Leveraging the preprocessor to lessen boilerplate. L(X) is the crux. It allows the me to apply different transformations over the same list.

gistfile1.txt

#define S(_s) @selector(application##_s:) // selector generator
#define N(_n) UIApplication##_n##Notification // name generator
#define L(X) X(WillResignActive), X(DidEnterBackground), \
X(WillEnterForeground), X(DidBecomeActive), X(WillTerminate) // apply X over list
#define C(_a) sizeof((_a))/sizeof((_a)[0]) // array count

- (void)monitorAppState:(BOOL)start
{
    
    NSNotificationCenter* nc = [NSNotificationCenter defaultCenter];
    SEL s[] = { L(S) };
    NSString* n[] = { L(N) };
    NSAssert(C(n) == C(s), @"Name and selector arrays have same size.");
    for(NSUInteger i = 0; i < C(n); ++i)
        if([self respondsToSelector:s[i]])
        {
            if(start)
                [nc addObserver:self selector:s[i] name:n[i] object:nil];
            else
                [nc removeObserver:self name:n[i] object:nil];
        }
}