Question

Im just wondering whether there is a way (in Objective-C and iPad) to call a factory method where I build the name of the object on the fly with a string.

e.g. I have a class XYZ and several factory methods

 +(XYZ *) A;
 +(XYZ *) B;
 +(XYX *) C

I would normally call it like

 [XYZ A];
 [XYZ B];
 [XYZ C];

But I want to be able to call it dynamically with a string e.g.

 NSString *s; 
 ...
 s = @"B";
 [XYZ s];

I hope you get my point.

Thank you.

Answer 1

Yes. You can use NSSelectorFromString() to convert an NSString* into a SEL (which is the same type that @selector() gives you). You can then call this with -performSelector: and its variants.

[XYZ performSelector:NSSelectorFromString(s)];

-performSelector: is useful for methods that take no arguments and return id or void. -performSelector:withObject: and -performSelector:withObject:withObject: are variants that take 1 or 2 id-typed parameters. If you need more parameters than that, or you need a parameter or return value that isn’t id, then you can use NSInvocation instead to set up the method call. Note that NSInvocation is (relatively) expensive, so it should only be used when there’s no other way.