关于iOS13 UITextField 通过KVC改变 plac

iOS13不再允许UITextField的placeholder属性通过 kvc 的方式更改颜色和字体等属性，替换方法为：

    UITextField * searchField = self.searchTextField;
    NSString *placeHolder = @"请输入关键词搜索";
    NSMutableAttributedString *placeHolderStr = [[NSMutableAttributedString alloc]initWithString:placeHolder];

    NSDictionary *attrDic = @{@"NSFontAttributeName":[UIFont systemFontOfSize:15],
                              @"NSForegroundColorAttributeName":[UIColor grayColor]};

    [placeHolderStr addAttributes:attrDic} range:NSMakeRange(0, placeHolder.length)];
    searchField.attributedPlaceholder = placeHolderStr;