Linked List in swift

LinkedList.swift

import Foundation

class LinkedList<T> {
    private class Link<T> {
        var prev: Link?
        var next: Link?
        let value: T
        
        init(_ value: T) {
            self.value = value
        }
    }

    private var _first: Link<T>?
    private var _last: Link<T>?
    
    var first: T { get { return _first!.value }}
    var last: T { get { return _last!.value }}
    
    func isEmpty() -> Bool {
        return _first == nil
    }
    func hasContent() -> Bool {
        return _first != nil
    }
    
    func add(beforeFirst value: T) {
        let newFirst = Link(value)
        if isEmpty() {
            _first = newFirst
            _last = newFirst
        } else {
            newFirst.prev = _first
            _first!.next = newFirst
            _first = newFirst
        }
    }
    
    func add(afterLast value: T) {
        let newLast = Link(value)
        if isEmpty() {
            _last = newLast
            _first = newLast
        } else {
            newLast.next = _last
            _last!.prev = newLast
            _last = newLast
        }
    }

    @discardableResult
    func removeFirst() -> T {
        let value = _first!.value
        _first = _first!.prev
        return value
    }
    
    @discardableResult
    func removeLast() -> T {
        let value = _last!.value
        _last = _last!.next
        return value
    }
    
    func enumerate(_ enumeratorFn: (T) -> Void) {
        var link = _last
        while link != nil {
            enumeratorFn(link!.value)
            link = link!.next
        }
    }
}