CocoaDev

Recently I needed to bind an NSTextCell NSTableView column to a to-many property of a CoreData entity. After banging my head against the wall for a bit when Cocoa refused to bind to the relationship as it can’t possibly be KVC-compliant, I came up with the idea of using an NSValueTransformer to transform the set to a string. Here’s my value transformer:

% // CDSetToStringTransformer.h #import <Cocoa/Cocoa.h>

@interface CDSetToStringTransformer : NSValueTransformer { NSString *_keyPath; }

• (id)initWithKeyPath:(NSString *)keyPath;

• (NSString *)keyPath;
• (void)setKeyPath:(NSString *)keyPath;

@end

// CDSetToStringTransformer.m #import “CDSetToStringTransformer.h”

@implementation CDSetToStringTransformer

• (Class)transformedValueClass { return [NSString class]; }

• (BOOL)allowsReverseTransformation { return NO; }

• (id)initWithKeyPath:(NSString *)keyPath { if(self = [super init]) [self setKeyPath:keyPath];

  return self; }

• (void)dealloc { [self setKeyPath:nil];

  [super dealloc]; }

• (NSString *)keyPath { return _keyPath; }

• (void)setKeyPath:(NSString *)newKeyPath { newKeyPath = [newKeyPath copy]; [_keyPath release]; _keyPath = newKeyPath; }

• (id)transformedValue:(id)value { NSSet *sourceSet = (NSSet *)value; if([sourceSet count] == 0) return [NSNull null];

  NSString *result; NSEnumerator *objectEnum = [sourceSet objectEnumerator]; result = objectEnum nextObject] valueForKeyPath:[self keyPath;

  id currentObject; while(currentObject = [objectEnum nextObject]) result = [result stringByAppendingFormat:@”, %@”, [currentObject valueForKeyPath:[self keyPath]]];

  return result; }

@end

Put the instantiation in the proper place (I’m using + [AppDelegate initialize]). Initialize the transformer using - [CDSetToStringTransformer initWithKeyPath:], where keyPath is the key path relative to each object in the set that you want to coalesce into one string.

Beware that this class is not localized, but localization should be trivial.