3156:Creating a DataAware Control for Browsing Data
KEYWORDS: Data, aware, control, component creation, panel, datasource AREA: Da
OVERVIEW
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 panel with DataSource and
DataField properties similar to the TDBText component. See the
Component Writer's Guide "Making a Control Data-Aware" for further
examples.
WHO SHOULD USE THIS DOCUMENT
To get the best use of this document, you should be familiar
with data aware control functionality and fundamental component
creation tasks, such as
-- deriving components from existing components
-- overriding constructor and destructors
-- creating new properties
-- getting and setting property values
-- assigning event handlers
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.
The TCustomPanel 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.
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 components 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 DataSource. 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: 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
datalink'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 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',
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.
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