例子 ：
UILabel *label = [[UILabel alloc]initWithFrame:CGRectMake(10, 100, 200, 30)];
label.text = @"SAFDFDSFSDFDAFDFLINAJIDN;OHAFIlsjfgdgljdfgSAFDFDSFSDFDAFDFLINAJIDN;OHAFIlsjfgdgljdfgSAFDFDSFSDFDAFDFLINAJIDN;OHAFIlsjfgdgljdfgSAFDFgfhfgjlinjiasdfsddfgdgdfgdfgfdgdgdwerterteteternlgonhjgjgjjg";
label.backgroundColor = [UIColor yellowColor];
//拿到label的字体大小
UIFont *font = label.font;
//当label.numberOfLines 设置为 0时及不固定label的行数
label.numberOfLines = 0 ;
NSDictionary *dic = [NSDictionary dictionaryWithObjectsAndKeys:font,NSFontAttributeName, nil];
CGSize size = [label.text boundingRectWithSize:CGSizeMake(32, 1000) options:NSStringDrawingUsesFontLeading|NSStringDrawingUsesLineFragmentOrigin attributes:dic context:nil].size;
label.frame = CGRectMake(0, 100, 320, size.height);