计算字符串内容所需高度

-(CGFloat) heightWithText:(NSString *) text font:(UIFont *) font width:(CGFloat) width{
    NSDictionary * attributes = @{NSFontAttributeName : font};
    CGSize contentSize = [text boundingRectWithSize:CGSizeMake(width, MAXFLOAT)
                                                options:(NSStringDrawingUsesLineFragmentOrigin|NSStringDrawingUsesFontLeading)
                                                attributes:attributes
                                                context:nil].size;
    return contentSize.height;
}