How to Intercept and Monitor Volume and Screen Brightness Function Keys on macOS?

Question

I am currently using CGEventTapCreate on macOS to intercept certain key operations and event responses. As I am developing a remote desktop tool, I need to capture all key presses, including those of the Fn extended function keys (volume, brightness, etc.).

My current challenge lies in accurately capturing the state of Fn extended function keys (volume, brightness, etc.). For instance, when I press the volume increase key, I should be able to receive an event indicating "Volume Increase key pressed", and correspondingly, I should receive an event when the key is released. However, my current code is unable to distinguish the state of the key.

    CGEventMask eventMask = CGEventMaskBit(NX_APPDEFINED) | \
                        CGEventMaskBit(NX_KITDEFINED) | \
                        CGEventMaskBit(NX_SYSDEFINED) | \
                        CGEventMaskBit(kCGEventKeyDown) | \
                        CGEventMaskBit(kCGEventKeyUp) | \
                        CGEventMaskBit(kCGEventFlagsChanged);

CFMachPortRef eventTap = CGEventTapCreate(kCGSessionEventTap, kCGHeadInsertEventTap, kCGEventTapOptionDefault, eventMask,
    [](CGEventTapProxy proxy, CGEventType type, CGEventRef pEventRef, void *refcon) -> CGEventRef {
        CGKeyCode keyCode = (CGKeyCode)CGEventGetIntegerValueField(pEventRef, kCGKeyboardEventKeycode);
        CGEventFlags flags = CGEventGetFlags(pEventRef);
        int64_t SourceStateID = CGEventGetIntegerValueField(pEventRef, kCGEventSourceStateID);
        int64_t Autorepeat = CGEventGetIntegerValueField(pEventRef, kCGKeyboardEventAutorepeat);
        int64_t KeyboardType = CGEventGetIntegerValueField(pEventRef, kCGKeyboardEventKeyboardType);
        int64_t UserData = CGEventGetIntegerValueField(pEventRef, kCGEventSourceUserData);
        int64_t SourceUnixProcessID = CGEventGetIntegerValueField(pEventRef, kCGEventSourceUnixProcessID);
        int64_t TargetUnixProcessID = CGEventGetIntegerValueField(pEventRef, kCGEventTargetUnixProcessID);

        qInfo() << "CGEvent type=" << type \
                << "flags=" << QString::number(flags, 16) \
                << "keyCode=" << QString::number(keyCode, 16) \
                << "Autorepeat=" << Autorepeat \
                << "KeyboardType=" << KeyboardType \
                << "UserData=" << UserData \
                << "SourceUnixProcessID=" << SourceUnixProcessID \
                << "TargetUnixProcessID=" << TargetUnixProcessID \
                << "SourceStateID=" << SourceStateID;

        return NULL;
    }, nullptr);

The following image displays the results obtained on my test machine. I pressed eight different keys by combining the Fn key with (F1, F2, F7~F12) individually：