performSelector消息会将讲接收者的引用计数retainCount加一
NSUInteger c = [self retainCount];//c为1
[self performSelector:@selector(process_one_job_item) withObject:nil afterDelay:0];
c = [self retainCount];//c为2
这将确保process_one_job_item运行时self不为空
转载于:https://blog.51cto.com/arduino/1154776