计算文字的尺寸(单行)

    NSDictionary *btAtt = @{NSFontAttributeName : [UIFont systemFontOfSize:KCHANGEHEIGHTVALUE(14)]};
    CGSize btSize = [_messageList.bt sizeWithAttributes:btAtt];

计算文字的尺寸(多行)

    NSDictionary *btAtt = @{NSFontAttributeName : [UIFont systemFontOfSize:KCHANGEHEIGHTVALUE(14)]};
    /**
     限制文本宽高

     @param 300 最大宽度
     @param MAXFLOAT 高度不限
     @return CGSize
     */
    CGSize textSize = CGSizeMake(300, MAXFLOAT);
    CGRect textRect = [_messageList.bt boundingRectWithSize:textSize options:NSStringDrawingUsesLineFragmentOrigin attributes:btAtt context:nil];