Question

Intro

I am new to GTK+ programming and am having difficulty understanding why the following does not block.

if self.journal:
    # Pump GTK messages.
    while gtk.events_pending(): gtk.main_iteration()

According to the PyGtk reference on gtk.main_iteration(), the default value for block is True, which should, seemingly, prevent the local code from running until events have been processed.

Yet, I don’t see this happening. Perhaps, a clue is in the # Pump GTK messages. comment. But I don’t understand what the comment is trying to convey to me.

My Question

So, my question is What does it mean to “Pump GTK messages?” Please be as low-level as possible. I understand that GTK is my widget toolkit and is designed to handle events.

Background

I am following a Sugar tutorial at flossmanuals using the SimCom activity. This is the section of code (to provide context) from which the snippet above is pulled.

#!/usr/bin/python
# SimCom.py
"""
    Copyright (C) 2011  Peter Hewitt

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

"""
import g,pygame,utils,sys,buttons,slider,load_save
try:
    import gtk
except:
    pass
import sim

class SimCom:

    def __init__(self):
        self.journal=True # set to False if we come in via main()
        self.canvas=None # set to the pygame canvas if we come in via activity.py
.
.
.

    def run(self):
        g.init()
        if not self.journal: utils.load()
        self.sim=sim.Sim()
        load_save.retrieve()
        self.buttons_setup()
        if g.saved_n==0: buttons.off('cyan')
        self.slider=slider.Slider(g.sx(23.4),g.sy(20.2),5,utils.GREEN)
        if self.canvas<>None: self.canvas.grab_focus()
        ctrl=False
        pygame.key.set_repeat(600,120); key_ms=pygame.time.get_ticks()
        going=True
        while going:
            if self.journal:
                # Pump GTK messages.
                while gtk.events_pending(): gtk.main_iteration()

            # Pump PyGame messages.
            for event in pygame.event.get():
                if event.type==pygame.QUIT:
                    if not self.journal: utils.save()
                    going=False
                elif event.type == pygame.MOUSEMOTION:
                    g.pos=event.pos
                    g.redraw=True
                    if self.canvas<>None: self.canvas.grab_focus()
                elif event.type == pygame.MOUSEBUTTONDOWN:
                    g.redraw=True
                    if g.help_on: g.help_on=False 
                    elif event.button==1:
                        if self.do_click():
                            pass
                        elif self.slider.mouse():
                            pass # level changed
                        else:
                            bu=buttons.check()
                            if bu!='': self.do_button(bu); self.flush_queue()
                    elif event.button==3:
                        self.right_click()
                elif event.type == pygame.KEYDOWN:
                    # throttle keyboard repeat
                    if pygame.time.get_ticks()-key_ms>110:
                        key_ms=pygame.time.get_ticks()
                        if ctrl:
                            if event.key==pygame.K_q:
                                if not self.journal: utils.save()
                                going=False; break
                            else:
                                ctrl=False
                        if event.key in (pygame.K_LCTRL,pygame.K_RCTRL):
                            ctrl=True; break
                        self.do_key(event.key); g.redraw=True
                        self.flush_queue()
                elif event.type == pygame.KEYUP:
                    ctrl=False
            if not going: break
            self.update()
            if g.redraw:
                self.display()
                if g.version_display: utils.version_display()
                g.screen.blit(g.pointer,g.pos)
                pygame.display.flip()
                g.redraw=False
            g.clock.tick(40)

if __name__=="__main__":
    pygame.init()
    pygame.display.set_mode()
    game=SimCom()
    game.journal=False
    game.run()
    pygame.display.quit()
    pygame.quit()
    sys.exit(0)

Answer 1

According to the PyGtk reference on gtk.main_iteration(), the default value for block is True,
which should, seemingly, prevent the local code from running until events have been processed.

Right, But the code:

while gtk.events_pending(): gtk.main_iteration()

first checks if there are any events, and only runs gtk.main_iteration() if there are. So there’s always at least one event to process, which means it will never block.

gtk.main_iteration() does one iteration of the gtk main loop, which will process and handle the pending events (mouse events, screen redraw requests, key input, audio input/output, whatever gtk needs to do to drive the UI and handle requests from the operating system/graphics interface(The X11 server in case of *nix)). Often such events results in dispatching callbacks that you, or a library like pygame, have registred.