The penultimate feature is the ability to add new items to the Add list.
My initial idea was to allow you to add a new item to the Add list inline but off the top of my head I can’t think of a way to do it with out-of-the-box Xamarin.Forms (one of it’s minor weaknesses) so I’m thinking a about adding another screen that’s displayed when you tap on the Add button in the upper right corner.
On this screen to add a new item to a list you type its name in the text field at the top of the screen and then tap Save. By default the new item is added to the Add list but but you can change the destination to the Buy list by tapping the switch below the text field before you tap Save.
Once the item is added to the selected list the text field is cleared but still has focus so you can add another item.
I probably should add some validation in there to prevent adding the same item to both lists.
I think I’m going to try to use XAML to create this screen (assuming both methods can coexist in the same project).