Question

The below protocol doesn’t have any methods. Is it possible to use it like this? What happens if we declare an empty protocol?

@class CircularPickerView;
@protocol CircularPickerViewDataSource <UIPickerViewDataSource>
@end

Answer 1

Is it possible to use it like this?

Yes.

What happens if we declare an empty protocol?

Strictly, this is not empty – it is also a UIPickerViewDataSource. Similar to subclassing, you may pass CircularPickerViewDataSource as a UIPickerViewDataSource. You may also expect it to adopt UIPickerViewDataSource‘s methods, and you will receive warnings if you do not implement the required interface. It may serve as documentation/intent, platform compatibility, or it may be a placeholder for future extension.

A protocol that declares no required or optional methods simply specifies adopters need not define any methods in their implementation. Since this is also a UIPickerViewDataSource, that’s not exactly the case.