I've written a protocol, allowing to link objects in a simple chain with predecessors and successors.

I'm trying to use this protocol with my Core Data entity "Event", but I'm getting this error:

Error:

Protocol 'Chainable' requirement 'chain' cannot be satisfied by a non-final class ('Event') because it uses 'Self' in a non-parameter, non-result type position.

How can I make this class final (I wouldn't like to set the Codegen to manual), or rewrite the var chain: [Self] definition?

Thank you in advance.

extension Event: Chainable {}

protocol Chainable where Self: NSManagedObject {

    var chain: [Self] { get }

    var predecessor: Self? { get set }
    var successor: Self? { get set }

    var selfIndexInChain: Int { get }

    mutating func moveInChain(to index: Int)
    mutating func removeSelfFromChain()

    mutating func addSelfAsSuccessor(of object: Self)
    mutating func addSelfAsPredecessor(of object: Self)
}

extension Chainable {
   
    var chain: [Self] {
    
        var _chain: [Self] = [self]
    
        // go right
        var current: Self = self
        while let successor = current.successor {
        
            _chain.append(successor)
            current = successor
        }
    
        // go left
        current = self
        while let predecessor = current.predecessor {
        
            _chain.insert(predecessor, at: 0)
            current = predecessor
        }
    
        // TODO: - Compare speed with the alternative realization: Find the first element, then populate the tasks array.
    
        return _chain
    }

    // Self Index in the chain
    var selfIndexInChain: Int { return self.chain.index(of: self)! }


    // Change place in the cahain
    mutating func moveInChain(to index: Int) {
    
        guard index != selfIndexInChain else { return } // Check the index isn't same
        guard 0...(chain.count-1) ~= index else { return }
    
        let taskAtDestination = chain[index]
    
        if index > selfIndexInChain {
        
            removeSelfFromChain()
            addSelfAsSuccessor(of: taskAtDestination)
       } else {
            removeSelfFromChain()
            addSelfAsPredecessor(of: taskAtDestination)
       }
    }

    mutating func removeSelfFromChain() {
    
        let successor = self.successor
        let predecessor = self.predecessor
    
        self.predecessor = nil
        self.successor = nil
    
        self.predecessor?.successor = successor
        self.successor?.predecessor = predecessor
    }

    mutating func insertSelfBetween(lhs: Self, rhs: Self) {
        // self shouldn't be linked
        guard self.predecessor == nil && self.successor == nil else { return }
    
        guard (lhs.successor == rhs && rhs.predecessor == lhs) ||
            (lhs.successor == nil && rhs.predecessor == nil ) else { return } // If both are nil, they will be connected thru self
    
        self.predecessor = lhs
        self.successor = rhs
    
    }

    mutating func addSelfAsSuccessor(of object: Self) {
        // self shouldn't be linked
        guard self.predecessor == nil && self.successor == nil else { return } // TODO: Add error support
        // self shouldn't be already the successor 
        guard object.successor != self else { return }
    
        let previousSuccessor = object.successor
        self.predecessor = object
        self.successor = previousSuccessor
    
    }

    mutating func addSelfAsPredecessor(of object: Self) {
        // self shouldn't be linked
        guard self.predecessor == nil && self.successor == nil else { return }
        // self shouldn't be the task successor already
        guard object.predecessor != self else { return }
    
        let previousPredecessor = object.predecessor
        self.successor = object
        self.predecessor = previousPredecessor
    }

}