How check for Command-Period in a tight loop? - cocoa

Question

I'm implementing a scripting language where the user might be causing an endless loop by accident. I want to give the user the opportunity to cancel such a runaway loop by holding down the command key while typing the period (".") key.
Currently, once for every line, I check for cancellation with this code:
NSEvent * evt = [[NSApplication sharedApplication] nextEventMatchingMask: NSKeyDownMask untilDate: [NSDate date] inMode: WILDScriptExecutionEventLoopMode dequeue: YES];
if( evt )
{
NSString * theKeys = [evt charactersIgnoringModifiers];
if( (evt.modifierFlags & NSCommandKeyMask) && theKeys.length > 0 && [theKeys characterAtIndex: 0] == '.' )
{
// +++ cancel script execution here.
}
}
The problem with this is that it eats any keyboard events that the user might be typing while the script is running, even though scripts should be able to check for keypresses. Also, it doesn't dequeue the corresponding NSKeyUp events. But if I tell it to dequeue key up events as well, it might dequeue the keyUp for a keypress that was being held before my script started and my application might never find out the key was released.
Also, I would like to not dequeue any events until I know it is actually a cancel event, but there is no separate dequeue call, and it feels unreliable to just assume the frontmost event on a second call will be the same one. And even if it is guaranteed to be the first, that would mean that the user typing an 'a' and then Cmd-. would mean I only ever see the 'a' and never the Cmd-. behind it if I don't dequeue events.
Is there a better option than going to the old Carbon stand-by GetKeys()? Fortunately, that seems to be available in 64 bit.
Also, I'm thinking about adding an NSStatusItem that adds a button to cancel the script to the menu bar or so. But how would I process events in a way that doesn't let the user e.g. select a menu while a script expects to be ruler of the main thread?
Any suggestions? Recommendations?

Answer 1

Using -addLocalMonitorForEventsMatchingMask: as Dave suggests is probably the easiest way to go about this, yes.
I just wanted to add that despite your unreliable feeling, the event queue is really a queue, and events don't change order. It is perfectly safe (and standard practice in event loops) to call -nextEventMatchingMask:inMode:dequeue:NO, examine the event, determine that it is one you want to deal with, and then call -nextEventMatchingMask:inMode:dequeue:YES in order to consume it. Just make sure that your mask and mode are identical between the two calls.

Answer 2

I would suggest using an event monitor. Since you're asking NSApp for events, it would seem that you're running the script in the current process, so you only have to monitor events in your own process (and not globally).
There are several ways to do this (subclassing NSApplication and overriding -sendEvent:, putting in an event tap, etc), but the easiest way to do this would be with a local event monitor:
id eventHandler = [NSEvent addLocalMonitorForEventsMatchingMask:NSKeyDown handler:^(NSEvent *event) {
// check the runloop mode
// check for cmd-.
// abort the script if necessary
return event;
}];
When you're all done monitoring for events, don't forget to unregister your monitor:
[NSEvent removeMonitor:eventHandler];

Answer 3

So there's +[NSEvent modifierFlags] which is intended as a replacement for GetKeys(). It doesn't cover your use case of the period key though, sadly.
The core problem here with the event queue is you want to be able to search it, which isn't something the API exposes. The only workaround to that I can think of is to dequeue all events into an array, checking for a Command-. event, and then re-queue them all using postEvent:atStart:. Not pretty.
Perhaps as an optimisation you could use +[NSEvent modifierFlags] to only check the event queue when the command key is held down, but that sounds open to race conditions to me.
So final suggestion, override -postEvent:atStart: (on either NSApplication or NSWindow) and see if you can fish out the desired info there. I think at worst it could be interesting for debugging.