IOS UILabel自動換行

 {
            UILabel *lb=[[UILabel alloc]initWithFrame:CGRectMake(offset_xx, offset_yy, 0, 0)];
            [lb setText:@"fawfwefwaefaewwewaefewafwefawefawefawefwaefwaefawefawefawefawafewfawefawfweafwaefawefawefawfewafawefawfawfweafwafawefwaefawefwaefaweewafwefawefawefawefwaefwaefaewafwefawefawefawefwaefwaefaewafwefawefawefawefwaefwaefaewafwefawefawefawefwaefwaefaewafwefawefawefawefwaefwaefa"];
            [lb setTextColor:[UIColor grayColor]];
            [lb setNumberOfLines:0];
            
            UIFont *font=[UIFont systemFontOfSize:13];
            [lb setFont:font];
            //設置行高上限
            CGSize size=CGSizeMake(320, 4000);
            
            //計算frame的高度
            CGRect lb_size=[lb.text boundingRectWithSize:size
                                                 options:NSStringDrawingUsesLineFragmentOrigin
                                              attributes:[NSDictionary dictionaryWithObjectsAndKeys:font,NSFontAttributeName, nil] context:nil];
            [lb setFrame:lb_size];
            [sv addSubview:lb];
        }