Spying on notifications.

notification.m

#import <Cocoa/Cocoa.h>

// clang -Weverything -fobjc-arc -framework Cocoa -o notification notification.m

void QuietLog (NSString *format, ...);
void StartSpying (void);
void StopSpying (void);


void QuietLog (NSString *format, ...) {
    va_list args;

    va_start (args, format);

    NSString *string;

    string = [[NSString alloc] initWithFormat: format  arguments: args];

    va_end (args);

    fprintf (stderr, "%s\n", [string UTF8String]);

} // QuietLog

NSMutableDictionary *g_notificationTokens;

void StartSpying (void) {
    g_notificationTokens = [NSMutableDictionary dictionary];
    id token;

    NSNotificationCenter *center;
    center = [NSNotificationCenter defaultCenter];
    token = [center addObserverForName: nil
                    object: nil
                    queue: nil
                    usingBlock: ^(NSNotification *notification) {
            QuietLog (@"NOTIFICATION %@ -> %@", notification.name, notification.userInfo);
        }];
    [g_notificationTokens setObject: token  forKey: @"defaultCenter"];
    [center removeObserver: token];
    
    center = [NSDistributedNotificationCenter defaultCenter];
    token = [center addObserverForName: nil
                    object: nil
                    queue: nil
                    usingBlock: ^(NSNotification *notification) {
            QuietLog (@"DISTRIBUTED %@ -> %@", notification.name, notification.userInfo);
        }];
    [g_notificationTokens setObject: token  forKey: @"distributedCenter"];
    [center removeObserver: token];

    center = [[NSWorkspace sharedWorkspace] notificationCenter];

    // Hack to get the notification working with blocks
#ifndef RADAR_11827110
    [center addObserver: @"hi"
            selector: @selector(self)
            name: nil
            object: nil];
#endif

    token = [center addObserverForName: nil
                    object: nil
                    queue: nil
                    usingBlock: ^(NSNotification *notification) {
            QuietLog (@"WORKSPACE %@ -> %@", notification.name, notification.userInfo);
        }];

    [g_notificationTokens setObject: token  forKey: @"workspaceCenter"];

} // StartSpying


void StopSpying (void) {
    id token;
    NSNotificationCenter *center;

    token = [g_notificationTokens objectForKey: @"defaultCenter"];
    center = [NSNotificationCenter defaultCenter];
    [center removeObserver: token];
    
    token = [g_notificationTokens objectForKey: @"distributedCenter"];
    center = [NSDistributedNotificationCenter defaultCenter];
    [center removeObserver: token];

    token = [g_notificationTokens objectForKey: @"workspaceCenter"];
    center = [[NSWorkspace sharedWorkspace] notificationCenter];
    [center removeObserver: token];
    
} // StopSpying


int main (void) {
    @autoreleasepool {
        StartSpying ();
        
        [[NSRunLoop currentRunLoop] run];

        // Won't get called, but added for completeness.
        StopSpying ();
    }

    return 0;
} // main

Thanks

Distributed notifications cannot be spied on anymore 😢
https://mjtsai.com/blog/2019/10/04/nsdistributednotificationcenter-no-longer-supports-nil-names/