Question

I would like to create a user agent in Objective-C that listens for notifications from the default NSDistributedNotificationCenter. The agent will not have a GUI. When I create a Cocoa application (I will also be using Distributed Objects, which I think is only in Cocoa) in Xcode, however, Xcode sets the project as a GUI application.

In the main function, I remove the NSApplicationMain(...) function call to remove the GUI elements from the application. However, now I can’t get the thread to wait (listen for) notifications coming in from the NSDistributedNotificationCenter. The app just starts and quits immediately.

I looked into using the NSRunLoop from the current NSThread, however, it seems that NSRunLoops only wait on NSPorts. There’s no mention of waiting on NSNotifications.

Answer 1

NSDistributedNotificationCenter is Foundation, so you don’t need to create a GUI app. You can create a command line template, for example, and run it from terminal. As a very simple example, you could create an example that just prints out every distributed notification it receives below.

To build, copy into an Xcode template for a Foundation command line app, or simply copy into a text file named something like test_note.m and build according to the comments. In this example, the application will never end (CFRunLoopRun() never returns) and you will have to kill it by hitting CTRL+C from the terminal or killing it with something like kill or the activity monitor.

// test_build.m
// to build: clang -o test_build test_build.m -framework foundation

#import <Foundation/Foundation.h>

@interface Observer : NSObject

- (void)observeNotification:(NSNotification*)note;

@end

@implementation Observer

- (void)observeNotification:(NSNotification*)note
{
  NSLog(@"Got Notification: %@", note);
}

@end

int main (int argc, char const *argv[])
{
  @autoreleasepool {
    Observer* myObserver = [[Observer alloc] init];
    [[NSDistributedNotificationCenter defaultCenter] addObserver:myObserver selector:@selector(observeNotification:) name:nil object:nil];
    CFRunLoopRun();
  }
  return 0;
}