TClientDataSet[1]: 浏览测试数据

想学 TClientDataSet 是在 2009 年 5 月, 但当时学不动; 现在好了, 有源码了(DBClient.pas).

希望这次学习能对其内存协调方式有所了解, 顺便学点数据库的知识.

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TClientDataSet 是一个内存数据集(说"数据表"对不住它), 其内存数据可存取到本地(*.cds 或 *.xml 格式).

用 TDBGrid 可方便查看其内存数据, 但需要用数据源组件(如: TDataSource)桥接一下:
TDBGrid.DataSource ← TDataSource.DataSet ← TClientDataSet

Program Files\Common Files\CodeGear Shared\Data 下有官方提供的测试数据, 下面程序可浏览这些数据:

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//假定已在设计时挂接好: ClientDataSet1、DataSource1、DBGrid1, 并添加一个 ListBox1



uses IOUtils, Types;



var DataPath: string;



procedure TForm1.FormCreate(Sender: TObject);

var

  sArr: TStringDynArray;

  s: string;

begin

  { 获取测试数据所在的路径 }

  DataPath := GetEnvironmentVariable('COMMONPROGRAMFILES') + '\CodeGear Shared\Data\';

  { 获取路径下所有 cds 文件 }

  sArr := TDirectory.GetFiles(DataPath, '*.cds');

  { 添加到列表 }

  for s in sArr do ListBox1.Items.Add(ExtractRelativePath(DataPath, s));

end;



procedure TForm1.ListBox1Click(Sender: TObject);

begin

  ClientDataSet1.LoadFromFile(DataPath + ListBox1.Items[ListBox1.ItemIndex]);

end;


 
   

其中的:

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procedure TForm1.ListBox1Click(Sender: TObject);

begin

  ClientDataSet1.LoadFromFile(DataPath + ListBox1.Items[ListBox1.ItemIndex]);

end;



//可换成:

procedure TForm1.ListBox1Click(Sender: TObject);

begin

  ClientDataSet1.Active := False;

  ClientDataSet1.FileName := DataPath + ListBox1.Items[ListBox1.ItemIndex];

  ClientDataSet1.Active := True;

end;



//或换成:

procedure TForm1.ListBox1Click(Sender: TObject);

begin

  ClientDataSet1.Close;

  ClientDataSet1.FileName := DataPath + ListBox1.Items[ListBox1.ItemIndex];

  ClientDataSet1.Open;

end;


 
   

从源码中看 Open/Close 方法和 Active 属性的关系:

---

{ TClientDataSet 的继承关系: TDataSet - TCustomClientDataSet - TClientDataSet }

procedure TDataSet.Open;

begin

  Active := True;

end;



procedure TDataSet.Close;

begin

  Active := False;

end;


 
   

从源码中查看 LoadFromFile 对 Open/Close 方法的调用:

---

procedure TCustomClientDataSet.LoadFromFile(const FileName: string = '');

var

  Stream: TStream;

begin

  Close;

...

  LoadFromStream(Stream); { LoadFromFile 调用了 LoadFromStream}

...

end;



procedure TCustomClientDataSet.LoadFromStream(Stream: TStream);

begin

  Close;

  ReadDataPacket(Stream, False);

  Open;

end;