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Builder pattern
Origin This builder pattern is based on that described in [Gam+,
pages 97..106]. The Delphi implementation for the pattern is from
Shaun Parry.
Intent 'Define an interface for creating an object, but let
subclasses decide which class to instantiate' [Gam+,p97].
Motivation The motivation is based on [Gam+, page 97]. Builder
has a similar motivation to the abstract factory but, whereas in
that pattern, the client uses the abstract factory class methods to
create its own object, in Builder the client instructs the builder
class on how to create the object and then asks it for the result.
How the class is put together is up to the Builder class. It's a
subtle difference.
Implementation The following is an example of making a panel
with three edit boxes within it, as with the Abstract Factory
pattern. Again we are going to use a method called CreatePanel
within a class called PanelMaker:
type TPanelMaker = class (TObject) public function CreatePanel(
aOwner : TWinControl; PanelBuilder : TBuilder ): TPanel; end;
implementation
function TPanelMaker.CreatePanel(aOwner : TWinControl;
PanelBuilder : TBuilder ): TPanel; // Notice that thereare no //
local variables such as // TempPanel and TempButton // These are
kept local to the builder object begin PanelBuilder.BuildPanel(
aOwner, 100, 200 );
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PanelBuilder.BuildEdit( 10, 10, 21, 100 );
PanelBuilder.BuildEdit( 40, 10, 21, 100 ); PanelBuilder.BuildEdit(
70, 10, 21, 100 ); CreatePanel := PanelBuilder.Panel; end;
Notice that all we are doing is telling the builder what to do
with the panel, rather than asking it for various components.
The builder class looks like this:
type TPanelBuilder = class (TObject) protected FButtonList :
TList; FEditList : TList; FPanel: TPanel; function GetPanel:
TPanel; virtual; public procedure BuildButton(Caption : string; Top
: Integer; Left : Integer; Height : Integer; Width: Integer);
virtual; procedure BuildEdit(Top : Integer; Left : Integer; Height
: Integer; Width : Integer); virtual; procedure BuildPanel(aOwner :
TWinControl; Height : Integer; Width : Integer); virtual; property
Panel: TPanel read GetPanel; end;
implementation
procedure TPanelBuilder.BuildButton(Caption : string; Top :
Integer; Left : Integer; Height : Integer; Width: Integer); var
TempButton: TButton; begin TempButton := TButton.Create( FPanel
);
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TempButton.Parent := FPanel; TempButton.Caption := Caption;
TempButton.Top := Top; TempButton.Left := Left; TempButton.Height
:= Height; TempButton.Width := Width; end;
procedure TPanelBuilder.BuildEdit(Top : Integer; Left : Integer;
Height : Integer; Width : Integer); var TempEdit: TEdit; begin
TempEdit := TEdit.Create( FPanel ); TempEdit.Parent := FPanel;
TempEdit.Top := Top; TempEdit.Left := Left; TempEdit.Height :=
Height; TempEdit.Width := Width; end;
procedure TPanelBuilder.BuildPanel(aOwner : TWinControl; Height
: Integer; Width : Integer); begin FPanel := TPanel.Create( aOwner
); FPanel.Parent := aOwner; FPanel.Height := Height; Fpanel.Width
:= Width; end;
function TPanelBuilder.GetPanel: TPanel; begin GetPanel :=
FPanel; end;
Say that we want to change this, such that we want to make the
default panel one with OK and Cancel buttons. All we have to do is
sub class TPanelBuilder and override BuildPanel, thus:
type TButtonPanelBuilder = class (TPanelBuilder) public
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procedure BuildPanel(aOwner : TWinControl; Height : Integer;
Width : Integer); override; end;
procedure TButtonPanelBuilder.BuildPanel(aOwner : TWinControl;
Height : Integer; Width : Integer); var OKButton: TButton;
CancelButton: TButton; begin inherited BuildPanel(aOwner, Height,
Width); FPanel.Width := FPanel.Width + 60; // increase the size of
the // panel to fit the buttons OKButton := TButton.Create( FPanel
); OKButton.Parent := FPanel; OKButton.Caption := 'OK';
OKButton.Top := FPanel.Top + 21; OKButton.Left := FPanel.Width -
60; OKButton.Width := 50; CancelButton := TButton.Create( FPanel );
CancelButton.Parent := FPanel; CancelButton.Caption := 'Cancel';
CancelButton.Top := OKButton.Top + OKButton.Top + 21;
CancelButton.Left := FPanel.Width - 60; CancelButton.Width := 50;
end;
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