UIButton的sizeToFit方法无效

可以通过boundingRectWithSize计算size再进行设值；

例：

NSDictionary *attribute = @{NSFontAttributeName: [UIFont systemFontOfSize:16.0f]};

CGSize size = [model.title boundingRectWithSize:CGSizeMake(300, 30) options:NSStringDrawingUsesLineFragmentOrigin attributes:attribute context:nil].size;