DrawingArea

The DrawingArea widget is used for creating custom user interface elements.

It’s essentially a blank widget; you can draw on it. After creating a drawing area, the application may want to connect to:

• Mouse and button press signals to respond to input from the user. (Use add_events to enable events you wish to receive.)
• The realize signal to take any necessary actions when the widget is instantiated on a particular display. (Create GDK resources in response to this signal.)
• The size_allocate signal to take any necessary actions when the widget changes size.
• The draw signal to handle redrawing the contents of the widget.

The following code portion demonstrates using a drawing area to display a circle in the normal widget foreground color.

Note that GDK automatically clears the exposed area before sending the expose event, and that drawing is implicitly clipped to the exposed area. If you want to have a theme-provided background, you need to call render_background in your draw method.

Simple GtkDrawingArea usage

gboolean
draw_callback (GtkWidget *widget, cairo_t *cr, gpointer data)
{
  guint width, height;
  GdkRGBA color;
  GtkStyleContext *context;

  context = gtk_widget_get_style_context (widget);

  width = gtk_widget_get_allocated_width (widget);
  height = gtk_widget_get_allocated_height (widget);

  gtk_render_background (context, cr, 0, 0, width, height);

  cairo_arc (cr,
             width / 2.0, height / 2.0,
             MIN (width, height) / 2.0,
             0, 2 * G_PI);

  gtk_style_context_get_color (context,
                               gtk_style_context_get_state (context),
                               &color);
  gdk_cairo_set_source_rgba (cr, &color);

  cairo_fill (cr);

 return FALSE;
}
[...]
  GtkWidget *drawing_area = gtk_drawing_area_new ();
  gtk_widget_set_size_request (drawing_area, 100, 100);
  g_signal_connect (G_OBJECT (drawing_area), "draw",
                    G_CALLBACK (draw_callback), NULL);

Draw signals are normally delivered when a drawing area first comes onscreen, or when it’s covered by another window and then uncovered. You can also force an expose event by adding to the “damage region” of the drawing area’s window; queue_draw_area and invalidate_rect are equally good ways to do this. You’ll then get a draw signal for the invalid region.

The available routines for drawing are documented on the GDK Drawing Primitives page and the cairo documentation.

To receive mouse events on a drawing area, you will need to enable them with add_events. To receive keyboard events, you will need to set the “can-focus” property on the drawing area, and you should probably draw some user-visible indication that the drawing area is focused. Use gtk_widget_has_focus in your expose event handler to decide whether to draw the focus indicator. See render_focus for one way to draw focus.

Example: DrawingArea:

public class Application : Gtk.Window {
	public Application () {
		// Prepare Gtk.Window:
		this.title = "My Gtk.DrawingArea";
		this.window_position = Gtk.WindowPosition.CENTER;
		this.destroy.connect (Gtk.main_quit);
		this.set_default_size (400, 400);

		// The drawing area:
		Gtk.DrawingArea drawing_area = new Gtk.DrawingArea ();
		drawing_area.draw.connect ((context) => {
			// Get necessary data:
			weak Gtk.StyleContext style_context = drawing_area.get_style_context ();
			int height = drawing_area.get_allocated_height ();
			int width = drawing_area.get_allocated_width ();
			Gdk.RGBA color = style_context.get_color (0);

			// Draw an arc:
			double xc = width / 2.0;
			double yc = height / 2.0;
			double radius = int.min (width, height) / 2.0;
			double angle1 = 0;
			double angle2 = 2*Math.PI;

			context.arc (xc, yc, radius, angle1, angle2);
			Gdk.cairo_set_source_rgba (context, color);
			context.fill ();
			return true;
		});
		this.add (drawing_area);
	}

	public static int main (string[] args) {
		Gtk.init (ref args);

		Application app = new Application ();
		app.show_all ();
		Gtk.main ();
		return 0;
	}
}

valac --pkg gtk+-3.0 Gtk.DrawingArea.vala