(* Drawing Area
 *
 * GtkDrawingArea is a blank area where you can draw custom displays
 * of various kinds.
 *
 * This demo has two drawing areas. The checkerboard area shows
 * how you can just draw something; all you have to do is write
 * a signal handler for expose_event, as shown here.
 *
 * The "scribble" area is a bit more advanced, and shows how to handle
 * events such as button presses and mouse motion. Click the mouse
 * and drag in the scribble area to draw squiggles. Resize the window
 * to clear the area.
 *)


var
  da_window : PGtkWidget;

(* Pixmap for scribble area, to store current scribbles *)
  da_pixmap : PGdkPixmap;

(* Create a new pixmap of the appropriate size to store our scribbles *)

function scribble_configure_event (widget : PGtkWidget;
                                   event  : PGdkEventConfigure;
                                   data   : gpointer): gboolean; cdecl;

begin
  if da_pixmap <> NULL then
    g_object_unref (G_OBJECT (da_pixmap));

  da_pixmap := gdk_pixmap_new (widget^.window,
                   widget^.allocation.width,
                   widget^.allocation.height,
                   -1);

  (* Initialize the pixmap to white *)
  gdk_draw_rectangle (da_pixmap,
                      widget^.style^.white_gc,
                      gint(gTRUE),
                      0, 0,
                      widget^.allocation.width,
                      widget^.allocation.height);

  (* We've handled the configure event, no need for further processing. *)
  scribble_configure_event := TRUE;

end;

(* Redraw the screen from the pixmap *)
function scribble_expose_event (widget : PGtkWidget;
                                event  : PGdkEventExpose;
                                data   : gpointer): gboolean; cdecl;

begin
  (* We use the "foreground GC" for the widget since it already exists,
   * but honestly any GC would work. The only thing to worry about
   * is whether the GC has an inappropriate clip region set.
   *)

  gdk_draw_drawable (widget^.window,
                     widget^.style^.fg_gc[GTK_WIDGET_STATE (widget)],
                     da_pixmap,
                     (* Only copy the area that was exposed. *)
                     event^.area.x, event^.area.y,
                     event^.area.x, event^.area.y,
                     event^.area.width, event^.area.height);

  scribble_expose_event := FALSE;
end;

(* Draw a rectangle on the screen *)
procedure draw_brush (widget : PGtkWidget;
                      x, y   : gdouble);
var
  update_rect : TGdkRectangle;

begin
  update_rect.x := round (x - 3);
  update_rect.y := round (y - 3);
  update_rect.width := 6;
  update_rect.height := 6;

  (* Paint to the pixmap, where we store our state *)
  gdk_draw_rectangle (da_pixmap,
              widget^.style^.black_gc,
                      gint(gTRUE),
                      update_rect.x, update_rect.y,
                      update_rect.width, update_rect.height);

  (* Now invalidate the affected region of the drawing area. *)
  gdk_window_invalidate_rect (widget^.window,
                              @update_rect,
                              FALSE);
end;

function scribble_button_press_event (widget : PGtkWidget;
                                      event  : PGdkEventButton;
                                      data   : gpointer): gboolean; cdecl;
begin
  if da_pixmap = NULL then
    exit (FALSE); (* paranoia check, in case we haven't gotten a configure event *)

  if event^.button = 1 then
    draw_brush (widget, event^.x, event^.y);

  (* We've handled the event, stop processing *)
  exit (TRUE);
end;

function scribble_motion_notify_event (widget : PGtkWidget;
                                       event  : PGdkEventButton;
                                       data   : gpointer): gboolean; cdecl;
var
  x, y   : gint;
  state  : TGdkModifierType;

begin
  if da_pixmap = NULL then
    exit (FALSE);  (* paranoia check, in case we haven't gotten a configure event *)

  (* This call is very important; it requests the next motion event.
   * If you don't call gdk_window_get_pointer() you'll only get
   * a single motion event. The reason is that we specified
   * GDK_POINTER_MOTION_HINT_MASK to gtk_widget_set_events().
   * If we hadn't specified that, we could just use event->x, event->y
   * as the pointer location. But we'd also get deluged in events.
   * By requesting the next event as we handle the current one,
   * we avoid getting a huge number of events faster than we
   * can cope.
   *)

  gdk_window_get_pointer (event^.window, @x, @y, @state);

  if (state and GDK_BUTTON1_MASK) <> 0 then
    draw_brush (widget, x, y);

  (* We've handled it, stop processing *)
  exit (TRUE);
end;


const
  CHECK_SIZE = 10;
  SPACING    =  2;

function checkerboard_expose (da     : PGtkWidget;
                              event  : PGdkEventButton;
                              data   : gpointer): gboolean; cdecl;
var
  i, j,
  xcount, ycount : gint;
  gc1, gc2, gc   : PGdkGc;
  color          : TGdkColor;

begin
  (* At the start of an expose handler, a clip region of event->area
   * is set on the window, and event->area has been cleared to the
   * widget's background color. The docs for
   * gdk_window_begin_paint_region() give more details on how this
   * works.
   *)

  (* It would be a bit more efficient to keep these
   * GC's around instead of recreating on each expose, but
   * this is the lazy/slow way.
   *)
  gc1 := gdk_gc_new (da^.window);
  color.red   := $7530;
  color.green := $0;
  color.blue  := $7530;

  gdk_gc_set_rgb_fg_color (gc1, @color);

  gc2 := gdk_gc_new (da^.window);
  color.red   := $ffff;
  color.green := $ffff;
  color.blue  := $ffff;

  gdk_gc_set_rgb_fg_color (gc2, @color);

  xcount := 0;
  i := SPACING;
  while i < da^.allocation.width do
  begin
    j := SPACING;
    ycount := xcount mod 2; (* start with even/odd depending on row *)
    while j < da^.allocation.height do
    begin
          if (ycount mod 2) <> 0 then
            gc := gc1
          else
            gc := gc2;

          (* If we're outside event->area, this will do nothing.
           * It might be mildly more efficient if we handled
           * the clipping ourselves, but again we're feeling lazy.
           *)
          gdk_draw_rectangle (da^.window,
                              gc,
                              gint(gTRUE),
                              i, j,
                              CHECK_SIZE,
                              CHECK_SIZE);

          j := j + CHECK_SIZE + SPACING;
          inc (ycount);
    end;

      i := i + CHECK_SIZE + SPACING;
      inc (xcount);
  end;

  g_object_unref (G_OBJECT (gc1));
  g_object_unref (G_OBJECT (gc2));

  (* return TRUE because we've handled this event, so no
   * further processing is required.
   *)
  checkerboard_expose := TRUE;
end;

function do_drawingarea    : PGtkWidget;
var
  frame,
  vbox,
  da,
  thelabel  : PGtkWidget;

begin
  if da_window = NULL then
  begin
    da_window := gtk_window_new (GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title (GTK_WINDOW (da_window), 'Drawing Area');

    g_signal_connect (da_window, 'destroy', G_CALLBACK (@gtk_widget_destroyed), @da_window);

    gtk_container_set_border_width (GTK_CONTAINER (da_window), 8);

    vbox := gtk_vbox_new (FALSE, 8);
    gtk_container_set_border_width (GTK_CONTAINER (vbox), 8);
    gtk_container_add (GTK_CONTAINER (da_window), vbox);

    (*
     * Create the checkerboard area
     *)

    thelabel := gtk_label_new (NULL);
    gtk_label_set_markup (GTK_LABEL (thelabel),
                          '<u>Checkerboard pattern</u>');
    gtk_box_pack_start (GTK_BOX (vbox), thelabel, FALSE, FALSE, 0);

    frame := gtk_frame_new (NULL);
    gtk_frame_set_shadow_type (GTK_FRAME (frame), GTK_SHADOW_IN);
    gtk_box_pack_start (GTK_BOX (vbox), frame, TRUE, TRUE, 0);

    da := gtk_drawing_area_new ();
    (* set a minimum size *)
    gtk_widget_set_size_request (da, 100, 100);

    gtk_container_add (GTK_CONTAINER (frame), da);

    g_signal_connect (da, 'expose_event',
                      G_CALLBACK (@checkerboard_expose), NULL);

    (*
     * Create the scribble area
     *)

    thelabel := gtk_label_new (NULL);
    gtk_label_set_markup (GTK_LABEL (thelabel),
                          '<u>Scribble area</u>');
    gtk_box_pack_start (GTK_BOX (vbox), thelabel, FALSE, FALSE, 0);

    frame := gtk_frame_new (NULL);
    gtk_frame_set_shadow_type (GTK_FRAME (frame), GTK_SHADOW_IN);
    gtk_box_pack_start (GTK_BOX (vbox), frame, TRUE, TRUE, 0);

    da := gtk_drawing_area_new ();
    (* set a minimum size *)
    gtk_widget_set_size_request (da, 100, 100);

    gtk_container_add (GTK_CONTAINER (frame), da);

    (* Signals used to handle backing pixmap *)

    g_signal_connect (da, 'expose_event',
                      G_CALLBACK (@scribble_expose_event), NULL);
    g_signal_connect (da,'configure_event',
                      G_CALLBACK (@scribble_configure_event), NULL);

    (* Event signals *)

    g_signal_connect (da, 'motion_notify_event',
                        G_CALLBACK (@scribble_motion_notify_event), NULL);
    g_signal_connect (da, 'button_press_event',
                        G_CALLBACK (@scribble_button_press_event), NULL);


    (* Ask to receive events the drawing area doesn't normally
     * subscribe to
     *)
      gtk_widget_set_events (da, gtk_widget_get_events (da)
                             or GDK_LEAVE_NOTIFY_MASK
                             or GDK_BUTTON_PRESS_MASK
                             or GDK_POINTER_MOTION_MASK
                             or GDK_POINTER_MOTION_HINT_MASK);

  end;

  if not GTK_WIDGET_VISIBLE (da_window) then
    gtk_widget_show_all (da_window)
  else begin
    gtk_widget_destroy (da_window);
    da_window := NULL;
  end;

  do_drawingarea := da_window;
end;