I'm working on a simple design that raises not-so-simple discussion. This is my UI:
User can add new items (country or region, i.e. State/DMA) using the combo above. When using combo at first, it shows all countries and regions. Our system has the following limitation: You can either select countries, or you can select regions inside one country. You can't do both.
There are two approaches:
Don't allow the user to make mistakes. Lead him through the UI. This means that if the system sees that user selects only countries, remove regions from combo - we don't allow them anyway (the "Apple" approach).
Don't change the content of the combo. Allow the user to select a non-allowed selection (e.g. select a region after he selected a few countries) - and then show a warning with explanation of the limitation and actions to fix the problem (the "Android" approach).
I'll assume the users have a selection in their mind when they approach the UI, and they may not be aware about our limitations. What would be the better approach in this case?
Is there a 3rd way - to not allow user to err but also help him understand the rules?