How to capture keyboard & mouse events even when the Qt app is minimized / in background / out of focus?

Question

Here the requirement is just to check if any key was pressed or any mouse click or movement has happened. Capturing other specific details is not required.
I am fine to do every 1 second polling as well to see if any mouse or key event happened.

Will QAbstractNativeEventFilter be of any help?
Any other platform independent C++ library also will be useful.

---

Below is a sample code to capture mouse & keyboard events only when the app is in focus:

#include<QApplication>
#include<QDebug>
#include<QKeyEvent>
#include<QWidget>

struct Widget : public QWidget
{
  Widget ()
  {
    installEventFilter(this);
    grabKeyboard();
    grabMouse();
    setMouseTracking(true);
  }
  ~Widget () { qDebug() << "~Event()"; }

  bool eventFilter (QObject* const pObject,
                    QEvent* const pEvent) override
  {
    qDebug() << "Event: " << pEvent->type();
    if(pEvent->type() == QEvent::KeyPress)
    {
      QKeyEvent* const pKeyEvent = static_cast<QKeyEvent*>(pEvent);
      qDebug() << "Key event: " << pKeyEvent->key();
    }
    return false; //QObject::eventFilter(pObject, pEvent);
  }
};


int main (int argc, char *argv[])
{
  QApplication application(argc, argv);
  Widget widget;
  widget.show();
  return application.exec();
}

Answer 1

Have written minimal working code for the 3 platforms below. Using Qt is optional.

#include<iostream>
#include<thread>
#include<chrono>

struct Activity
{
  Activity ();
};

// You may need to put Qt equivalent part here; The core logic is after main()
#include<QGuiApplication>
int
main (int argc, char *argv[])
{  // Qt specific; but it can be anything here
  QGuiApplication application(argc, argv);
  Activity activity;
  return application.exec();
}

#ifdef __linux__  // add in .pro file "linux: QT += x11extras" and "linux: LIBS += -lX11 -lXtst"
#include<X11/Xlib.h>
#include<X11/extensions/record.h>

#define CHECK(EVENT) if(*pDatum == EVENT) std::cout << #EVENT
void Handle (XPointer, XRecordInterceptData *pRecord)
{
  using XRecordDatum = char;
  std::cout << pRecord->category << "---" << pRecord->data;
  if(auto* const pDatum = reinterpret_cast<XRecordDatum*>(pRecord->data))
  { CHECK(KeyPress); else CHECK(KeyRelease); else CHECK(ButtonPress); else CHECK(ButtonRelease); }
  ::XRecordFreeData(pRecord);
}

Activity::Activity ()
{
  if(auto* const pDisplay = XOpenDisplay(nullptr))
  {
    XRecordClientSpec clients = XRecordAllClients;
    auto* pRange = ::XRecordAllocRange();
    pRange->device_events = XRecordRange8{KeyPress, ButtonRelease};
    auto context = ::XRecordCreateContext(pDisplay, 0, &clients, 1, &pRange, 1);
    ::XRecordEnableContextAsync(pDisplay, context, Handle, nullptr); // use with/without `...Async()`

    //  ::XRecordProcessReplies(pDisplay);
    ::XFlush(pDisplay);
    ::XSync(pDisplay, true);
  }
// Use below functions by putting variables in global scope to stop capturing
// Also refer: https://stackoverflow.com/questions/69711608/why-xrecorddisablecontext-is-not-working
// ::XRecordDisableContext(pDisplay, context);
//  ::XRecordFreeContext(pDisplay, context);
//  ::XFree(pRange);
}
#endif

#ifdef WIN32
#include<Windows.h> // add in .pro file "win32: LIBS += -luser32"

HHOOK sHookKeyboard, sHookMouse;
Activity::Activity ()
{
    sHookKeyboard = ::SetWindowsHookExA(WH_KEYBOARD_LL,
                     [] (int i, WPARAM w, LPARAM l)
                     { std::cout << i << w << l; return ::CallNextHookEx(sHookKeyboard, i, w, l); },
                                        0, 0);
    sHookMouse = ::SetWindowsHookExA(WH_MOUSE_LL,
                     [] (int i, WPARAM w, LPARAM l)
                     { std::cout << i << w << l; return ::CallNextHookEx(sHookMouse, i, w, l); },
                                        0, 0);
}
#endif

#ifdef __APPLE__ // add in .pro file "mac: LIBS += -framework ApplicationServices"
#include<ApplicationServices/ApplicationServices.h>
// Add binary (& if you are it via running Qt, then that too) into the "privacy" settings of the ...
// ... Mac system; System Preferences > Privacy settings; Without this the code will not work

// https://developer.apple.com/forums/thread/109283
// https://stackoverflow.com/questions/4556278/cgeventtapcreates-watching-keyboard-input-in-cocoa
Activity::Activity ()
{
  CGEventMask mask = CGEventMaskBit(kCGEventKeyDown) | CGEventMaskBit(kCGEventScrollWheel) |
                     CGEventMaskBit(kCGEventLeftMouseDown) | CGEventMaskBit(kCGEventRightMouseDown);
  auto eventTap = CGEventTapCreate(kCGSessionEventTap, kCGHeadInsertEventTap,
                                   kCGEventTapOptionDefault, mask,
                       [] (CGEventTapProxy, CGEventType type, CGEventRef event, void*)
                       { std::cout << "captured: " << type; return event; }, this);
  if(eventTap == nullptr)
    return;

  CFRunLoopAddSource(CFRunLoopGetCurrent(),
                     CFMachPortCreateRunLoopSource(kCFAllocatorDefault, eventTap, 0),
                     kCFRunLoopCommonModes);
  // Enable the event tap.
  CGEventTapEnable(eventTap, true);
}
#endif