Custom Uninstall page (not MsgBox)

You can modify the uninstall form to behave like the install form (with pages and the Next/Back buttons).

In the InitializeUninstallProgressForm:

• Create the new pages and insert them to the UninstallProgressForm.InnerNotebook (or the .OuterNotebook).
• Implement the “Next” and the “Back” buttons.
• You can also make the “Cancel” button working.
• Run modal loop of the form using UninstallProgressForm.ShowModal.
• Only after the modal loop exits, restore original layout of the form and let the uninstallation continue.

[Code]

var
  UninstallFirstPage: TNewNotebookPage;
  UninstallSecondPage: TNewNotebookPage;
  UninstallBackButton: TNewButton;
  UninstallNextButton: TNewButton;

procedure UpdateUninstallWizard;
begin
  if UninstallProgressForm.InnerNotebook.ActivePage = UninstallFirstPage then
  begin
    UninstallProgressForm.PageNameLabel.Caption := 'First uninstall wizard page';
    UninstallProgressForm.PageDescriptionLabel.Caption :=
      'Lorem ipsum dolor sit amet, consectetur adipiscing elit.';
  end
    else
  if UninstallProgressForm.InnerNotebook.ActivePage = UninstallSecondPage then
  begin
    UninstallProgressForm.PageNameLabel.Caption := 'Second uninstall wizard page';
    UninstallProgressForm.PageDescriptionLabel.Caption :=
      'Sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.';
  end;
  
  UninstallBackButton.Visible :=
    (UninstallProgressForm.InnerNotebook.ActivePage <> UninstallFirstPage);

  if UninstallProgressForm.InnerNotebook.ActivePage <> UninstallSecondPage then
  begin
    UninstallNextButton.Caption := SetupMessage(msgButtonNext);
    UninstallNextButton.ModalResult := mrNone;
  end
    else
  begin
    UninstallNextButton.Caption := 'Uninstall';
    { Make the "Uninstall" button break the ShowModal loop }
    UninstallNextButton.ModalResult := mrOK;
  end;
end;  

procedure UninstallNextButtonClick(Sender: TObject);
begin
  if UninstallProgressForm.InnerNotebook.ActivePage = UninstallSecondPage then
  begin
    UninstallNextButton.Visible := False;
    UninstallBackButton.Visible := False;
  end
    else
  begin
    if UninstallProgressForm.InnerNotebook.ActivePage = UninstallFirstPage then
    begin
      UninstallProgressForm.InnerNotebook.ActivePage := UninstallSecondPage;
    end;
    UpdateUninstallWizard;
  end;
end;

procedure UninstallBackButtonClick(Sender: TObject);
begin
  if UninstallProgressForm.InnerNotebook.ActivePage = UninstallSecondPage then
  begin
    UninstallProgressForm.InnerNotebook.ActivePage := UninstallFirstPage;
  end;
  UpdateUninstallWizard;
end;

procedure InitializeUninstallProgressForm();
var
  PageText: TNewStaticText;
  PageNameLabel: string;
  PageDescriptionLabel: string;
  CancelButtonEnabled: Boolean;
  CancelButtonModalResult: Integer;
begin
  if not UninstallSilent then
  begin
    { Create the first page and make it active }
    UninstallFirstPage := TNewNotebookPage.Create(UninstallProgressForm);
    UninstallFirstPage.Notebook := UninstallProgressForm.InnerNotebook;
    UninstallFirstPage.Parent := UninstallProgressForm.InnerNotebook;
    UninstallFirstPage.Align := alClient;
  
    PageText := TNewStaticText.Create(UninstallProgressForm);
    PageText.Parent := UninstallFirstPage;
    PageText.Top := UninstallProgressForm.StatusLabel.Top;
    PageText.Left := UninstallProgressForm.StatusLabel.Left;
    PageText.Width := UninstallProgressForm.StatusLabel.Width;
    PageText.Height := UninstallProgressForm.StatusLabel.Height;
    PageText.AutoSize := False;
    PageText.ShowAccelChar := False;
    PageText.Caption := 'Press Next to proceeed with uninstallation.';
  
    UninstallProgressForm.InnerNotebook.ActivePage := UninstallFirstPage;

    PageNameLabel := UninstallProgressForm.PageNameLabel.Caption;
    PageDescriptionLabel := UninstallProgressForm.PageDescriptionLabel.Caption;
  
    { Create the second page }

    UninstallSecondPage := TNewNotebookPage.Create(UninstallProgressForm);
    UninstallSecondPage.Notebook := UninstallProgressForm.InnerNotebook;
    UninstallSecondPage.Parent := UninstallProgressForm.InnerNotebook;
    UninstallSecondPage.Align := alClient;
  
    PageText := TNewStaticText.Create(UninstallProgressForm);
    PageText.Parent := UninstallSecondPage;
    PageText.Top := UninstallProgressForm.StatusLabel.Top;
    PageText.Left := UninstallProgressForm.StatusLabel.Left;
    PageText.Width := UninstallProgressForm.StatusLabel.Width;
    PageText.Height := UninstallProgressForm.StatusLabel.Height;
    PageText.AutoSize := False;
    PageText.ShowAccelChar := False;
    PageText.Caption := 'Press Uninstall to proceeed with uninstallation.';
  
    UninstallNextButton := TNewButton.Create(UninstallProgressForm);
    UninstallNextButton.Parent := UninstallProgressForm;
    UninstallNextButton.Left :=
      UninstallProgressForm.CancelButton.Left -
      UninstallProgressForm.CancelButton.Width -
      ScaleX(10);
    UninstallNextButton.Top := UninstallProgressForm.CancelButton.Top;
    UninstallNextButton.Width := UninstallProgressForm.CancelButton.Width;
    UninstallNextButton.Height := UninstallProgressForm.CancelButton.Height;
    UninstallNextButton.OnClick := @UninstallNextButtonClick;

    UninstallBackButton := TNewButton.Create(UninstallProgressForm);
    UninstallBackButton.Parent := UninstallProgressForm;
    UninstallBackButton.Left :=
      UninstallNextButton.Left - UninstallNextButton.Width -
      ScaleX(10);
    UninstallBackButton.Top := UninstallProgressForm.CancelButton.Top;
    UninstallBackButton.Width := UninstallProgressForm.CancelButton.Width;
    UninstallBackButton.Height := UninstallProgressForm.CancelButton.Height;
    UninstallBackButton.Caption := SetupMessage(msgButtonBack);
    UninstallBackButton.OnClick := @UninstallBackButtonClick;
    UninstallBackButton.TabOrder := UninstallProgressForm.CancelButton.TabOrder;

    UninstallNextButton.TabOrder := UninstallBackButton.TabOrder + 1;

    UninstallProgressForm.CancelButton.TabOrder :=
      UninstallNextButton.TabOrder + 1;

    { Run our wizard pages } 
    UpdateUninstallWizard;
    CancelButtonEnabled := UninstallProgressForm.CancelButton.Enabled
    UninstallProgressForm.CancelButton.Enabled := True;
    CancelButtonModalResult := UninstallProgressForm.CancelButton.ModalResult;
    UninstallProgressForm.CancelButton.ModalResult := mrCancel;

    if UninstallProgressForm.ShowModal = mrCancel then Abort;

    { Restore the standard page payout }
    UninstallProgressForm.CancelButton.Enabled := CancelButtonEnabled;
    UninstallProgressForm.CancelButton.ModalResult := CancelButtonModalResult;

    UninstallProgressForm.PageNameLabel.Caption := PageNameLabel;
    UninstallProgressForm.PageDescriptionLabel.Caption := PageDescriptionLabel;

    UninstallProgressForm.InnerNotebook.ActivePage :=
      UninstallProgressForm.InstallingPage;
  end;
end;

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See also Inno Setup – How to create a OuterNotebook/welcome page in the uninstaller?