创建一个UIImageView对象：1UIImageView * poImgView = [[UIImageView alloc] init];
GCD：串行/并行/主队列下，同步/异步的执行方式
GCD中获取各种类型的队列：12345678//获取串行的队列 dispatch_queue_t singalQueue = dispatch_queue_create("single",DISPATCH_QUEUE_SERIAL);//获取并发执行的队列dispatch_queue_t concrtQueue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);//获取主队列dispatch_queue_t mainQueue = dispatch_get_main_queue();//获取全局的队列（并发的）dispatch_queue_t gobalqueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);