Is there Windows system event on active window changed?

The desktop application I'm d开发者_运维百科eveloping 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.

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 */
}

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