How do I position a TOpenDialog

I have got a Delphi application which uses TOpenDialog to let the user select a file. By default, the open dialog is displayed centered on the current monitor which nowadays can be "miles" away from the application's window. I would like the dialog to be displayed centered on the TOpenDialog's owner control, failing that, I'd settle for the application's main window.

The following code kind of works, it is derived from TJvOpenDialog which gave me some hint on how to do it:

type
  TMyOpenDialog = class(TJvOpenDialog)
  private
    procedure SetPosition;
  protected
    procedure DoFolderChange; override;
    procedure WndProc(var Msg: TMessage); override;
  end;

procedure TMyOpenDialog.SetPosition;
begin
var
  Monitor: TMonitor;
  ParentControl: TWinControl;
  Res: LongBool;
begin
  if (Assigned(Owner)) and (Owner is TWinControl) then
    ParentControl := (Owner as TWinControl)
  else if Application.MainForm <> nil then
    ParentControl := Application.MainForm
  else begin
    // this code was already in TJvOpenDialog
    Monitor := Screen.Monitors[0];
    Res := SetWindowPos(ParentWnd, 0,
      Monitor.Left + ((Monitor.Width - Width) div 2),
      Monitor.Top + ((Monitor.Height - Height) div 3),
      Width, Height,
      SWP_NOACTIVATE or SWP_NOZORDER);
    exit; // =>
  end;
  // this is new
  Res := SetWindowPos(GetParent(Handle), 0,
    ParentControl.Left + ((ParentControl.Width - Width) div 2),
    ParentControl.Top + ((ParentControl.Height - Height) div 3),
    Width, Height,
    SWP_NOACTIVATE or SWP_NOZORDER);
end;

procedure TMyOpenDialog.DoFolderChange
begin
  inherited DoFolderChange;  // call inherited first, it sets the dialog style etc.
  SetPosition;
end;

procedure TMyOpenDialog.WndProc(var Msg: TMessage);
begin
  case Msg.Msg of
    WM_ENTERIDLE: begin
      // This has never been called in my tests, but since TJVOpenDialog
      // does it I figured there may be some fringe case which requires
      // SetPosition being called from here.
      inherited; // call inherited first, it sets the dialog style etc.
      SetPosition;
      exit;
    end;
  end;
  inherited;
end;

"kind of works" meaning that the first time the dialog is opened, it is displayed centered on the owner form. But, if I then close the dialog, move the window and open the dialog again, SetWindowPos doesn't seem to have any effect even though it does return true. The dialog gets opened at the same position as the first time.

This is with Delphi 2007 running on Windows XP, the target box is also running Windows XP.

Solution

TJvOpenDialog is a descendant of TOpenDialog, hence you should run your placement call after the VCL centers the dialog. The VCL does it in response to a CDN_INITDONE notification. Responding to a WM_SHOWWINDOW message is too early, and in my tests the window procedure never receives a WM_ENTERIDLE message.

uses
  commdlg;

[...]

procedure TJvOpenDialog.DoFolderChange;
begin
  inherited DoFolderChange;  
//  SetPosition; // shouldn't be needing this, only place the dialog once
end;

procedure TJvOpenDialog.WndProc(var Msg: TMessage);
begin
  case Msg.Msg of
    WM_NOTIFY: begin
      if POFNotify(Msg.LParam)^.hdr.code = CDN_INITDONE then begin
        inherited;    // VCL centers the dialog here
        SetPosition;  // we don't like it ;)
        Exit;
      end;
  end;
  inherited;
end;

or,

procedure TJvOpenDialog.WndProc(var Msg: TMessage);
begin
  case Msg.Msg of
    WM_NOTIFY: if POFNotify(Msg.LParam)^.hdr.code = CDN_INITDONE then
                 Exit;
  end;
  inherited;
end;

to have the dialog where the OS puts it, it actually makes sense.