Delphi Change main form while application is running

Question

I have this problem. When I hide my main form, then the taskbar icon of my application is also hidden. I saw a new question about this problem as well and the answers didn't really help. They suggested to minimize it, but I do not want to minimize the application.

Is it possible to change the main form while the application is already running?

for instance. I have two forms. when I want to hide the one form and show the other form, then the taskbar icon should stay at the taskbar and the main form should switch to the other form.

I am using Delphi XE6 and it is a VCL Forms application.

I have also seen a different old question about changing main form while runtime, but it is very old and still based on Delphi 6.

Answer 1

You can change main form. Do a variable F: ^TForm, then set it to @Application.MainForm. After that you can set main form as via F^ := YourAnotherForm.

Answer 2

As David Heffernan already said it is not possible to change the Main Form of an already running application. This is the limitation of the windows itself.

What you can do is cheat and never actually change the Main Form but only make it look like you did.
How do you achieve that?

Step 1: Add code to the second Form to make its own Taskbar button

procedure TWorkForm.CreateParams(var Params: TCreateParams);
begin
  inherited;
  Params.ExStyle := Params.ExStyle or WS_EX_APPWINDOW;
end;

Step 2: Dynamically create the second Form just before switching to it. Upon its creation previously added code will create a new Taskbar button for your second form.

Step 3: Now hide you actual Main Form. Hiding it will also hide the Taskbar button belonging to it. So you end up still having one Taskbar button shown and it is the one belonging to your second Form.

Step 4: In order to allow your second Form to terminate your application at its closure call Close method of your true Main Form from your second Forms OnClose or OnFormCloseQuery event.
If you wanna be able to switch back to your true Main Form call Show method of your Main Form instead of Close method.

This approach allows us of swapping forms pretty quickly so only most keen users will notice short animation of Taskbar button.
NOTE: If your second for is a complex one and because of that takes some time to create you might wanna create it hidden and then once its creation process is finished show it and do the swap. Otherwise you might end up with two Taskbar buttons being shown at same time which I believe you wanna avoid.

Here is a short example:
- LoginForm is a true Main Form that is created at the application startup - WorkForm is the Form on which user will spend most of time after logging in and this one is created in login process

Login Form code:

unit ULoginForm;

interface

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

type
  TLoginForm = class(TForm)
    BLogIn: TButton;
    procedure BLogInClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  LoginForm: TLoginForm;

  //Global variable to tell us if we are only logging out or closing our program
  LoggingOut: Boolean;

implementation

uses Unit2;

{$R *.dfm}

procedure TLoginForm.BLogInClick(Sender: TObject);
begin
  //Create second Form
  Application.CreateForm(TWorkForm, WorkForm);
  //Hide Main Form
  Self.Hide;
  //Don't forget to clear login fields
end;

end.

Work form code:

unit UWorkForm;

interface

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

type
  TWorkForm = class(TForm)
    BLogOut: TButton;
    //Used in overriding forms creating parameters so we can add its own Taskbar button
    procedure CreateParams(var Params: TCreateParams); override;
    procedure FormCloseQuery(Sender: TObject; var CanClose: Boolean);
    procedure BLogOutClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  WorkForm: TWorkForm;

implementation

uses Unit1;

{$R *.dfm}

procedure TWorkForm.BLogOutClick(Sender: TObject);
begin
  //Set to true so we know we are in the process of simply logging out
  LoggingOut := True;
  //Call close method to begin closing the current Form
  Close;
end;

procedure TWorkForm.CreateParams(var Params: TCreateParams);
begin
  inherited;
  Params.ExStyle := Params.ExStyle or WS_EX_APPWINDOW;
end;

procedure TWorkForm.FormCloseQuery(Sender: TObject; var CanClose: Boolean);
begin
  //Check to see if we are in the process of simply logging out
  if not LoggingOut then
  begin
    //If we are not in the process of logging out close the Main Form
    LoginForm.Close;
    //and then allow closing of current form
    CanClose := True;
  end
  else
  begin
    //But if we are in the process of simply logging out show the Main Form
    LoginForm.Show;
    //Reset the LoggingOut to false
    LoggingOut := False;
    //and then alow closing of current form
    CanClose := True;
  end;
end;

end.

Answer 3

Is it possible to change the main form while the application is already running?

It is not possible to change the VCL main form whilst the program is running. This property is determined once and for all when the program starts.

One possible way to proceed for you is to arrange for the secondary form, the form that is not the main form, to have a button on the taskbar. Do that by making it unowned, or by using the WS_EX_APPWINDOW extended window style.

Update

Well, you can change Application.MainForm, but it requires you to destroy the current main form, and then create a new one.