Mega Code Archive
Limit applications instances to just on
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{
JustOne v1.1 - A Delphi Component
By: Steven L. Keyser
email: 71214,3117@compuserve.com
JustOne v1.1 - Added the ABOUT property
(1/14/96) - Eliminated the AllowMultInst property
- Eliminated the EXECUTE property
- Added JUSTONE.HLP
- Added JUSTONE.KWF
Notes: The ABOUT property was added simply as a learning
exercise. The EXECUTE property was removed due to an
improvement in the component's design. With special
thanks to Russ Chinoy, the JustOne component no
longer requires any code to be added to the user's
application. Dropping the JustOne component onto the startup
form is all that is required now to make JustOne work for you.
JustOne v1.0 - The basic stuff.
(Oct '95)
Purpose: JustOne is a component which makes it easy to limit the
number of your application's instances to just one. If a second
instance of your application starts, the first instance is brought
to the front and given the focus (or restored if it was minimized
to an icon). The second instance then halts.
Credit where credit is due...
Some of the source code for this component came from a Help file
I downloaded from the Delphi Forum on CompuServe (LDELPHI.ZIP). This
Help file, called Lloyd's Delphi Notes (Lloyd Linklater), lists many
tips on using Delphi. One of the items addressed is how to add code
to your application which will allow just one instance to run. In the
Help file, that code is further credited to Pat Ritchey.
I took that snippet of code and put it into an easily re-usable
component.
Additional ideas came from Russ Chinoy (RC Software) on a way to
have JustOne perform its function without the user having to put
any code into their application.
JustOne is released as Freeware. If you use it, you do so at your
own risk. Feel free to modify this source code to suit your own
purposes. If you enhance JustOne, I'd like to see your work.
}
unit Justone;
interface
uses
SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
Forms, Dialogs, DsgnIntf;
type
TMyDataType = record
Name : string;
end;
type
TJustOne = class(TComponent)
private
FAbout: string;
public
constructor Create(AOwner:TComponent); override;
destructor Destroy; override;
procedure GoToPreviousInstance;
procedure ShowAbout;
published
property About: string read FAbout write FAbout stored False;
end;
procedure Register;
type
PHWND = ^HWND;
function EnumFunc(Wnd:HWND; TargetWindow:PHWND): boolean; export;
implementation
{########################################################################}
type
TAboutProperty = class(TPropertyEditor)
public
procedure Edit; override;
function GetAttributes: TPropertyAttributes; override;
function GetValue:string; override;
end;
{########################################################################}
procedure TAboutProperty.Edit;
{Invoke the about dialog when clicking on ... in the Object Inspector}
begin
TJustOne(GetComponent(0)).ShowAbout;
end;
{########################################################################}
function TAboutProperty.GetAttributes: TPropertyAttributes;
{Make settings for just displaying a string in the ABOUT property in the
Object Inspector}
begin
GetAttributes := [paDialog, paReadOnly];
end;
{########################################################################}
function TAboutProperty.GetValue: String;
{Text in the Object Inspector for the ABOUT property}
begin
GetValue := '(About)';
end;
{########################################################################}
procedure TJustOne.ShowAbout;
var
msg: string;
const
carriage_return = chr(13);
copyright_symbol = chr(169);
begin
msg := 'JustOne v1.1';
AppendStr(msg, carriage_return);
AppendStr(msg, 'A Freeware component');
AppendStr(msg, carriage_return);
AppendStr(msg, carriage_return);
AppendStr(msg, 'Copyright ');
AppendStr(msg, copyright_symbol);
AppendStr(msg, ' 1995, 1996 by Steven L. Keyser');
AppendStr(msg, carriage_return);
AppendStr(msg, 'e-mail 71214.3117@compuserve.com');
AppendStr(msg, carriage_return);
ShowMessage(msg);
end;
{########################################################################}
procedure Register;
{If you want, replace 'SLicK' with whichever component page you want
JustOne to show up on.}
begin
RegisterComponents('SLicK', [TJustOne]);
RegisterPropertyEditor(TypeInfo(String), TJustOne, 'About',
TAboutProperty);
end;
{########################################################################}
function EnumFunc(Wnd:HWND; TargetWindow:PHWND): boolean;
var
ClassName : array[0..30] of char;
begin
result := TRUE;
if GetWindowWord(Wnd,GWW_HINSTANCE) = hPrevInst then
begin
GetClassName(Wnd,ClassName,30);
if StrIComp(ClassName,'TApplication') = 0 then
begin
TargetWindow^ := Wnd;
result := FALSE;
end;
end;
end;
{########################################################################}
procedure TJustOne.GotoPreviousInstance;
var
PrevInstWnd : HWND;
begin
PrevInstWnd := 0;
EnumWindows(@EnumFunc,longint(@PrevInstWnd));
if PrevInstWnd <> 0 then
if IsIconic(PrevInstWnd) then
ShowWindow(PrevInstWnd,SW_RESTORE)
else
BringWindowToTop(PrevInstWnd);
end;
{########################################################################}
constructor TJustOne.Create(AOwner:TComponent);
begin
inherited Create(AOwner);
if hPrevInst <> 0 then
begin
GotoPreviousInstance;
halt;
end;
end;
{########################################################################}
destructor TJustOne.Destroy;
begin
inherited Destroy;
end;
{########################################################################}
end.
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