I’ve recently open-sourced a small but handy Objective-C library called MCEModelEditingProxy.
Often times when presenting values from a model you want to make them editable but don’t want to store the changes back immediately, but only after a confirmation from the user (e.g. pressing a Save button). MCEModelEditingProxy does precisely that. It stands as a transparent layer between your model and your controller, intercepting writes to the model.
If this sounds too abstract, check out the README in the project repo where you can find example use cases. I hope you’ll find the library useful and include it in your projects too.