This is the scenario: your user interface comprises an outline view and a hierarchical data model. You want the outline view to display the hierarchy and remember the expansion state automatically in preferences. Hence when the user re-runs the application, the items that were expanded are still expanded, and vice versa: what was collapsed remains so. Outline view, tree controller, hierarchical model, bindings. That’s the recipe.
According to some, storing expansion state of an outline view when used with a tree controller is not just difficult: it’s impossible! But is it? No, is the simple answer. It’s actually quite easy. In this article I introduce a new helper class called RROutlineViewExpandedItemsAutosaver! Original aren’t I? It does not involve sub-classing or access to private methods. The solution presented uses only documented interfaces and only requires a small stateless class instance for handling all outline view auto-saving technicalities. It does assume Core Data use for modelling. But you can easily adapt the technique for other data-model implementations.
The last article on this subject (Organising view controllers) started looking at an example. Apple’s ViewController sample illustrates basic use of multiple view controllers. This article completes the work.
Download the source if you would rather skip ahead. Sometimes it’s easier that way. You can see the code in its full context. Snippets and extracts don’t always tell you everything you need to know.