CODEWITHSUNDEEP
c++windowswinapi
twig
twig

Reputation: 4184

Windows/C++: detect when focus has changed between windows (globally)

I'm trying to find a way to detect when focus is changed to another window (without having to poll every X ms).

I've already figured out a way to detect when focus is switched between applications using WH_SHELL and HSHELL_ACTIVATESHELLWINDOW.

The problem is I want to detect when focus is switched between dialog/windows within the same app.

ie. In Notepad, I can determine when the app switches to Notepad, but I cannot detect when the "Open" or "Save" dialogs appear because the focus is still within the same application.

Upvotes: 1

Views: 4789

Answers (3)

twig
twig

Reputation: 4184

Sorry for the delay, I don't have VS installed on this computer so it's a bit hard to find the code.

Use this to hook your code into the system.

HHOOK oldShellHook = SetWindowsHookEx(WH_SHELL, BCK_WndShellProc, hDll, NULL);

if (!info->oldShellHook) {
    MessageBox(m_hwnd, L"Failed to load global hook.", strTitle, MB_OK | MB_ICONERROR);
    return;
}

And this your hook. Depending on what you're doing, use nMsg to figure out when you want to apply your custom logic.

LRESULT CALLBACK BCK_WndShellProc(int nMsg, WPARAM wParam, LPARAM lParam) {
    ...
}

Read the MSDN docs for SetWindowsHookEx() to make sure you return the expected values, otherwise you can lock up the whole system.

I use code similar to this on my Breadcrumb Killer and Spasm (Show all programs on Start menu) programs and it seems to work fine.

Upvotes: 1

Alex K.
Alex K.

Reputation: 175748

How about the "Computer Based Training API"; SetWindowsHookEx with WH_CBT which will enable you to receive HCBT_SETFOCUS (among others)

Upvotes: 0

Phil Devaney
Phil Devaney

Reputation: 17997

You can use SetWindowsHookEx with a WH_CBT hook type. If you just want to detect focus changes within an application, pass GetCurrentThreadId() as the last parameter, otherwise the hook will be for all threads on the current desktop.

Note that using windows hooks can have an adverse effect on system performance, so the hook should only be installed when necessary and you should do a minimum amount of work in the hook procedure.

Upvotes: 3

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