Question

For example, get the notification that another Application is becoming Active on the screen, or resign active state.

Answer 1

Sure. In your app delegate class, you can use NSWorkspace to get notified when an app becomes active (NSWorkspaceDidActivateApplicationNotification) or resigns active (NSWorkspaceDidDeactivateApplicationNotification). See the documentation on NSWorkspace for more info.

In your controller class, you’d do something like this:

- (id)init {
   if ((self = [super init])) {
       [[[NSWorkspace sharedWorkspace] notificationCenter] addObserver:self
                             selector:@selector(appDidActivate:)
                            name:NSWorkspaceDidActivateApplicationNotification
                              object:nil];
   }
   return self;
}

- (void)dealloc {
    [[[NSWorkspace sharedWorkspace] notificationCenter] removeObserver:self];
    [super dealloc];
}

- (void)appDidActivate:(NSNotification *)notification {
   NSDictionary *userInfo = [notification userInfo];
   NSLog(@"userInfo == %@", userInfo);

}

The key points are basically that you need to register to receive the notifications like shown in -init. You’d repeat the code to add another observer for each additional notification name that you want (e.g NSWorkspaceDidDeactivateApplicationNotification).

Another important thing to remember is to remove yourself as an observer in -dealloc (or elsewhere), so that NSWorkspace doesn’t try to notify your controller object after it’s been released+dealloc’d (and would no longer be valid).

In the specified -appDidActivate: method, do whatever you need to with the info about the app in question.