使用多线程创建单例对象

#import "ZKZApplication.h"

@implementation ZKZApplication

+(instancetype)allocWithZone:(struct _NSZone *)zone
{
static ZKZApplication *application;
// dispatch_once是线程安全的，onceToken默认为0
static dispatch_once_t onceToken;
// dispatch_once宏可以保证块代码中的指令只被执行一次
dispatch_once(&onceToken, ^{
application=[self allocWithZone:zone];
});
return application;
}

+(instancetype)shareApplication
{
return [[self alloc]init];
}
@end