Question

I am handling from my Application associated extension files from Windows. So when you double click a file from Windows it will execute my program, and I handle the file from there, something like:

procedure TMainForm.FormCreate(Sender: TObject);
var
  i: Integer;
begin
  for i := 0 to ParamCount -1 do
  begin
    if SameText(ExtractFileExt(ParamStr(i)), '.ext1') then
    begin
      // handle my file..

      // break if needed
    end else
    if SameText(ExtractFileExt(ParamStr(i)), '.ext2') then
    begin
      // handle my file..

      // break if needed
    end else
  end;
end;

That works pretty much how I want it to, but when I was testing I realised it does not consider using only one instance of my program.

So for example, if I selected several Files from Windows and opened them all at the same time, this will create the same number of instances of my program with the number of Files being opened.

What would be a good way to approach this, so that instead of several instances of my program being opened, any additional Files from Windows being opened will simply focus back to the one and only instance, and I handle the Files as normal?

Thanks

UPDATE

I found a good article here: http://www.delphidabbler.com/articles?article=13&part=2 which I think is what I need, and shows how to work with the Windows API as mentioned by rhooligan. I am going to read through it now..

Answer 1

Here is some simple example code that gets the job done. I hope it is self-explanatory.

program StartupProject;

uses
  SysUtils,
  Messages,
  Windows,
  Forms,
  uMainForm in 'uMainForm.pas' {MainForm};

{$R *.res}

procedure Main;
var
  i: Integer;
  Arg: string;
  Window: HWND;
  CopyDataStruct: TCopyDataStruct;
begin
  Window := FindWindow(SWindowClassName, nil);
  if Window=0 then begin
    Application.Initialize;
    Application.MainFormOnTaskbar := True;
    Application.CreateForm(TMainForm, MainForm);
    Application.Run;
  end else begin
    FillChar(CopyDataStruct, Sizeof(CopyDataStruct), 0);
    for i := 1 to ParamCount do begin
      Arg := ParamStr(i);
      CopyDataStruct.cbData := (Length(Arg)+1)*SizeOf(Char);
      CopyDataStruct.lpData := PChar(Arg);
      SendMessage(Window, WM_COPYDATA, 0, NativeInt(@CopyDataStruct));
    end;
    SetForegroundWindow(Window);
  end;
end;

begin
  Main;
end.

 

unit uMainForm;

interface

uses
  Windows, Messages, SysUtils, Classes, Controls, Forms, StdCtrls;

type
  TMainForm = class(TForm)
    ListBox1: TListBox;
    procedure FormCreate(Sender: TObject);
  protected
    procedure CreateParams(var Params: TCreateParams); override;
    procedure WMCopyData(var Message: TWMCopyData); message WM_COPYDATA;
  public
    procedure ProcessArgument(const Arg: string);
  end;

var
  MainForm: TMainForm;

const
  SWindowClassName = 'VeryUniqueNameToAvoidUnexpectedCollisions';

implementation

{$R *.dfm}

{ TMainForm }

procedure TMainForm.CreateParams(var Params: TCreateParams);
begin
  inherited;
  Params.WinClassName := SWindowClassName;
end;

procedure TMainForm.FormCreate(Sender: TObject);
var
  i: Integer;
begin
  for i := 1 to ParamCount do begin
    ProcessArgument(ParamStr(i));
  end;
end;

procedure TMainForm.ProcessArgument(const Arg: string);
begin
  ListBox1.Items.Add(Arg);
end;

procedure TMainForm.WMCopyData(var Message: TWMCopyData);
var
  Arg: string;
begin
  SetString(Arg, PChar(Message.CopyDataStruct.lpData), (Message.CopyDataStruct.cbData div SizeOf(Char))-1);
  ProcessArgument(Arg);
  Application.Restore;
  Application.BringToFront;
end;

end.