On Key Bindings

For my R GUI I want user customizable key bindings. I know for a fact that there are customizable key bindings built into the Cocoa text system, but they aren't user accessible and the only response from Apple was a message posted to a mailing list about 2 years ago to the effect of "I have something." Two years later, well, surprise, surprise. So what to do?

Well, for the first pass (found in the R-Multi git repository) I class-dumped AppKit and found NSKeyBindingManager. It turns out that this class has a shared initializer and a means for setting a new key binding dictionary. So, I did the simple thing and just changed that dictionary during app initialization to support some new keybindings similar to the ones found in TextMate (which doesn't use NSTextView as far as I know):

NSMutableDictionary *newBindings = [[NSMutableDictionary alloc] 
    initWithDictionary:origBindings];
[[NSKeyBindingManager sharedKeyBindingManager] setDictionary:newBindings];
[newBindings release];

About as simple as you can possibly get. Unfortunately, this approach has several drawbacks primarily due to the fact that it is a global change to the entire application (which is more useful that the SYSTEM global version mostly recommended). Can we do better? Let's find out.

For a clue, let's look at one of the R GUI's own crash traces:

[NSTextView keyDown:]
...[NSView interpretKeyEvents:]
......[NSTSMInputContext interpretKeyEvents:]
.........[NSKeyBindingManager(NSKeyBindingManager_MultiClients) flushTextForClient:]

Excellent. So it looks like we'll be needing to subclass and hook into interpretKeyEvents: to get things to work properly in our system. However, we cannot simply use an NSKeyBindingManager because the manager doesn't return a BOOL so we can't tell when we haven't handled a binding. Or does it?

So, it turns out there is a MultiClient category on NSKeyBindingManager that lets us do just that using interpretEventAsCommand:forClient:

So, my interpretKeyEvents now looks like:

- (void)interpretKeyEvents:(id)sender {
 if([(NSArray*)sender count] == 1) {
  if(YES == [manager interpretEventAsCommand:[(NSArray*)sender objectAtIndex:0] 
                   forClient:self]) return;
 }
 [super interpretKeyEvents:sender];
}

So far it seems to work in testing, but I'll have to do more testing with it over time. For example, I don't know if I'll ever see more than one event in that array.