Now that we've covered the basics of drawing with Cairo, let's try to put it all together and create a simple application that actually does something. The following example uses Cairo to create a custom Clock widget. The clock has a second hand, a minute hand, and an hour hand, and updates itself every second.

Source Code

File: clock.h

#ifndef GTKMM_EXAMPLE_CLOCK_H
#define GTKMM_EXAMPLE_CLOCK_H

#include <gtkmm/drawingarea.h>

class Clock : public Gtk::DrawingArea
{
    public:
        Clock();
        virtual ~Clock();

    protected:
        //Override default signal handler:
        virtual bool on_expose_event(GdkEventExpose* event);
        double m_radius;
        double m_lineWidth;
        bool onSecondElapsed(void);
};

#endif // GTKMM_EXAMPLE_CLOCK_H

File: clock.cc

#include <ctime>
#include <cairomm/context.h>
#include "clock.h"

Clock::Clock()
    : m_radius(0.42), m_lineWidth(0.05)
{
    Glib::signal_timeout().connect(
                sigc::mem_fun(*this, &Clock::onSecondElapsed), 1000);
}

Clock::~Clock()
{
}

bool Clock::on_expose_event(GdkEventExpose* event)
{
  // This is where we draw on the window
  Glib::RefPtr<Gdk::Window> window = get_window();
  if(window)
  {
    Gtk::Allocation allocation = get_allocation();
    const int width = allocation.get_width();
    const int height = allocation.get_height();

    Cairo::RefPtr<Cairo::Context> cr = window->create_cairo_context();

    if (event)
    {
        // clip to the area indicated by the expose event so that we only
        // redraw the portion of the window that needs to be redrawn
        cr->rectangle(event->area.x, event->area.y,
                event->area.width, event->area.height);
        cr->clip();
    }

    // scale to unit square and translate (0, 0) to be (0.5, 0.5), i.e.
    // the center of the window
    cr->scale(width, height);
    cr->translate(0.5, 0.5);
    cr->set_line_width(m_lineWidth);

    cr->save();
    cr->set_source_rgba(0.337, 0.612, 0.117, 0.9);   // green
    cr->paint();
    cr->restore();
    cr->arc(0, 0, m_radius, 0, 2 * M_PI);
    cr->save();
    cr->set_source_rgba(1.0, 1.0, 1.0, 0.8);
    cr->fill_preserve();
    cr->restore();
    cr->stroke_preserve();
    cr->clip();

    //clock ticks
    for (int i = 0; i < 12; i++)
    {
        double inset = 0.05;

        cr->save();
        cr->set_line_cap(Cairo::LINE_CAP_ROUND);

        if (i % 3 != 0)
        {
            inset *= 0.8;
            cr->set_line_width(0.03);
        }

        cr->move_to(
                (m_radius - inset) * cos (i * M_PI / 6),
                (m_radius - inset) * sin (i * M_PI / 6));
        cr->line_to (
                m_radius * cos (i * M_PI / 6),
                m_radius * sin (i * M_PI / 6));
        cr->stroke();
        cr->restore(); /* stack-pen-size */
    }

    // store the current time
    time_t rawtime;
    time(&rawtime);
    struct tm * timeinfo = localtime (&rawtime);

    // compute the angles of the indicators of our clock
    double minutes = timeinfo->tm_min * M_PI / 30;
    double hours = timeinfo->tm_hour * M_PI / 6;
    double seconds= timeinfo->tm_sec * M_PI / 30;

    cr->save();
    cr->set_line_cap(Cairo::LINE_CAP_ROUND);

    // draw the seconds hand
    cr->save();
    cr->set_line_width(m_lineWidth / 3);
    cr->set_source_rgba(0.7, 0.7, 0.7, 0.8); // gray
    cr->move_to(0, 0);
    cr->line_to(sin(seconds) * (m_radius * 0.9), 
            -cos(seconds) * (m_radius * 0.9));
    cr->stroke();
    cr->restore();

    // draw the minutes hand
    cr->set_source_rgba(0.117, 0.337, 0.612, 0.9);   // blue
    cr->move_to(0, 0);
    cr->line_to(sin(minutes + seconds / 60) * (m_radius * 0.8),
            -cos(minutes + seconds / 60) * (m_radius * 0.8));
    cr->stroke();

    // draw the hours hand
    cr->set_source_rgba(0.337, 0.612, 0.117, 0.9);   // green
    cr->move_to(0, 0);
    cr->line_to(sin(hours + minutes / 12.0) * (m_radius * 0.5),
            -cos(hours + minutes / 12.0) * (m_radius * 0.5));
    cr->stroke();
    cr->restore();

    // draw a little dot in the middle
    cr->arc(0, 0, m_lineWidth / 3.0, 0, 2 * M_PI);
    cr->fill();

  }

  return true;
}


bool Clock::onSecondElapsed(void)
{
    // force our program to redraw the entire clock.
    Glib::RefPtr<Gdk::Window> win = get_window();
    if (win)
    {
        Gdk::Rectangle r(0, 0, get_allocation().get_width(),
                get_allocation().get_height());
        win->invalidate_rect(r, false);
    }
    return true;
}

File: main.cc

#include "clock.h"
#include <gtkmm/main.h>
#include <gtkmm/window.h>

int main(int argc, char** argv)
{
   Gtk::Main kit(argc, argv);

   Gtk::Window win;
   win.set_title("Cairomm Clock");

   Clock c;
   win.add(c);
   c.show();

   Gtk::Main::run(win);

   return 0;
}

As before, almost all of the interesting stuff is done in the expose event handler on_expose_event(). Before we dig into the expose event handler, notice that the constructor for the Clock widget connects a handler function onSecondElapsed() to a timer with a timeout period of 1000 milliseconds (1 second). This means that onSecondElapsed() will get called once per second. The sole responsibility of this function is to invalidate the window so that gtkmm will be forced to redraw it.

Now let's take a look at the code that performs the actual drawing. The first section of on_expose_event() should be pretty familiar by now as it's mostly 'boilerplate' code for getting the Gdk::Window, creating a Cairo::Context, and clipping to the area that we want to re-draw. This example again scales the coordinate system to be a unit square so that it's easier to draw the clock as a percentage of window size so that it will automatically scale when the window size is adjusted. Furthermore, the coordinate system is scaled over and down so that the (0, 0) coordinate is in the very center of the window.

The function Cairo::Context::paint() is used here to set the background color of the window. This function takes no arguments and fills the current surface (or the clipped portion of the surface) with the source color currently active. After setting the background color of the window, we draw a circle for the clock outline, fill it with white, and then stroke the outline in black. Notice that both of these actions use the _preserve variant to preserve the current path, and then this same path is clipped to make sure than our next lines don't go outside the outline of the clock.

After drawing the outline, we go around the clock and draw ticks for every hour, with a larger tick at 12, 3, 6, and 9. Now we're finally ready to implement the time-keeping functionality of the clock, which simply involves getting the current values for hours, minutes and seconds, and drawing the hands at the correct angles.