Detecting NSKeyUp of the Shift key - macos

Question

I am using this to detect keystrokes on my app...
[NSEvent addLocalMonitorForEventsMatchingMask:NSEventMaskKeyDown
handler:^NSEvent * (NSEvent * theEvent)
OK I can use theEvent to know what characters were typed and know if a shift was pressed using this:
NSString *typedKey = theEvent.charactersIgnoringModifiers;
BOOL shiftDetected = [theEvent modifierFlags] & NSShiftKeyMask;
My App has an interface displayed with some buttons and I am allowing the keyboard to be used instead of clicking on the buttons. This interface has 3 buttons in particular that has a second function.
For example: the first button has 2 functions, A and B but just the A label is displayed on that button. Lets say I specify that the letter Q is the keyboard shortcut for that button. If the user presses Q function A is executed. If the user presses Shift Q then function B is executed.
But this is the problem. I need to detect all presses or releases of the Shift, because the moment the user presses Shift I have to change the label of that button from A to B, so the user knows that now that button will lead to the execution of function B instead of A. Like a keyboard that will change from lowercase to uppercase while Shift is being hold and change back to lowercase the moment Shift is released.
How do I do that?

Answer 1

I created a simple project using addLocalMonitorForEvents function. Please check my code, it is Swift code but I think it should be as same as objective c.
func applicationDidFinishLaunching(_ aNotification: Notification) {
// Insert code here to initialize your application
NSEvent.addLocalMonitorForEvents(matching: [.flagsChanged, .keyDown]) { (theEvent) -> NSEvent? in
if theEvent.modifierFlags.contains(.shift) {
if theEvent.keyCode == 56 { // this is Shif key
print("Shift START!")
}
else {
print("Shift pressed with keycode \(theEvent.keyCode)")
}
}
else {
if theEvent.keyCode == 56 { // this is Shif key
print("Shift END!")
}
else {
print("Normal keycode \(theEvent.keyCode)")
}
}
return theEvent
}
}
This is Objective c:
[NSEvent addLocalMonitorForEventsMatchingMask:NSEventMaskFlagsChanged|NSEventMaskKeyDown handler:^NSEvent * (NSEvent * theEvent) {
if ([theEvent modifierFlags] & NSEventModifierFlagShift) {
if (theEvent.keyCode == 56) { // this is Shif key
NSLog(#"Shift START");
}
else {
NSLog(#"Shift pressed with keycode %d", theEvent.keyCode);
}
}
else {
if (theEvent.keyCode == 56) { // this is Shif key
NSLog(#"Shift END");
}
else {
NSLog(#"Normal keycode %d", theEvent.keyCode);
}
}
return theEvent;
}];
Just copy and paste this part to your AppDelegate for quick testing.