How to observe notifications from a different application? - objective-c

Question

I'd like to get notified when a certain application triggers an event. I'm not an Objective-C developer nor do I know the OS X APIs—so I hope this question is not too basic.
My goal is to write the meta info of the currently playing song to a log file. For iTunes, I got this working with the following Objective-C line:
[[NSDistributedNotificationCenter defaultCenter]
addObserver: myObserver selector: #selector(observeNotification:)
name: #"com.apple.iTunes.playerInfo" object:nil];
However, I also need this for AirServer (which is an software AirPlay receiver). Unfortunately, the following doesn't work—the observer never gets called:
[[NSDistributedNotificationCenter defaultCenter]
addObserver: myObserver selector: #selector(observeNotification:)
name: #"com.pratikkumar.airserver-mac" object:nil];
So apparently, AirServer does not send these type of notifications. However, there's a notification in the Notification Center when a new song starts playing.
My next step would be to periodically check for new notifications in the OS X Notification Center (as described here: https://stackoverflow.com/a/25930769/1387396). This is not too clean though—so my questions is: is there another option in this particular case?

Answer 1

Catalina silently changed the addObserver behavior - you can no longer use a nil value for the name to observe all notifications - you have to pass in a name. This makes discovery of event names more difficult.
Firstly, you have to understand that while the NSDistributedNotificationCenter has the word Notification in it; it's not related. From the About Local Notifications and Remote Notifications, it does state:
Note: Remote notifications and local notifications are not related to broadcast notifications (NSNotificationCenter) or key-value observing notifications.
So, at that point, I'm going to answer from the perspective of NSDistributedNotificationCenter, and not about Remote/Local notifications - you already have a potential solution in the answer you linked for observing the DB file that contains a record of the notifications in that way.
This example code will not work on Catalina (10.15) because of API behavior changes
The first thing you need to do is listen for the correct notification. Creating a simple application that listens for all events; e.g. using:
NSDistributedNotificationCenter *center = [NSDistributedNotificationCenter defaultCenter];
[center addObserver:self
selector:#selector(notificationEvent:)
name:nil
object:nil];
-(void)notificationEvent:(NSNotification *)notif {
NSLog(#"%#", notif);
}
It indicates that the notifications are:
__CFNotification 0x6100000464b0 {name = com.airserverapp.AudioDidStart; object = com.pratikkumar.airserver-mac; userInfo = {
ip = "2002:d55e:dbb2:1:10e0:1bfb:4e81:b481";
remote = YES;
}}
__CFNotification 0x618000042790 {name = com.airserverapp.AudioDidStop; object = com.pratikkumar.airserver-mac; userInfo = {
ip = "2002:d55e:dbb2:1:10e0:1bfb:4e81:b481";
remote = YES;
}}
This indicates that the name parameter in the addObserver call should be com.airserverapp.AudioDidStart and com.airserverapp.AudioDidStop.
You can use code like that to determine all notifications, which will allow you to add the relevant observers when you want the particular observer. This is probably the easiest way to observe notifications of this type.