C++ Global Hotkeys with platform APIs

Question

I'm working on an application for taking screenshots on Windows, OSX and Linux in C++/Qt. Now I need to set global hotkeys, so the user can take screenshots when the application is running in the background. I tried with Qxt and UGlobalHotkey, which are both Qt libraries, but neither of them seemed to work.

I tried to implement it for OSX with Carbon (tutorial), but I need to call a class member function, which just doesn't work. Could someone provide me with an example? You can find my code here. The function i need to call is new_screenshot().

Or is there any other way to achieve something like this? I really need my application to take a screenshot from the background, otherwise it's pretty useless (yes, I should probably have implemented it at the very beginning to see if it even works). Would it maybe be better to have a separate client for every platform (Cocoa Swift for OSX, GTK for Linux, C# client for Windows)? I have often thought about this the past few days.

Answer 1

Do I understand correctly that you want to call new_screenshot from the hot key event handler? If so, InstallApplicationEventHandler lets you pass a pointer to user data in 4th argument. Pass a pointer to your MainWindow instance (based on code from the tutorial):

MainWindow *mainWindow = ... // get main window somehow
InstallApplicationEventHandler(&MyHotKeyHandler,1,&eventType,mainWindow,NULL);

Then you can use it in the event handler.

OSStatus MyHotKeyHandler(EventHandlerCallRef nextHandler,EventRef theEvent, void *userData)
{
//Do something once the key is pressed
static_cast<MainWindow*>(userData)->new_screenshot();
return noErr;
}

Answer 2

I did something in the past with MFC and WIN32 API....so it only works on Windows...but pressing ALT+F10 was able to hide/show a window...

void CWinHideDlg::OnButtonActive() 
{
    CString tmp;
    GetDlgItemText(IDC_BUTTON_ACTIVE,tmp);
    if(0 == strcmp(tmp.GetBuffer(tmp.GetLength()),"Activate"))
    {
        m_myAtom=GlobalAddAtom("MY_GLOBAL_HOT_HIDE_KEY");
        int err=RegisterHotKey(this->GetSafeHwnd(),m_myAtom,MOD_ALT,VK_F10);
        SetDlgItemText(IDC_BUTTON_ACTIVE,"Stop");
        CButton *pBtn = (CButton *)GetDlgItem(IDC_BUTTON_UNHIDE);
        pBtn->EnableWindow(TRUE);
        SetDlgItemText(IDC_STATIC_INFO,"Set the mouse over the window \nand press ALT + F10 to hide it...");
    }
    else
    {
        UnregisterHotKey(this->GetSafeHwnd(),m_myAtom);
        GlobalDeleteAtom(m_myAtom);     
        CButton *pBtn = (CButton *)GetDlgItem(IDC_BUTTON_UNHIDE);
        pBtn->EnableWindow(FALSE);
        SetDlgItemText(IDC_BUTTON_ACTIVE,"Activate");

    }   
}

Basically this code activates/deactivates the hot key ALT+F10, once it activates you can hide/unhide a running window on the system by setting the mouse pointer over the window and press ALT+F10...

This is from the WindowProc function:

if(message == WM_HOTKEY)
    {
        CString tmp;
        POINT pc;
        GetCursorPos(&pc);


        if(GetAsyncKeyState(VK_F10))
        {
            HWND hwnd=::WindowFromPoint(pc);            
            if(hwnd)
            {
                tmp.Format("%08Xh",hwnd);
                m_HideWins.InsertString(m_HideWins.GetCount(),tmp);
                ::ShowWindow(hwnd,SW_HIDE);
            }
        }
    } 

You can use the code to register your own HOT Key and use it to take a screenshot...

Hope it helps...