由于oc是不允许直接修改frame的所以
 _contentLabel = [[UILabel alloc]initWithFrame:CGRectMake(30, 30, 300-25, 20)];
   CGRect floatCont = _contentLabel.frame;
    floatCont.size.height = CGRectGetHeight(rect);
    _contentLabel.frame = floatCont;