What should variables contained in the protocol implement?

Variables declared in a protocol in iOS (Swift) define the requirements for properties that a class, structure, or enumeration conforming to this protocol must implement. Such variables must have a specified type and can be read-only (get) or read-write (get set).

This means that any type implementing the protocol is obliged to provide these properties with the required accessibility. Thus, the protocol sets a contract that guarantees the presence of certain data or behavior.

Example:

protocol Identifiable {
    var id: String { get }
}

struct User: Identifiable {
    var id: String
}