How to detect when a user switches between windows (in WinXP)?

[Tim Golden]

Tim Roberts wrote:
> mangoldproject at gmail.com wrote:
>> I'm a beginning-to-intermediate Python programmer with some experience
>> in other languages. At the moment I am trying to write a Python
>> program that will run in the background and execute a series of
>> commands whenever I switch between windows (my OS is Windows XP). For
>> example, I want my program to do something when I switch to my Firefox
>> browser, and then do something else when I switch to a certain sub-
>> window in Photoshop (which has a certain title, and a certain photo
>> associated with it), then do yet another thing when I switch to
>> another sub-window (with a different title and photo) within
>> Photoshop.
>> The particular actions will be based on the window's title. I've
>> already figured out how to retrieve that, using
>> GetWindowText(GetForegroundWindow()) from the win32gui module.
>> My question is, how can I detect when the user switches between
>> windows? I haven't been able to figure that part out yet.
>> Searching the group didn't give me any answers.
> 
> The way you do this is to write a Windows hook.  The WH_CBT hook intercepts
> WM_ACTIVATE and WM_DEACTIVATE calls system-wide.
> 
> However, that requires injecting the hook DLL into every process with a
> Windows, and you certainly don't want to do that in Python.  Write a
> minimal C DLL to be the hook, and have it send messages to your Python
> process.

Also, aside from the technicalities of how you do this, be sure
you've thought through the effect on user-interaction (if the
user isn't you). Switching between windows is one of those things
a user has a very definite expectation about. If you subvert that
by doing "magic things" when you switch between this and that
window then you might end up alienating the user.

But, as Tim points out, system hooks are the way to go.

TJG