How exactly to subclass CALayer and use a custom property?

I am trying to create a subclass of CALayer with a custom index property that I can both animate and change directly in order to display a different picture based on the index.

In the header, I declared:

@property NSUInteger index;

In the implementation, I overrode needDisplayForKey:

+ (BOOL)needsDisplayForKey:(NSString *)key
{
    if ([key isEqualToString:@"index"])
        return YES;
    else
        return [super needsDisplayForKey:key];
}

Now, in the display method, I want to display a picture based on the index. However, during an animation, the value of self.index never changes, so I have to query the value of the presentation layer, contrary to the example on Providing CALayer Content by Subclassing:

- (void)display
{
    NSUInteger presentationIndex = [(CustomLayer *)[self presentationLayer] index];
    self.contents = (id)[[images objectAtIndex:presentationIndex] CGImage];
}

The problem is, if I do that, I cannot set the index value directly outside of an animation, because it will only change the model layer, and the display method explicitly queries the presentation layer.

If I add an initialization method that copies the value of index, it works:

- (id)initWithLayer:(id)layer
{
    self = [super initWithLayer:layer];
    if (self) {
        if ([layer isKindOfClass:[CustomLayer class]])
            self.index = [(CustomLayer *)layer index];
    }
    return self;
}

However, after or before an animation, there is always a 1 image glitch because the presentation or the model value don't match (depending if I