NULL] init];
    return  defaultManager;
}

@end

2.使用gcd

#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