方法1（系统）:CGSize declabesize = [_questionDecLabel.text boundingRectWithSize:CGSizeMake(CGRectGetWidth(_questionDecLabel.frame), MAXFLOAT) options:NSStringDrawingUsesLineFragmentOrigin attributes:@{NSFontAttributeName:_questionDecLabel.font} context:nil].size;
原文：http://www.cnblogs.com/wq-gril/p/4439775.html