Sketch of aspect-oriented programming in Objective-C

aspects.m

/* Logger.m */
@aspect Logger
// called for any selector
- (void)before {
  	// '_cmd' is special -- the selector for the actual method with the advice
	// applied to it
	NSLog(@"starting -[%@ %@]", self, _cmd);
}

// only called for 'deposit:'
- (void)beforeDeposit:(NSDecimalNumber *)amount {
	NSLog(@"depositing %@ to %@", amount, self);
}

// called for any selector
- (void)after {
	NSLog(@"done with -[%@ %@]", self, _cmd);
}
@end

/* BankAccount.h */
@interface BankAccount : NSObject <Logger> {
}

- (void)deposit:(NSDecimalNumber *)amount;
- (void)withdraw:(NSDecimalNumber *)amount;
@end

/* Transaction.m */
@aspect Transaction
- (void)before {
  	// blah blah blah transaction code here
}

- (void)after {
  	// blah blah blah transaction code here
}
@end

/* BankAccount+TransactionExtensions.h */
@interface BankAccount (TransactionExtensions) <Transaction>
@end