How to create data-aware components #194

This document describes minimal steps necessary to create a data-aware browsing component that displays data for a single field. The example component is a TPanel descendant with DataSource and DataField properties similar to the TDBText component. "Making a Control Data-Aware" in the Component Writer's Guide in the Delphi help file for further examples.

Basic steps to create a data-browsing component

Create or derive a component that allows the display, but not the entry of data. For instance, you could use a TMemo with ReadOnly set to true. In the example outlined in this document, we'll use a TCustomPanel. This will allow display, but not data entry.

• Add a data-link object to your component. This object manages communication between the component and the database table.
• Add DataField and DataSource properties to the component.
• Add methods to get and set the DataField and DataSource.
• Add a DataChange method the component to handle the data-link object's OnDataChange event.
• Override the component constructor to create the datalink and hook up the DataChange method.
• Override the component destructor to cleanup the datalink.

Creating the TDBPanel

Create or derive a component that allows the display, but not the entry of data. We'll be using a TCustomPanel as a starting point for this example.

In Delphi, choose the appropriate menu option to create a new component (this will vary between editions of Delphi), and specify TDBPanel as the class name, and TCustomPanel as the ancestor type. You may specify any palette page.

Add DB and DBTables to your uses clause.

Add a data-link object to the component's private section. This example will display data for a single field, so we will use a TFieldDataLink to provide the connection between our new component and a data-source. Name the new data-link object FDataLink. Example:

private
  FDataLink: TFieldDataLink;

Add DataField and DataSource properties to the component. We will add supporting code for the get and set methods in following steps. Note that our new component will have DataField and DataSource properties and FDataLink will also have its own DataField and Datasource properties. Example:

published
  property DataField: string read GetDataField write SetDataField;
  property DataSource: TDataSource read GetDataSource write SetDataSource;

Add private methods to get and set the DataField and DataSource property values to and from the DataField and DataSource for FDataLink. Example:

...
private
  FDataLink: TFieldDataLink;
  function GetDataField: String;
  function GetDataSource: TDataSource;
  procedure SetDataField(Const Value: string);
  procedure SetDataSource(Value: TDataSource);
...
 
implementation    
 
function TDBPanel.GetDataField: String;
begin
  Result := FDataLink.FieldName;
end;
 
function TDBPanel.GetDataSource: TDataSource;
begin
  Result := FDataLink.DataSource;
end;
 
procedure TDBPanel.SetDataField(Const Value: string);
begin
  FDataLink.FieldName := Value;
end;
 
procedure TDBPanel.SetDataSource(Value: TDataSource);
begin
  FDataLink.DataSource := Value;
end;

Add a private DataChange method to be assigned to the data-link's OnDataChange event. In the DataChange method add code to display actual database field data provided by the data-link object. In this example, we assign FDataLink's field value to the panel's caption. Example:

...
private
  procedure DataChange(Sender: TObject);
...
 
implementation
 
procedure TDBPanel.DataChange(Sender: TObject);
begin
  if FDataLink.Field = nil then
    Caption := '';
  else
    Caption := FDataLink.Field.AsString;
end;

Override the component constructor Create method. In the implementation of Create, create the FDataLink object, and assign the private DataChange method to FDataLink's OnDataChange event. Example:

...
public
  constructor Create(AOwner: TComponent); override;
...
 
implementation
 
constructor TMyDBPanel.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FDataLink := TFieldDataLink.Create;
  FDataLink.OnDataChange := DataChange;
end;

Override the component destructor Destroy method. In the implementation of Destroy, set OnDataChange to nil (avoids a GPF), and free FDatalink. Example:

...
public
  destructor Destroy; override;
...
 
implementation
 
destructor TDBPanel.Destroy;
begin
  FDataLink.OnDataChange := nil;
  FDataLink.Free;
  inherited Destroy;
end;

Save the unit and install the component (see the Users Guide, and the Component Writers Guide in Delphi Help for more on saving units and installing components).

To test the functionality of the component, add a TTable, TDatasource, TDBNavigator and TDBPanel to a form. Set the TTable DatabaseName and TableName to 'DBDemos' and 'BioLife' respectively, and the Active property to True. Set the TDatasource.Dataset property to Table1. Set the TDBNavigator and TDBPanel.DataSource property to Datasource1. The TDBPanel.DataField name should be set as 'Common_Name'. Run the application and use the navigator to move between records to demonstrate the TDBPanel's ability to detect the change in data and display the appropriate field value.

Full source listing

unit Mydbp;
 
interface
 
uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics,
  Controls, Forms, Dialogs, ExtCtrls, DB, DBTables;
 
type
  TDBPanel = class(TCustomPanel)
  private
    FDataLink: TFieldDataLink;
    function GetDataField: String;
    function GetDataSource: TDataSource;
    procedure SetDataField(Const Value: string);
    procedure SetDataSource(Value: TDataSource);
    procedure DataChange(Sender: TObject);
public
  constructor Create(AOwner: TComponent); override;
  destructor Destroy; override;
published
  property DataField: string read GetDataField write SetDataField;
  property DataSource: TdataSource read GetDataSource write SetDataSource;
end;
 
procedure Register;
 
implementation
 
procedure Register;
begin
  RegisterComponents('Samples', [TDBPanel]);
end;
 
function TDBPanel.GetDataField: String;
begin
  Result := FDataLink.FieldName;
end;
 
function TDBPanel.GetDataSource: TDataSource;
begin
  Result := FDataLink.DataSource;
end;
 
procedure TDBPanel.SetDataField(Const Value: string);
begin
  FDataLink.FieldName := Value;
end;
 
procedure TDBPanel.SetDataSource(Value: TDataSource);
begin
  FDataLink.DataSource := Value;
end;
 
procedure TDBPanel.DataChange(Sender: TObject);
begin
  if FDataLink.Field = nil then
    Caption := ''
  else
    Caption := FDataLink.Field.AsString;
end;
 
constructor TDBPanel.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FDataLink := TFieldDataLink.Create;
  FDataLink.OnDataChange := DataChange;
end;
 
destructor TDBPanel.Destroy;
begin
  FDataLink.Free;
  FDataLink.OnDataChange := nil;
  inherited Destroy;
end;
 
end.