FieldByName, FindField确实太好用了

2016-08-22 10:23:30来源:作者:jyxy521人点击


作者 François Gaillard

Google翻译 整理


while not ADataSet.Eof do begin[...]ADataSet.FieldByName('MyFieldName1').SomePropertyOrMethod;ADataSet.FieldByName('MyFieldName2').SomePropertyOrMethod;[...]ADataSet.Next; end;


*清晰度和可读性。代码的意图是明显的,数据正是你需要的,这是毫无疑问。 *接近。你创造了你您需要的字段的引用。 *灵活性。你不会被设计时字段阻碍了,你的DataSet可以代表多个查询,你只需要知道内容中有一个MyFieldName的列即可。 *安全。 FieldByName永远不会返回nil,如果没有确实实地字段存在,因为它会引发一个异常。所以,你直接使用属性或方法时,确信不会获得一个av(地址访问错误)。


*这是搜索,每一个DataSet的记录都要查询已经找到的同一字段。这样的代码有老年痴呆症。 *他们知道每调用FieldByName主要是调用FindField,会在主循环引进3个新子循环。象上面的两个字段,就会引入6个子循环,而且是每个数据行都这样。


事实上,我可以告诉你,这(无数)公司的产品代码实际上包含了如下(匿名,但真实的)代码数十例: aQuery.ParamByName('Zone_mask').DataType := ftLargeint; aQuery.ParamByName('Zone_mask').Value:= aZoneMask; aQuery.ParamByName('Start_date').AsDateTime := fStartDate; aQuery.ParamByName('End_date').AsDateTime:= fEndDate;

aQuery.Open; while not aQuery.EOF do begineffectiveDeviceInfo := TEffectiveDeviceInfo.Create( aQuery.FieldByName('Device_code_name').Value, aQuery.FieldByName('Device_descr').Value, aQuery.FieldByName('Toggle_times').Value, aQuery.FieldByName('Comments').Value, aQuery.FieldByName('Effective_start_date').Value, aQuery.FieldByName('Effective_end_date').Value);

fChannelDevice.AddObject( self.MakeSortKey(aQuery.FieldByName('Device_code_id').Value,aQuery.FieldByName('Channel_int').Value), effectiveDeviceInfo);

aQuery.Next; end; aQuery.Close;


所以,当我看到有人发布的代码片段,看起来很不错(包括verify Active, DisableControls, try..finally等),一看到循环内使用FieldByName,就很泄气:

begin Assert(AdoQuery1.Active, 'Dataset is not active!'); tryAdoQuery1.DisableControls;AdoQuery1.First;while not AdoQuery1.Eof dobegin AdoQuery1.Edit; AdoQuery1.FieldByName('MyFieldName').Value := Edit1.Text; AdoQuery1.Post; AdoQuery1.Next;end; finallyAdoQuery1.EnableControls; end;end;


var AField : TField; // <= 增加的行begin Assert(AdoQuery1.Active, 'Dataset is not active!'); tryAdoQuery1.DisableControls;AField := AdoQuery1.FieldByName('MyFieldName'); // <= 增加的行AdoQuery1.First;while not AdoQuery1.Eof dobegin AdoQuery1.Edit; AField.Value := Edit1.Text; // <= 这行修改了 AdoQuery1.Post; AdoQuery1.Next;end; finallyAdoQuery1.EnableControls; end;end;


我很好奇,想知道如果你在你的代码库中找到一个循环FieldByName或FindField grep的时候,你会得到什么... 你还有什么?


I don’t know about you, but I can’t count anymore the number of times I’ve seen this code pattern (in code snippets online as well as in production code):

whilenot ADataSet.Eof dobegin[...]ADataSet.FieldByName('MyFieldName1').SomePropertyOrMethod;ADataSet.FieldByName('MyFieldName2').SomePropertyOrMethod;[...]ADataSet.Next;end;

Using FieldByName in a case like this is certainly very convenient and has a lot of advantages:

Clarity and readability. The intent of the code is obvious and there is no question about which Data point you work with.

Proximity. You create your Field reference exactly where you need it.

Flexibility. You’re not stuck with design-time Fields, your DataSet can represent multiple queries, you just need to know that you have a ‘MyFieldName’ column in this context.

Security. FieldByName never returns nil because it raises an exception if the Field does no exist. So, you’re sure not to get an AV when directly using a property or method.

But is has a major problem:

It is searching -again- for the same already found Field at each and every record of the DataSet. Like this code has Alzheimer.

Knowing that each call to FieldByName is mainly a call to FindField, that can introduce up to 3 new sub-loops within your main loop. With our 2-Field example above, that’s 6 sub-loops that can be added for every row.

It does not seem that bad if it is just an example or some demo code, but as soon as it is published, someone will grab a snippet and a few copy’n'haste later it ends up in production code.

And indeed, I can tell you that this (untold) company’s production code actually contained dozens of cases like the following (anonymized but real) code:

aQuery.ParamByName('Zone_mask').DataType := ftLargeint;aQuery.ParamByName('Zone_mask').Value:= aZoneMask;aQuery.ParamByName('Start_date').AsDateTime := fStartDate;aQuery.ParamByName('End_date').AsDateTime := fEndDate;aQuery.Open;whilenot aQuery.EOF dobegineffectiveDeviceInfo :=TEffectiveDeviceInfo.Create( aQuery.FieldByName('Device_code_name').Value, aQuery.FieldByName('Device_descr').Value, aQuery.FieldByName('Toggle_times').Value, aQuery.FieldByName('Comments').Value, aQuery.FieldByName('Effective_start_date').Value, aQuery.FieldByName('Effective_end_date').Value);fChannelDevice.AddObject(self.MakeSortKey(aQuery.FieldByName('Device_code_id').Value,aQuery.FieldByName('Channel_int').Value),effectiveDeviceInfo);aQuery.Next;end;aQuery.Close;

That’s between 8 and 24 added loops for each record in this dataset which could have tens of thousands of records!Granted, that would be small loops if the number of fields is reasonable, but still…

So, I cringe when I see someone posting on StackOverflow a code snippet that looks good (verify Active, DisableControls, try..finally) but has the FieldByName inside the loop:

beginAssert(AdoQuery1.Active,'Dataset is not active!');tryAdoQuery1.DisableControls;AdoQuery1.First;whilenot AdoQuery1.Eof dobeginAdoQuery1.Edit;AdoQuery1.FieldByName('MyFieldName').Value := Edit1.Text;AdoQuery1.Post;AdoQuery1.Next;end;finallyAdoQuery1.EnableControls;end;end;

And kudos to the poster who changed it after just a little nudge to create a local Field variable before the loop. It adds 2 more lines per Field, but the code is much better:

varAField : TField;// <= line addedbeginAssert(AdoQuery1.Active,'Dataset is not active!');tryAdoQuery1.DisableControls;AField := AdoQuery1.FieldByName('MyFieldName');// <= line addedAdoQuery1.First;whilenot AdoQuery1.Eof dobeginAdoQuery1.Edit;AField.Value := Edit1.Text;AdoQuery1.Post;AdoQuery1.Next;end;finallyAdoQuery1.EnableControls;end;end;

Now, don’t go “premature optimization” on me, as the cases where the number of records is too small to benefit from ousting the FieldByName from the DataSet loop are pretty rare.

I would be curious to know what you get if you do a grep in your code base to find FieldByName or FindField in loops…Do you have any?