I don't know if what I see with a popup button populated by bindings with a value transformer is the way it's supposed to be or not -- the unusual thing I'm seeing (at least with respect to what I've seen with value transformers and table views) is that the "value" parameter in the transformedValue: method is the whole array bound to the array controller, not the individual strings in the array. When I've done this with table views, the transformer is called once for each displayed row in the table, and the "value" parameter is whatever object is bound to that row and column, not the whole array that serves as the content array for the array controller.

I have a very simple app to test this. In the app delegate there is this:

+(void)initialize {
    RDTransformer *transformer = [[RDTransformer alloc] init];
    [NSValueTransformer setValueTransformer:transformer forName:@"testTransformer"];
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    self.theData = @[@{@"name":@"William", @"age":@"24"},@{@"name":@"Thomas", @"age":@"23"},@{@"name":@"Alexander", @"age":@"64"},@{@"name":@"James", @"age":@"47"}];
}

In the RDTransformer class is this:

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


+(BOOL)allowsReverseTransformation {
    return NO;
}

-(id)transformedValue:(id)value {
    NSLog(@"%@",value);
    return value;
}

In IB, I added an NSPopupButton to the window and an array controller to the objects list. The content array of the controller is bound to App Delegate.theData, and the Content Values of the popup button is bound to Array Controller.arrangedObjects.name with the value transformer, testTransformer.

When I run the program, the log from the transformedValue: method is this:

2012-09-19 20:31:39.975 PopupBindingWithTransformer[793:303] (
)
2012-09-19 20:31:40.019 PopupBindingWithTransformer[793:303] (
    William,
    Thomas,
    Alexander,
    James
)

This doesn't seem to be other people's experience from what I can see on SO. Is there something I'm doing wrong with either the bindings or the value transformer?