How does Apple update the Airport menu while it is open? (How to change NSMenu when it is already open)

Menu mouse tracking is done in a special run loop mode (NSEventTrackingRunLoopMode). In order to modify the menu, you need to dispatch a message so that it will be processed in the event tracking mode. The easiest way to do this is to use this method of NSRunLoop:

[[NSRunLoop currentRunLoop] performSelector:@selector(updateTheMenu:) target:self argument:yourMenu order:0 modes:[NSArray arrayWithObject:NSEventTrackingRunLoopMode]]

You can also specify the mode as NSRunLoopCommonModes and the message will be sent during any of the common run loop modes, including NSEventTrackingRunLoopMode.

Your update method would then do something like this:

- (void)updateTheMenu:(NSMenu*)menu
{
    [menu addItemWithTitle:@"Foobar" action:NULL keyEquivalent:@""];
    [menu update];
}

(If you want to change the layout of the menu, similar to how the Airport menu shows more info when you option click it, then keep reading. If you want to do something entirely different, then this answer may not be as relevant as you'd like.)

The key is -[NSMenuItem setAlternate:]. For an example, let's say we're going to build an NSMenu that has a Do something... action in it. You'd code that up as something like:

NSMenu * m = [[NSMenu alloc] init];

NSMenuItem * doSomethingPrompt = [m addItemWithTitle:@"Do something..." action:@selector(doSomethingPrompt:) keyEquivalent:@"d"];
[doSomethingPrompt setTarget:self];
[doSomethingPrompt setKeyEquivalentModifierMask:NSShiftKeyMask];

NSMenuItem * doSomething = [m addItemWithTitle:@"Do something" action:@selector(doSomething:) keyEquivalent:@"d"];
[doSomething setTarget:self];
[doSomething setKeyEquivalentModifierMask:(NSShiftKeyMask | NSAlternateKeyMask)];
[doSomething setAlternate:YES];

//do something with m

Now, you'd think that that would create a menu with two items in it: "Do something..." and "Do something", and you'd be partly right. Because we set the second menu item to be an alternate, and because both menu items have the same key equivalent (but different modifier masks), then only the first one (ie, the one that is by default setAlternate:NO) will show. Then when you have the menu open, if you press the modifier mask that represents the second one (ie, the option key), then the menu item will transform in real time from the first menu item to the second.

This, for example, is how the Apple menu works. If you click once on it, you'll see a few options with ellipses after them, such as "Restart..." and "Shutdown...". The HIG specifies that if there's an ellipsis, it means that the system will prompt the user for confirmation before executing the action. However, if you press the option key (with the menu still open), you'll notice they change to "Restart" and "Shutdown". The ellipses go away, which means that if you select them while the option key is pressed down, they will execute immediately without prompting the user for confirmation.

The same general functionality holds true for the menus in status items. You can have the expanded information be "alternate" items to the regular info that only shows up with the option key is pressed. Once you understand the basic principle, it's actually quite easy to implement without a whole lot of trickery.

The problem here is that you need your callback to get triggered even in menu tracking mode.

For example, -[NSTask waitUntilExit] "polls the current run loop using NSDefaultRunLoopMode until the task completes". This means that it won't get run until after the menu closes. At that point, scheduling updateTheMenu to run on NSCommonRunLoopMode doesn't help—it can't go back in time, after all. I believe that NSNotificationCenter observers also only trigger in NSDefaultRunLoopMode.

If you can find some way to schedule a callback that gets run even in the menu tracking mode, you're set; you can just call updateTheMenu directly from that callback.

- (void)updateTheMenu {
  static BOOL flip = NO;
  NSMenu *filemenu = [[[NSApp mainMenu] itemAtIndex:1] submenu];
  if (flip) {
    [filemenu removeItemAtIndex:[filemenu numberOfItems] - 1];
  } else {
    [filemenu addItemWithTitle:@"Now you see me" action:nil keyEquivalent:@""];
  }
  flip = !flip;
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
  NSTimer *timer = [NSTimer timerWithTimeInterval:0.5
                                           target:self
                                         selector:@selector(updateTheMenu)
                                         userInfo:nil
                                          repeats:YES];
  [[NSRunLoop currentRunLoop] addTimer:timer forMode:NSRunLoopCommonModes];
}

Run this and hold down the File menu, and you'll see the extra menu item appears and disappears every half second. Obviously "every half second" isn't what you're looking for, and NSTimer doesn't understand "when my background task is finished". But there may be some equally simple mechanism that you can use.

If not, you can build it yourself out of one of the NSPort subclasses—e.g., create an NSMessagePort and have your NSTask write to that when it's done.

The only case you're really going to need to explicitly schedule updateTheMenu the way Rob Keniger described above is if you're trying to call it from outside of the run loop. For example, you could spawn a thread that fires off a child process and calls waitpid (which blocks until the process is done), then that thread would have to call performSelector:target:argument:order:modes: instead of calling updateTheMenu directly.