How to use KVO to detect when an application gets active?

Question

I have the following code in a Cocoa application:

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
   NSArray* arrayAppList = [[NSWorkspace sharedWorkspace] runningApplications];
}

My intention is to use KVO to detect an application when changes its state between inactive to active.

I read that I have to use the instance method -addObserver:forKeyPath:options:context:

And then use -observeValueForKeyPath:ofObject:change:context: to respond to change notifications.

I understand that -observeValueForKeyPath is a callback method where I can write code to respond to the properties changes I am interested in.

Nevertheless, I feel confused in how I must to use the addObserver method in order to be notified when the active property of the runningApplications change. Now, I am wondering where is the place to make the registration, for now I am using -applicationDidFinishLaunching but not sure if is the right place to do it. Additionally if I use the -observeValueForKeyPath callback method, I have to implement it in the class that inherits from NSObject and is the same class where I am registering the notification?

Answer 1

1. You should call the addObserver:… method on each object in the runningApplications array (using isActive as the key path).

2. Starting the observing after your app finishes launching sounds about right. Time-wise, that is. As for the place, there should be a separate class dedicated to these observations. By implementing the observation code right in the app delegate you would violate the single-responsibility principle (and that means headache in the long term).

3. The observeValueForKeyPath:… callback should be implemented by the object that called the addObserver:… methods.