How to allow or disallow program shutdown in Delphi

Post date: Mar 15, 2010 9:50:56 PM

An example of trapping the closing of a program for example a user might accidentally try to maximize a form and close it instead, without saving their work ...

Make a change to the first edit box and then attempt to shut down the program, and a demo of a messagebox appears asking for confirmation and listing choices for possible actions you can also implement similar code yourself in a buttonclick event handler for an 'Exit' button (using 'Apllication.Terminate'). Remember to double click the event handler in the Object Inspector for the form's OnCloseQuery event and the Edit Box 'OnChange' event to initialize the event handlers.

unit Unit1;

interface

uses

Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,

StdCtrls;

type

  TForm1 = class(TForm)

  Edit1: TEdit;

  Edit2: TEdit;

  procedure Edit1Change(Sender: TObject);

  procedure FormCloseQuery(Sender: TObject; var CanClose: Boolean);

  private

  { Private declarations }

  public

  { Public declarations }

 end;

var

  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Edit1Change(Sender: TObject);

begin

  Form1.Tag := 1;

end;

{the var variable 'CanClose controls whether or not the user can shut

down the program, its default value is true}

procedure TForm1.FormCloseQuery(Sender: TObject; var CanClose: Boolean);

var

 TheAnswer : Word;

begin

  If Form1.Tag <> 0 then

  begin {the text in Edit box was changed}

   TheAnswer := MessageDlg('Save changes?', mtConfirmation,

   [mbYes, mbNo, mbCancel], 0);

   If TheAnswer = mrCancel then CanClose := false; {return to program}

   If TheAnswer = mrYes then {save the file routine}

   Edit2.Text := Edit1.Text; {in otherwords do the save here}

   {if the answer was no, since canclose defaults to true

   just let the program close}

  end;

end;

Possible MessageDlg Return value constants  Word

these return values indicate which button was pushed on the Dialog

these values correspond to the button type which is declared using

mb instead of mr ie mbOk pressed would return mrOk

mrNone mrAbort mrYes mrOk mrRetry mrNo mrCancel mrIgnore mrAll

the following constants indicate what type of dialog icon will appear

and the caption on the dialog and are in the second field TMsgDlgType

mtWarning A message box containing a yellow exclamation point symbol.

mtError A message box containing a red stop sign.

mtInformation A message box containing a blue "i".

mtConfirmation A message box containing a green question mark.

mtCustom A message box containing no bitmap.

The caption of the message box

is the name of the application's executable file.

end.