Sunday, December 13, 2009

The Responder Chain is a Collection

Timothy Wood voices some great ideas on modernizing the Cocoa responder chain. I'd like to venture that if we treat the Responder Chain as a simple collection, a singly-linked list, then such alternatives become easier to model and reason about.

NSEnumerator *responderEnumerator = [[firstResponder mapToNextObjectFromMessage] nextResponder];

I am currently abstracting from the intricate delegate mapping and other ops, these could be handled in an analog fashion. With the enumerator in place, we can obviously snapshot it to get the current state of the responder chain, and also log that.
NSArray *responders = [responderEnumerator allObjects];
NSLog(@"full responder chain:  %@",responders);

Now we can express both current features and possible variations of the Responder Chain architecture compactly as common collection operations. The current dispatch mechanism simply sends the message to the first object that is capable of responding. This corresponds to using the first object of a -select, which is expressed in the -selectFirst convenience method.

Current dispatch

[[[responders selectFirst] respondsToSelector:action] performSelector:action withObject:sender];

If I understood him correctly, Tim wants the objects in the responder chain to return an object that they would like to respond to the message. This turns the -select into a -collect (without a -collectFirst), but is otherwise very similar.

Tim's dispatch

possibleResponders = [[responders collect] responsibleTargetForAction:theAction sender:sender]];
[[possibleResponders objectAtIndex:0] performSelector:action withObject:sender];

I hope this does Tim's ideas justice, but I think the succinct formulation should make it easy to tell wether it does or not.

In terms of combining validation with target/action, I'd be somewhat wary of accidentally triggering actions when validation was meant, though I do appreciate the advantages of combining the two operations. I am not sure what value the block is adding over just having an additional BOOL parameter in the target/action method.

Combined action and validation

typedef BOOL IBAction;
-(IBAction)delete:sender  :(BOOL)onlyValidate
    NSArray *selection = [self selectedItems];

   if ( onlyValidate || [selection count] == 0 ) {
        return NO;
   // perform the action
// or if you're worried about the naming issues

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