Question

Newbie question here. I’d like to be able to specify through data (i.e. an XML file), the appropriate Objective-C message to send. Any advice on if this is possible or how I can do this?

The next best thing, if I can’t do this, would be some way to create a map object that would correlate a key (an int) with a function (I guess also a selector). Is that possible if the above isn’t?

If someone could point me to some tutorial or example code as reference, that’d be great. Right now I’m doing things with a big switch statement, and I don’t like it. (I’m switching on the id and in each case, explicitly calling the method relevant to the particular id.)

Answer 1

I love that you asked this question; too often, I see Satan’s Swollen Switch Statement. It’s nice to see someone wanting to using a function-table instead.

If you’re OK with using a property list file (which is usually encoded in XML), this is really easy.

Just make a property list where the root element is a dictionary, which maps from some keys to some selectors.

Key               Type         Value
----------------------------------------------
Root              Dictionary
  firstKey        String       someSelector
  secondKey       String       anotherSelector

Load the contents of your property list into an NSDictionary:

id path = [[NSBundle mainBundle] pathForResource:@"filename" ofType:@"plist"];
id dict = [NSDictionary dictionaryWithContentsOfFile:path];

SEL selector = NSSelectorFromString([dict objectForKey:@"firstKey"]);
if ([someObject respondsToSelector:selector]) {
    [someObject performSelector:selector];
}

Of course, you’ll want to refactor this logic into an appropriate method, and probably cache the property list as an instance variable.

Note: I personally think it’s better to just put this function table inline; property lists are cool, but I’m not sure that it is very helpful in this case. Also, if you are cool with using Objective-C++, std::map will allow you to get away with not wrapping and unwrapping the selectors in NSString objects, etc.