Python Cookbook

Processing Windows Messages Using MsgWaitForMultipleObjects

Credit: Michael Robin

Problem

In a Win32 application, you need to process messages, but you also want to wait for kernel-level waitable objects and coordinate several activities.

Solution

A Windows application message loop, also known as its message pump, is at the heart of Windows. It’s worth some effort to ensure that the heart beats properly and regularly:

import win32event
import pythoncom

TIMEOUT = 200 # ms

StopEvent = win32event.CreateEvent(None, 0, 0, None)
OtherEvent = win32event.CreateEvent(None, 0, 0, None)

class myCoolApp:
    def OnQuit(self):
          if areYouSure(  ):
              win32event.SetEvent(StopEvent) # Exit msg pump

def _MessagePump( ):
     waitables = StopEvent, OtherEvent
     while 1:
         rc = win32event.MsgWaitForMultipleObjects(
             waitables,
             0, # Wait for all = false, so it waits for anyone
             TIMEOUT, # (or win32event.INFINITE)
             win32event.QS_ALLEVENTS) # Accepts all input # You can call a function here, if it doesn't take too long. It will # be executed at least every 200ms -- possibly a lot more often, # depending on the number of Windows messages received. if rc == win32event.WAIT_OBJECT_0: # Our first event listed, the StopEvent, was triggered, so we must exit break elif rc == win32event.WAIT_OBJECT_0+1: # Our second event listed, "OtherEvent", was set. Do whatever needs # to be done -- you can wait on as many kernel-waitable objects as # needed (events, locks, processes, threads, notifications, and so on). pass elif rc == win32event.WAIT_OBJECT_0+len(waitables): ...