Difference Between Adapter and Delegation Design Patterns

I'll speak specifically to iOS as it has its own concepts and some language features make its way of doing things a good fit. Often in iOS the Controller - Model layer interactions are based on some variant of Observer pattern (key-value observing (KVO) but also NSNotification). I find the model layer often doesn't engage in the same type of interactive back and forth that happens in the View - Controller layer interface (think the complexity of a dynamic, fully-featured table view), so delegation is not the most natural fit. Usually something (user input, a network request, etc) changes the model, and the controller responds accordingly by updating the view. But delegation is still used in long-running, dynamic interactions such as a table view that displays the contents of a frequently updating data set (see NSFetchedResultsController).

As to your question of having the controller be the delegate of your view and your model, there's no inherent problem other than you'll degrade the cohesion of your controller, and it sometimes becomes a good choice to split the controller layer into view controllers and data controllers. Using KVO or NSNotification to observe changes in the model layer is usually straightforward enough that you won't find a pressing need to extract a new class.

Not entirely.

The primary purpose of the adapter pattern is to change the interface of class/library A to the expectations of client B. The typical implementation is a wrapper class or set of classes. The purpose is not to facilitate future interface changes, but current interface incompatibilities.

The proxy pattern also uses wrapper classes, but for a different purpose. The purpose of the proxy pattern is to create a stand-in for a real resource. Reasons for using a proxy can be

• The real resource resides on a remote computer (the proxy facilitates the interaction with the remote resource)
• The real resource is expensive to create (the proxy ensures the cost is not incurred unless/until really needed)

The most important thing is that a proxy provides a drop-in replacement for the real resource it is a stand-in for, so it must provide the same interface.