Getting array elements with valueForKeyPath

Question

Is there any way to access an NSArray element with valueForKeyPath? Google's reverse geocoder service, for example, returns a very complex data structure. If I want to get the city, right now I have to break it into two calls, like this:

NSDictionary *address = [NSString stringWithString:[[[dictionary objectForKey:@"Placemark"] objectAtIndex:0] objectForKey:@"address"]];
NSLog(@"%@", [address valueForKeyPath:@"AddressDetails.Country.AdministrativeArea.SubAdministrativeArea.Locality.LocalityName"]);

Just wondering if there's a way to shoehorn the objectAtIndex: call into the valueForKeyPath string. I tried a javascript-esque formulation like @"Placemark[0].address" but no dice.

Answer 1

Unfortunately, no. The full documentation for what's allowed using Key-Value Coding is here. There are not, to my knowledge, any operators that allow you to grab a particular array or set object.

Answer 2

Here's a category I just wrote for NSObject that can handle array indexes so you can access a nested object like this: "person.friends[0].name"

@interface NSObject (ValueForKeyPathWithIndexes)
   -(id)valueForKeyPathWithIndexes:(NSString*)fullPath;
@end


#import "NSObject+ValueForKeyPathWithIndexes.h"    
@implementation NSObject (ValueForKeyPathWithIndexes)

-(id)valueForKeyPathWithIndexes:(NSString*)fullPath
{
    NSRange testrange = [fullPath rangeOfString:@"["];
    if (testrange.location == NSNotFound)
        return [self valueForKeyPath:fullPath];

    NSArray* parts = [fullPath componentsSeparatedByString:@"."];
    id currentObj = self;
    for (NSString* part in parts)
    {
        NSRange range1 = [part rangeOfString:@"["];
        if (range1.location == NSNotFound)          
        {
            currentObj = [currentObj valueForKey:part];
        }
        else
        {
            NSString* arrayKey = [part substringToIndex:range1.location];
            int index = [[[part substringToIndex:part.length-1] substringFromIndex:range1.location+1] intValue];
            currentObj = [[currentObj valueForKey:arrayKey] objectAtIndex:index];
        }
    }
    return currentObj;
}
@end

Use it like so

NSString* personsFriendsName = [obj valueForKeyPathsWithIndexes:@"me.friends[0].name"];

There's no error checking, so it's prone to breaking but you get the idea.

Answer 3

You can intercept the keypath in the object holding the NSArray.

In your case the keypath would become Placemark0.address... Override valueForUndefinedKey; look for the index in the keypath; something like this:

-(id)valueForUndefinedKey:(NSString *)key
{
    // Handle paths like Placemark0, Placemark1, ...
    if ([key hasPrefix:@"Placemark"])
    {
        // Caller wants to access the Placemark array.
        // Find the array index they're after.
        NSString *indexString = [key stringByReplacingOccurrencesOfString:@"Placemark" withString:@""];
        NSInteger index = [indexString integerValue];

        // Return array element.
        if (index < self.placemarks.count)
            return self.placemarks[index];
    }

    return [super valueForUndefinedKey:key];
}

This works really well for model frameworks e.g. Mantle.