chanson

Cocoa supports automatic menu validation based on a few simple rules. However, it can be inconvenient to keep adding things to -validateMenuItem: all the time, particularly when there are inheritance hierarchies involved.

There's a pattern that can help you get it right much more easily using introspection. You can implement your -validateMenuItem: method to ask the receiver whether it can respond to a particular selector at the moment, like so:

- (BOOL)validateMenuItem:(id <NSMenuItem>)menuItem
{
    BOOL enabled = YES;
    
    SEL itemAction = [menuItem action];
    
    if (itemAction != nil) {
        if ([self respondsToSelector:itemAction]) {
            SEL canAction = [self canSelectorForAction:itemAction];
            
            if ((canAction != nil) && [self respondsToSelector:canAction]) {
                BOOL (*performCanAction)(id, SEL) = (BOOL (*)(id, SEL)) objc_msgSend;
                enabled = performCanAction(self, canAction);
            } else {
                enabled = YES;
            }
        }
    }
    
    return enabled;
}

- (SEL)canSelectorForAction:(SEL)action
{
    NSString *actionString = NSStringFromSelector(action);
        // actionString is guaranteed to be 2+ characters
    NSString *actionStringFirst = [actionString substringToIndex:1];
    NSString *actionStringBody = [actionString substringWithRange:
        NSMakeRange(1, [actionString length] - 2)];
    
    NSString *canString
    = [[@"can" stringByAppendingString:[actionStringFirst uppercaseString]]
        stringByAppendingString:actionStringBody];
    
    return NSSelectorFromString(canString);
}

The above checks whether or not the receiver responds to the item's action selector. If it does, it will check if the receiver also implements a -(BOOL)canActionName selector, and use that to tell whether the menu item should be enabled.

The -canSelectorForAction: method just takes an action selector and returns its equivalent "canAction" selector, by upper-casing the first letter and appending it to "can". This lets you write a pair of methods for each action, one to perform the action and one to tell whether the action can be performed at the moment, leading to easy-to-test and uncluttered validation code.

Update (June 19, 2008): Changed how the -(BOOL)canActionName selector is performed to be safe and correct on Intel, and other architectures where a (BOOL) return might not be exactly equivalent to an (id) return. Thanks to an anonymous commenter for the correction!