How to react event when window user enter focus into an input area?

I'm going to implementing an On Screen Keyboard for Window which requires the OSK app to show when an user enter an input area. For example, when a us开发者_C百科er open notepad.exe and start clicking on it, my OSK app (currently run in system tray) should be showed.

I have no idea how to catch those event (user click on notepad app) but I guess it relates to the Win32 things. Can anybody give me a hint for this ? Would be highly appreciated.

Thanks

You'll probably have to set a global hook, using e.g. SetWindowsHookEx(), in order to monitor the WM_SETFOCUS messages sent to all the edit controls in all the applications.

That's quite intrusive, and that won't be easy. Good luck.