Question

Does Objective C have a function like Smalltalk’s Block valueWithArguments?

I’m looking for a function with a signature like:

apply(^(), NSArray* args)

Alternatively, is there a way to call a selector over a list of arguments?

Answer 1

With blocks you probably have to do some va_args magic. Calling a selector with variable arguments can be done but you probably want some helpers to make it a bit less painful.

NSObject+performSelectorWithArgsArray.h

@interface NSObject (performSelectorWithArgsArray)
- (id)performSelector:(SEL)sel withArgsArray:(NSArray *)args;
@end

NSObject+performSelectorWithArgsArray.m

@implementation NSObject (performSelectorWithArgsArray)
- (id)performSelector:(SEL)sel withArgsArray:(NSArray *)args {
  NSInvocation *inv = [NSInvocation invocationWithMethodSignature:
                       [self methodSignatureForSelector:sel]];
  [inv setSelector:sel];
  [inv setTarget:self];
  for (int i = 0; i < args.count; i++) {
    id a = [args objectAtIndex:i];
    [inv setArgument:&a atIndex:2 + i]; // 0 is target, 1 is cmd-selector
  }
  [inv invoke];

  NSNumber *r;
  [inv getReturnValue:&r];
  return r;
}
@end

And then use it like this:

#import "NSObject+performSelectorWithArgs.h"

@interface SomeClass : NSObject
@end

@implementation SomeClass
- (NSNumber *)withA:(NSNumber *)a withB:(NSNumber *)b {
  return [NSNumber numberWithInt:a.intValue + b.intValue];
}
@end

- (void)someMethod {
  SomeClass *a = [[SomeClass alloc] init];
  SEL sel = @selector(withA:withB:);
  NSArray *args = [NSArray arrayWithObjects:
                   [NSNumber numberWithInt:2],
                   [NSNumber numberWithInt:3],
                   nil];
  NSNumber *r = [a performSelector:sel withArgsArray:args];
  NSLog(@"%d", r.intValue);
}

If you choose to do something like this I recommend that you really read up on how NSInvocation works as it can bite quite hard. Not sure if it is possible to do this without requiring that all arguments and return value are objects.