Autosave Expanded Items of NSOutlineView doesn't work

I've had the problem that my implementation of -outlineView:itemForPersistentObject: was not called at all. It turns out that this method is called when either "autosaveExpandedItems" or "autosaveName" is set. My solution was to set both properties in Code and NOT in InterfaceBuilder. When i set the properties after the delegate is assigned, the method gets called.

I got this to work - you need to return the corresponding tree node instead of "just" its represented object.

In itemForPersistentObject:, instead of return item; you need return [self itemForObject:item inNodes:[_treeController.arrangedObjects childNodes]];

with

- (id)itemForObject:(id)object inNodes:(NSArray *)nodes {
    for (NSTreeNode *node in nodes) {
        if ([node representedObject] == object)
            return node;

        id item = [self itemForObject:object inNodes:node.childNodes];
        if (item)
            return item;
    }

    return nil;
}

where _treeController is the NSTreeController instance that you use to populate the outline view.