How can the use of a protocol be restricted in Swift?

The protocol can be restricted in several ways:

1. Inheritance from a class: The protocol can inherit from a class, meaning only classes that are subclasses of this class can adopt this protocol.

   class BaseClass {}
   
   protocol RestrictedProtocol: BaseClass {
       // Protocol requirements
   }
   

2. Using AnyObject: Adding AnyObject to the inheritance list of the protocol means only classes can implement this protocol.

   protocol ClassOnlyProtocol: AnyObject {
       // Protocol requirements
   }
   

3. Restricting associated type: Using the where keyword, you can restrict the types that can be used for the associated type in the protocol.

   protocol Container {
       associatedtype Item
       mutating func append(_ item: Item)
       var count: Int { get }
       subscript(i: Int) -> Item { get }
   }
   
   protocol LimitedContainer: Container where Item: Equatable {
       // Additional requirements
   }
   

4. Specifying requirements for associated type: You can specify that the associated type must conform to a certain protocol or inherit from a class.

   protocol AnotherContainer {
       associatedtype Element: SomeOtherProtocol
       // ...
   }
   

These methods allow precise control over which types can conform to the protocol, increasing type safety and code predictability.