Title: Coding Tip..(pay it forward)
Question: How do handle dynamic object creation in a neat and effective way ?
Answer:
When one creates objects dynamically, as needed, your code inevitably becomes quite messy. Checking to see if the object exists before creating it, eg.
if FMyVariable = nil then
FMyVariable := TMyClass.Create;
Checking to see if the object exists before destroying it, e.g
if FMyVariable nil then
FreeAndNil(FMyVariable)
Checking to see if the object exists before using it, e.g
if FMyVariable = nil then
begin
FMyVariable := TMyClass.Create;
FMyVariable.SomeMethod;
end;
OR
if FMyVariable nil then
FMyVariable.SomeMethod;
Inevitably your code becomes scattered with code like the fragments shown above making maintenance a real difficult task. If you needed to change the create you would have to hunt all over looking for the "if nil then create" or "if = nil then create", with 4 simple methods you can neaten up your code and give maintenance programmers a break.
The four simple methods are :
procedure CreateObject;
procedure FreeObject;
function ObjectAllocated: Boolean;
procedure ObjectNeeded;
and this is how the code for each method looks;
procedure CreateObject;
begin
FObject := TObject.Create;
end;
procedure FreeObject;
begin
if ObjectAllocated then
FreeAndNil(FObject);
end;
function ObjectAllocated: Boolean;
begin
Result := (FObject nil);
end;
procedure ObjectNeeded;
begin
if not ObjectAllocated then
CreateObject;
end;
And thats it, so instead of using :
"If FObject nil then" you use "if ObjectAllocated then", when you need the object simply call ObjectNeeded, and to destroy the object simply call FreeObject. All the nitty-gritty if existance checking is already taken care of. Here is a simple demo program that uses this technique:
DEMO APP:
Main Form in Text::
object Form1: TForm1
Left = 371
Top = 213
Width = 236
Height = 146
Caption = 'Demo'
Color = clBtnFace
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -11
Font.Name = 'MS Sans Serif'
Font.Style = []
OldCreateOrder = False
OnDestroy = FormDestroy
PixelsPerInch = 96
TextHeight = 13
object Button1: TButton
Left = 8
Top = 8
Width = 100
Height = 25
Caption = 'Create Object'
TabOrder = 0
OnClick = Button1Click
end
object Button2: TButton
Left = 120
Top = 8
Width = 100
Height = 25
Caption = 'Free Object'
TabOrder = 1
OnClick = Button2Click
end
object Button3: TButton
Left = 8
Top = 44
Width = 100
Height = 25
Caption = 'Show Classname'
TabOrder = 2
OnClick = Button3Click
end
object Button4: TButton
Left = 120
Top = 44
Width = 100
Height = 25
Caption = 'Check Existance'
TabOrder = 3
OnClick = Button4Click
end
object Button5: TButton
Left = 76
Top = 84
Width = 75
Height = 25
Caption = '&Close'
Default = True
TabOrder = 4
OnClick = Button5Click
end
end
Main form unit ::
unit FMain;
interface
uses
Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
StdCtrls;
type
TForm1 = class(TForm)
Button1: TButton;
Button2: TButton;
Button3: TButton;
Button4: TButton;
Button5: TButton;
procedure Button5Click(Sender: TObject);
procedure Button1Click(Sender: TObject);
procedure Button2Click(Sender: TObject);
procedure Button3Click(Sender: TObject);
procedure Button4Click(Sender: TObject);
procedure FormDestroy(Sender: TObject);
private
FObject: TObject;
procedure CreateObject;
procedure FreeObject;
function ObjectAllocated: Boolean;
procedure ObjectNeeded;
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.DFM}
const
ExistIdents: array[Boolean] of string =
('Object does not exist', 'Object Exists');
{ TForm1 }
procedure TForm1.CreateObject;
begin
FObject := TObject.Create;
end;
procedure TForm1.FreeObject;
begin
if ObjectAllocated then
FreeAndNil(FObject);
end;
function TForm1.ObjectAllocated: Boolean;
begin
Result := (FObject nil);
end;
procedure TForm1.ObjectNeeded;
begin
if not ObjectAllocated then
CreateObject;
end;
procedure TForm1.Button5Click(Sender: TObject);
begin
Close;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
ObjectNeeded;
end;
procedure TForm1.Button2Click(Sender: TObject);
begin
FreeObject;
end;
procedure TForm1.Button3Click(Sender: TObject);
begin
ObjectNeeded;
ShowMessage(FObject.ClassName);
end;
procedure TForm1.Button4Click(Sender: TObject);
begin
// if ObjectAllocated then
// Showmessage('Object does not exist')
// else
// Showmessage('Object Exists');
// this does exactly the same as above, but is also a preferred technique
ShowMessage(ExistIdents[ObjectAllocated]);
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
FreeObject;
end;
end.
Good luck,
Derrick....(dnel@oldmutual.com)
"programming in VB is like kicking a dead whale down a beach !!".