unit OnlyOne;
{ * * * * * * * * * * * * * * * * * * * * * * * * * * * *
OnlyOne, version 1.00, Freeware
A Delphi 2.0 component
composed by Gary Nielsen, 3/26/96
70323.2610@compuserve.com
Drop the OnlyOne component onto a form and only one
instance of that window will occur. Any attempt to make
a second instance will restore the previous window.
caveat artifex:
Use this component at your own risk. OnlyOne may not
work with applications that change their title bar, or
with applications that have names longer than 20 chars.
I have only tested this component on a limited number
of programs, so treat it as 'alpha'-ware.
Acknowledgements:
To make this into a component, I used Steven L. Keyser's
JustOne component as a template. I also derived some code
from PC Mag's Michael J. Mefford's PicAlbum utility,
in which hPrevInst is used, but, according to the
documentation, hPrevInst always equals NULL with Delphi 2
and Win95.
Please, if you modify or enhance this code, drop me a
note so that I can learn from your work.
* * * * * * * * * * * * * * * * * * * * * * * * * * * * }
interface
uses
SysUtils, Windows, Messages, Classes, Graphics, Controls,
Forms, Dialogs;
type
TOnlyOne = class(TComponent)
private
public
constructor Create(AOwner:TComponent); override;
destructor Destroy; override;
published
end;
procedure Register;
procedure LookForPreviousInstance;
var
AtomText: array[0..31] of Char;
implementation
procedure Register;
begin
RegisterComponents('Win95', [TOnlyOne]);
end;
procedure LookForPreviousInstance;
var
PreviousInstanceWindow : hWnd;
AppName : array[0..30] of char;
FoundAtom : TAtom;
begin
{put the app name into AtomText}
StrFmt(AtomText, 'OnlyOne%s', [Copy(Application.Title,1,20)]);
{check to see if there's a global atom based on the app name }
FoundAtom := GlobalFindAtom(AtomText);
if FoundAtom <> 0 then {another instance exists}
begin
{get the app name into a pointer string }
StrFmt(AppName,'%s', [Application.Title]);
{change current title so that FindWindow doesn't see it }
Application.Title := 'destroy me';
{locate the previous instance of the app }
PreviousInstanceWindow := FindWindow(nil,AppName);
{give focus to the previous instance of the app }
if PreviousInstanceWindow <> 0 then
if IsIconic(PreviousInstanceWindow) then
ShowWindow(PreviousInstanceWindow,SW_RESTORE)
else
BringWindowToTop(PreviousInstanceWindow);
{stop the current instance of the application }
Application.Terminate;
end;
{ make the global atom so no other instances can occur }
FoundAtom := GlobalAddAtom(AtomText);
end;
constructor TOnlyOne.Create(AOwner:TComponent);
begin
inherited Create(AOwner);
LookForPreviousInstance;
end;
destructor TOnlyOne.Destroy;
var
FoundAtom : TAtom;
ValueReturned : word;
begin
{ must not forget to remove the global atom, so first
check to see if there's a global atom already }
FoundAtom := GlobalFindAtom(AtomText);
if FoundAtom <> 0 then {and now remove that atom}
ValueReturned := GlobalDeleteAtom(FoundAtom);
inherited Destroy;
end;
end.