Cannot invoke 'indexOf' with an argument list of type '(ChecklistItem)'

Question

When I am writing code for finding an item from the array with the use of indexOf it shows me the above stated error. Here is my code:-

func addItemViewContr         


        

Answer 1

In order to use indexOf the ChecklistItem must adopt Equatable protocol. Only by adopting this protocol the list can compare an item with other items to find the desired index

Answer 2

In Swift 4 and Swift 3, update your Data Model to conform to "Equatable" protocol, and implement the lhs=rhs method , only then you can use ".index(of:...)", because you are comparing your custom object

Eg:
class Photo : Equatable{
    var imageURL: URL?
    init(imageURL: URL){
        self.imageURL = imageURL
    }

    static func == (lhs: Photo, rhs: Photo) -> Bool{
        return lhs.imageURL == rhs.imageURL
    }
}

Usage:

let index = self.photos.index(of: aPhoto)

Answer 3

indexOf can only be applied to Collections of Equatable types, your ChecklistItem doesn't conform to Equatable protocol (have an == operator).

To be able to use indexOf add this to the file containing ChecklistItem class in the global scope :

func ==(lhs: ChecklistItem, rhs: ChecklistItem) -> Bool {
    return lhs === rhs
}

Swift3: 
public static func ==(lhs: Place, rhs: Place) -> Bool {
        return lhs === rhs
    }

Please note it will make comparison by comparing instances addresses in memory. You may want to check equality by comparing members of the class instead.

Answer 4

I realize this question already has an accepted answer, but I found another case that will cause this error so it might help someone else. I'm using Swift 3.

If you create a collection and allow the type to be inferred you may also see this error.

Example:

// UITextfield conforms to the 'Equatable' protocol, but if you create an
// array of UITextfields and leave the explicit type off you will
// also see this error when trying to find the index as below
let fields = [
        tf_username,
        tf_email,
        tf_firstname,
        tf_lastname,
        tf_password,
        tf_password2
    ]

// you will see the error here
let index = fields.index(of: textField)

// to fix this issue update your array declaration with an explicit type
let fields:[UITextField] = [
    // fields here
]

Answer 5

The possible reason is you didn't tell the ChecklistItem type that it is equatable, maybe you forgot to mention ChecklistItem class is inherited from NSObject.

import Foundation

class ChecklistItem: NSObject {
  var text = ""
  var checked = false

  func toggleChecked() {
    checked = !checked
  }
}

NSObject adopts or conforms to the equatable protocol