Cannot assign to property: 'point' is a 'let' constant

This is my func:

func adjustPointOffset(
    point: PointType,
    isBefore: Bool,
    key: Key,
    target: Node,
    textLength: Int) {
      if point.type == "text" {
        point.key = key// error: Cannot assign to property: 'point' is a 'let' constant
        if !isBefore {
          point.offset += textLength// error: Left side of mutating operator isn't mutable: 'point' is a 'let' constant
        }
      } else if point.offset > target.getIndexWithinParent() {
        point.offset -= 1// error: Left side of mutating operator isn't mutable: 'point' is a 'let' constant
      }
    }

This is PointType class:

enum SelectionType: String {
  case text = "text"
  case element = "element"
}

protocol PointType {
  var key: NodeKey { get set }
  var offset: Int { get set }
  var type: SelectionType { get set }

  func isPointType(point: PointType) -> Bool
  func isBefore(point: PointType) -> Bool
  func getNode() -> Node
  func setPointType(key: NodeKey, offset: Int, type: SelectionType)
  func getCharacterOffset() -> Int
  func isAtNodeEnd() -> Bool
} 
class Point: PointType {
  var key: Key
  var offset: Int
  var type: SelectionType

  init(key: Key, offset: Int, type: SelectionType) {
    self.key = key
    self.offset = offset
    self.type = type
  }
}

extension Point: Equatable {
  static func == (lhs: Point, rhs: Point) -> Bool {
    return lhs.key == rhs.key &&
    lhs.offset == rhs.offset &&
    lhs.type == rhs.type
  }
}

How do I solve these errors – error: Cannot assign to property: ‘point’ is a ‘let’ constant. I don’t understand ‘point’ is a protocol and the values are all ‘var’. But its pointing to ‘let’.

>Solution :

PointType as a protocol, as written, could apply to either a value type (such as a struct) or a reference type (such as a class). If it were a value type and passed to your function without inout, it would be immutable.

If your PointType is always going to be a reference type (and thus always mutable from inside a function where it’s pointed to by reference), you can fix your error by saying that it has to be AnyObject:

protocol PointType : AnyObject {