Count of tracks in playlists in a playlist folder

count.m

#import <Foundation/Foundation.h>

@interface PlaylistFolder : NSObject
@property (retain) NSArray *playlists;
-(id)trackCount;

@end
@interface Playlist : NSObject
@property (retain) NSArray *tracks;
@end
@interface Track : NSObject
@end



int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
	
	PlaylistFolder *folder = [PlaylistFolder new];
	Playlist *pl1 = [Playlist new], *pl2 = [Playlist new], *pl3 = [Playlist new];
	
	pl1.tracks = [NSArray arrayWithObjects:[Track new], nil];
	pl2.tracks = [NSArray arrayWithObjects:[Track new], [Track new], nil];
	pl3.tracks = [NSArray arrayWithObjects:[Track new], [Track new], [Track new], [Track new], nil];
	folder.playlists = [NSArray arrayWithObjects:pl1, pl2, pl3, nil];
	
	NSLog(@"Folder %@", [folder trackCount]);
	
    [pool drain];
    return 0;
}


@implementation Playlist
@synthesize tracks;
@end
@implementation Track
@end
@implementation PlaylistFolder
@synthesize playlists;
-(id)trackCount;
{
	/////// This is the part I can't figure out
	return [self valueForKeyPath:@"[email protected].@count"];
}
@end

2011-05-30 14:14:15.674 Foo[95534:a0f] -[NSCFNumber count]: unrecognized selector sent to instance 0x100106ae0
2011-05-30 14:14:15.676 Foo[95534:a0f] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSCFNumber count]: unrecognized selector sent to instance 0x100106ae0'
*** Call stack at first throw:
(
0 CoreFoundation 0x00007fff820eb7b4 exceptionPreprocess + 180
1 libobjc.A.dylib 0x00007fff84e7b0f3 objc_exception_throw + 45
2 CoreFoundation 0x00007fff82145110 +[NSObject(NSObject) doesNotRecognizeSelector:] + 0
3 CoreFoundation 0x00007fff820bd91f __forwarding + 751
4 CoreFoundation 0x00007fff820b9a68 _CF_forwarding_prep_0 + 232
5 CoreFoundation 0x00007fff82096c47 -[NSMutableArray addObjectsFromArray:] + 71
6 Foundation 0x00007fff800ef5b4 -[NSArray(NSKeyValueCoding) _unionOfArraysForKeyPath:] + 112
7 Foundation 0x00007fff8007ec2f -[NSArray(NSKeyValueCoding) valueForKeyPath:] + 566
8 Foundation 0x00007fff80022e62 -[NSObject(NSKeyValueCoding) valueForKeyPath:] + 376
9 Foo 0x0000000100001c56 -[PlaylistFolder trackCount] + 45
10 Foo 0x0000000100001b6c main + 616
11 Foo 0x00000001000018fc start + 52
12 ??? 0x0000000000000001 0x0 + 1
)
terminate called after throwing an instance of 'NSException'

I've tried a bunch of different variants but I can't seem to get it right.

I ended up doing

return [self valueForKeyPath:@"[email protected]"];

(since I had the attribute countOfTracksCached; still not sure how I would do it without exposing the count as a separate property)

It's [email protected].@count.

AFAIK, @unionOfArrays will evaluate the entire remaining key-path, then union. So the original code tried to union three counts, rather than count a union. (Does that make sense?)

The new code counts each array in playlists.tracks and sums the array of counts.

(This is me randomly reading other programmers' Twitter feeds...)

@sum! Of course! That's what I get for coding when I'm tired. Thanks!