How do I get the MessageDlgPos dimensions?

Question

I want to position a MessageBox in a particular position with respect to the active cell in a string grid and this is no problem using MessageDlgPos() except that I want to prevent the box running off the right or bottom of the screen when the active cell is close to the right or bottom. What I need for this is a way of getting the dimensions of the box but I cannot see a simple way of getting these. Anyone know how without creating my own box?

Answer 1

The MessageDlg...() functions do not support what you are asking for. The dimensions of the dialog are not known until the dialog is being displayed, and you have no way to access the dialog window directly to query/re-position it, except maybe with a WH_CBT hook from SetWindowsHookEx().

That being said...

On Windows Vista+ with Vcl.Dialogs.UseLatestCommonDialogs=true and Visual Styles enabled, the MessageDlg...() functions call the Win32 TaskDialogIndirect() API to display a message box. You have no control over that dialog's dimensions, so you would have to wait for that dialog to issue a TDN_DIALOG_CONSTRUCTED notification to then query its actual dimensions before it is displayed, so you can then adjust its position as needed. However, the MessageDlg...() functions do not provide access to any of TaskDialogIndirect()'s notifications (TCustomTaskDialog, which is used internally, does have an OnDialogConstructed event, amongst other events). So, if you wanted to reposition this dialog, you would have to call TaskDialogIndirect() yourself with a custom callback function (or, use the VCL's TTaskDialog wrapper).

On pre-Vista, or with UseLatestCommonDialogs=false or Visual Styles disabled, the MessageDlg...() functions display a custom VCL TForm via Vcl.Dialogs.CreateMessageDialog() instead, which you can call directly, and then pretty much query, manipulate, and show the returned TForm however you want. Just be sure to Free() it when you are done using it.

Answer 2

You could use an actual TTaskDialog. You can create you own version of it, add a TaskDialogConstructed procedure and get the dimension in the TaskDialogConstructed procedure. Something along the lines of the following.

type
  TTaskDialog = class(Vcl.Dialogs.TTaskDialog)
  protected
    procedure TaskDialogConstructed(Sender: TObject);
  end;

procedure TTaskDialog.TaskDialogConstructed(Sender: TObject);
var
  TaskDialog: TTaskDialog;
  R: TRect;
begin
  TaskDialog := Sender as TTaskDialog;
  Win32Check(GetWindowRect(TaskDialog.Handle, R));
  
  {... Do whatever with R ...}
  
end;
    
function ExecuteTaskDialog(AOwner: TComponent; ATitle, AText: string; ACommonButtons: TTaskDialogCommonButtons = [tcbOK]): integer;
var
  TaskDialog: TTaskDialog;
begin
  TaskDialog := TTaskDialog.Create(AOwner);
  with TaskDialog do
  begin
    Caption := Application.Title;
    Title := ATitle;
    Text := AText;
    MainIcon := tdiNone;
    Flags := Flags + [tfUseHiconMain];
    CommonButtons := ACommonButtons;
    CustomMainIcon.LoadFromResourceName(HInstance, 'MAINICON');
    OnDialogConstructed := TaskDialogConstructed;
    Execute;
    Result := ModalResult;
    Free;
  end;
end;

Answer 3

Create the MessageDlg yourself. Add an OnActivate or OnShow event. In this method, ask / change the properties of the dialog.

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.StdCtrls;

type
  Tfrm = class(TForm)
    btn: TButton;
    procedure btnClick(Sender: TObject);
  private
   procedure OnDlgActivate(Sender: TObject);
  public
    { Public-Deklarationen }
  end;

var
  frm: Tfrm;


implementation

uses
  Vcl.Dialogs, System.TypInfo;

{$R *.dfm}


procedure Tfrm.btnClick(Sender: TObject);
var
  Ldlg  : TForm;
  LiRet : integer;
begin
  Ldlg := CreateMessageDialog('Hallo World!', mtInformation,mbYesNo, mbYes);
  try
    Ldlg.OnActivate := OnDlgActivate;
    LiRet := Ldlg.ShowModal;
  finally
    Ldlg.free;
  end;

end;

procedure Tfrm.OnDlgActivate(Sender: TObject);
var
  Lfrm: TForm;
  LcTxt: string;
begin
  Lfrm  := Sender as TForm;
  LcTxt := Format('%s %sLeft: %d / Top: %d', [Lfrm.ClassName, sLineBreak, Lfrm.Left, Lfrm.Top]);

  ShowMessage(LcTxt);
end;

end.