Using selectors with NSPredicate

Question

I have an object which contains several different NSStrings. When displaying this object, depending on another attribute of the object, I will display one string or another. I have a function defined in the object that takes care of deciding which string to display. So, as a simple example:

@interface MyObject : NSObject {
    NSString* string1;
    NSString* string2;
    NSString* string3;
    int stringNum;
}

-(NSString)getDisplayString {
    if(stringNum == 1) {
        return string1; 
    } else if (stringNum == 2) {
        return string2;
    } else if (stringNum == 3) {
        return string3;
    }
}

Now, I would like to create an NSPredicate for searching an array of these objects. Is it possible to create one that will search on the results of getDisplayString? Obviously I could probably replicate the behaviour of getDisplayString within the predicate, but then I'll be doubling up on logic, and probably lead to an error somewhere down the line.