PCL: GUI programs in Pascal

The PXX Component Library (PCL) is PXX's own GUI library for Pascal programs, in the shape Lazarus and Delphi use: a TForm holds controls such as TButton and TEdit, an event is a method assigned to a property like OnClick, and a form can be designed in an .lfm file and streamed in at run time. It is written from scratch and is much smaller than Lazarus' LCL. Its units live in lib/pcl/.

Underneath, the PCL draws through GTK 3 on Linux. That is its only "widgetset" so far, and everything below was measured on Linux x86-64.

For a Nil Python program, use tkinter instead.

What you need

An example

A form built in code, without an .lfm file: an edit box, a check box, a button and a label in a vertical box. The button's OnClick is a method. Instead of waiting for a person, a TTimer types a name, clicks the button, ticks the box, prints what the controls now hold, and quits, so the program also runs headless.

program pcl_hello;

uses interfaces, uwidgetset, forms, controls, stdctrls, extctrls, gtk3_c;

type
  TApp = class
    Edit: TEdit;
    Check: TCheckBox;
    Button: TButton;
    Greeting: TLabel;
    procedure ButtonClick(Sender: TObject);
    procedure Tick(Sender: TObject);
  end;

procedure TApp.ButtonClick(Sender: TObject);
begin
  Greeting.Caption := 'Hello, ' + Edit.Text;
end;

procedure TApp.Tick(Sender: TObject);
begin
  Edit.Text := 'PXX';
  gtk_button_clicked(Button.Handle);   { what a real click does }
  Check.Checked := True;
  writeln('label: ', Greeting.Caption);
  writeln('checked: ', Check.Checked);
  WidgetSet.AppQuit;
end;

var
  Form: TForm;
  Box: TBox;
  App: TApp;
  Timer: TTimer;
begin
  Application.Initialize;
  Form := TForm.Create(nil);
  Form.Caption := 'PCL hello';
  Box := TBox.Create(nil);
  Box.Vertical := True;
  Box.Parent := Form;

  App := TApp.Create;
  App.Edit := TEdit.Create(nil);
  App.Edit.Parent := Box;
  App.Check := TCheckBox.Create(nil);
  App.Check.Caption := 'Remember me';
  App.Check.Parent := Box;
  App.Button := TButton.Create(nil);
  App.Button.Caption := 'Greet';
  App.Button.OnClick := @App.ButtonClick;
  App.Button.Parent := Box;
  App.Greeting := TLabel.Create(nil);
  App.Greeting.Caption := '(nobody yet)';
  App.Greeting.Parent := Box;

  Timer := TTimer.Create(nil);
  Timer.Interval := 300;
  Timer.OnTimer := @App.Tick;
  Timer.Enabled := True;

  Application.MainForm := Form;
  Application.Run;
  writeln('closed');
end.



./pxx pcl_hello.pas pcl_hello
GDK_BACKEND=x11 xvfb-run -a ./pcl_hello



label: Hello, PXX
checked: TRUE
closed

Built with pin v451 (compiler sha256 d9b7226769cc) through ./pxx, and with the pinned compiler called directly (the same binary), and run under xvfb-run on 2026-09-29. gtk_button_clicked comes from gtk3_c, the GTK binding, and stands in for a person clicking. Remove the timer to keep the window open. Closing the main form ends Application.Run, and WidgetSet.AppQuit (unit uwidgetset) ends it from code.

What works

Unit Classes and routines
forms TForm (Caption, Menu, Realize), TApplication (Initialize, CreateForm, Run, MainForm); the global Application
controls TControl: Left, Top, Width, Height, SetBounds, Caption, Parent, Handle, Show, Invalidate, and the events OnClick, OnMouseDown, OnMouseUp, OnMouseMove, OnKeyDown, OnResize
stdctrls TButton, TLabel, TEdit, TCheckBox, TMemo, TListBox, TComboBox (Text, Checked, AddItem, Item, Count, ItemIndex, OnChange)
extctrls TPanel, TTimer, TPaintBox (Canvas, OnPaint), TBox (a row or column), TPaned (a split with a movable divider), TTabBar
comctrls TTreeView and TTreeNode, TToolBar
menus TMainMenu, TMenuItem (Caption, Enabled, Visible, OnClick)
dialogs ShowMessage; TOpenDialog, TSaveDialog, TSelectDirectoryDialog (set the properties, call Execute, read FileName)
graphics TCanvas (MoveTo, LineTo, Rectangle, Ellipse, TextOut, Draw) with TPen, TBrush, TFont; TBitmap; colour constants such as clRed. Drawing goes through Cairo.
glarea TGLArea: an OpenGL drawing area with OnRender

A form can also be designed in a Lazarus-style .lfm file. With {$R *.lfm} in the program, Application.CreateForm(TForm1, Form1) creates the form, its controls and their event links from the file. With v451 on 2026-09-29, the test programs for these parts ran under Xvfb and printed their success lines: test/gui/test_pcl_lfm.pas and test/gui/test_pcl_helloworld.pas (.lfm streaming and ShowMessage), test/gui/test_pcl_widgets.pas, test/gui/test_pcl_drawing.pas and test/gui/test_pcl_menus.pas. tools/gui_suite.sh runs the whole set.

Larger programs built on the PCL are shown on the Examples page: an OpenGL triangle, the Game of Life, a Mandelbrot explorer, a ray tracer and Solitaire. All five build with v451. The Mandelbrot explorer renders on several threads, so it needs the flag: ./pxx --threadsafe examples/mandelbrot/mandelbrot_gui.pas mandelbrot_gui.

Limits

Measured with pin v451 on 2026-09-29: