Is there Windows system event on active window changed?

Question

The desktop application I'm developing need to know what windows were active while the application was run. Currently it performs GetForegroundWindow() call (of user32.dll) every 250 msec. The approach is not very accurate.

Is there any Windows (WINAPI?) event which fires every time the active (focused) window changed? I'd like to subscribe with my callback function.

Thanks.

Solution 1

Yes, you can use SetWinEventHook function.

hEvent = SetWinEventHook(EVENT_SYSTEM_FOREGROUND , 
    EVENT_SYSTEM_FOREGROUND , NULL, 
    WinEventProcCallback, 0, 0, 
    WINEVENT_OUTOFCONTEXT | WINEVENT_SKIPOWNPROCESS);

.......

VOID CALLBACK WinEventProcCallback ( HWINEVENTHOOK hWinEventHook, DWORD dwEvent, HWND hwnd, LONG idObject, LONG idChild, DWORD dwEventThread, DWORD dwmsEventTime)
{
    /* your code here */
}

Solution 2

There's the WM_ACTIVATE message, which is sent to the activated and deactivated windows.

Comments

[angularsen]

A very good answer is here: stackoverflow.com/a/11943387/134761

[Vasyl Boroviak]

This is not the one I'm looking for. In is unreasonable to subscribe to every window in the system. :)

[Vasyl Boroviak]

Looks like this is the one I was looking for. Let me check it out.

[Piskvor left the building]

@Vasiliy Borovyak: Ah, never mind then - I have misunderstood, thinking you only need the active state of a specific window.

[DReJ]

I have never tried EVENT_SYSTEM_FOREGROUND, but I hooked EVENT_OBJECT_LOCATIONCHANGE event to handle windows position change with SetWinEventHook and it worked fine for me.

[Chris]

Why is there a C/C++ answer to a C# question?

[pomeroy]

@Chris Knowing the Win32 call allows you to use p/invoke pinvoke.net/default.aspx/user32/SetWinEventHook.html

[Chris Vilches]

Downvote because it's pseudocode. Pretty much nothing, whether it's the method, or those parameters, and not even the "VOID CALLBACK" thing is found by the compiler. Nothing works.

[Chris Vilches]

(And if someone argues that this is C++, then hEvent has no type, so still pseudo-code)

[Werner Erasmus]

@DReJ, I've found that this doesn't work consistently. Sometimes my callback is called, other times not, depending from what to what focus is switched. Perhaps another event...