Check modifierFlags of NSEvent if a certain modifier was pressed but no other

Question

I just experimented with the addLocalMonitorForEventsMatchingMask:handler: method in NSEvent and came across the following question: How do I find out if only certain modifiers were pressed?

A short example to set this question into context: I wanted to listen for the shortcut "⌘+W". Therefore I wrote the following code:

[NSEvent addLocalMonitorForEventsMatchingMask:NSKeyDownMask handler:^(NSEvent *theEvent) {
    if ([theEvent modifierFlags] & NSCommandKeyMask && [theEvent keyCode] == 13) {
        [self.window performClose:self];
    }
    return theEvent;
}];

This works well, however the shortcut will be triggered, even if more modifier keys are pressed, e.g. "⌃+⌘+W" or "⇧+⌃+⌥+⌘+W". Is there a way to circumvent this?

A simple solution would be to check for all other modifier keys and ensure they are not pressed. This seems tedious and error prone - besides it's ugly enough as it is now with the unary "&". In addition you may get into trouble if (for some reason) another modifier key is added to keyboard layouts.

As always I'm thankful for any recommendations.

Answer 1

I think this'll do it:

// Mask out everything but the key flags
NSUInteger flags = [theEvent modifierFlags] & NSEventModifierFlagDeviceIndependentFlagsMask;
if( flags == NSCommandKeyMask ){
    // Got it!
}

Hat tip to SpaceDog for pointing out the deprecation of the original mask name, NSDeviceIndependentModifierFlagsMask.

Answer 2

Swift 5 version

if event.modifierFlags.intersection(.deviceIndependentFlagsMask) == .command {
    // Got it!
}