UserDefaults Unit testing in iOS

In this example, we’ll try to write a unit test case of UserDefaults in iOS.

Suppose that we are in a situation where we are saving some product IDs in user default for a shopping app. When the user moves an item into the cart, we save an array of IDs in UserDefaults and if the user removes an item from the cart then we fetch and update the saved IDs array. Later we save the updated IDs array in UserDefaults.

We can write an extension (UserDefaults+Extension.swift)of UserDefaults as below

import Foundation
protocol UserDefaultKey {
     var userDefaulKey: String {get set}
}
extension UserDefaults: UserDefaultKey {
  var userDefaulKey: String {
   get {
       Keys.cartItemIDS
      } set {
       Keys.cartItemIDS = newValue
     }
  }
  private enum Keys { 
    static var cartItemIDS = "cartItemIDs"
  }
  class var savedCartItems: [Int]? {
     get {
       return UserDefaults.standard.value(forKey: Keys.cartItemIDS)    as?   [Int]
     } set {
      UserDefaults.standard.set(newValue, forKey: Keys.cartItemIDS)
     }
   }
}

Now we have an extension of UserDefaults which confirms a protocol UserDefaultKey. Now in real-world, we’ll save an id as below

func manageProductInCart(product: MainProductModel) {
  // get all saved product ids and store in an array
     var availaleCartItemIDs = UserDefaults.savedCartItems ?? []
// if in case product is already available in cart, then dont add  item in array. MainProductModel has a prperty id
  guard availaleCartItemIDs.firstIndex(where: {$0 == product.id}) ==   nil else {
     print("Item already available in cart. Retunr from here")
     return
  }
  availaleCartItemIDs.append(product.id)
 // save in user deafults. This will update the array of IDs saved in UserDefaults 
  UserDefaults.savedCartItems = availaleCartItemIDs
}

Now let's come to the UnitTesting part

class UserDefaultsTest: XCTestCase {
// Create a KEY for Unit test 
     private var userDefaultTestKey = "userDefaultsTestKeyCart"
    var cartViewModel: ProductViewModelProtocol!
override func setUp() { 
    UserDefaults.standard.userDefaulKey = userDefaultTestKey
    let someFakeCartModel = ... Create a fake cart model
     cartViewModel = CartListViewModel(model: someFakeCartModel)
  }
  override func tearDown(){
    UserDefaults.standard.removeObject(forKey: userDefaultTestKey)  
  }
func testWhenItemRemovedFromCart() {
   UserDefaults.savedCartItems = getFakeStoredCartProductIDs() // 1. Get some data from your fake
   let beforeRemoving = UserDefaults.savedCartItems // 2.  Store items in an array 
   let indexPath = IndexPath(row: 0, section: 0) // get an item from given indexpath. We are displying all products in UICollectionView.
   let prouct = cartViewModel.product(at: indexPath) // Get a product from given indexpath from your ViewModel
   cartViewModel.removeACartProduct(from: prouct) //3.   Remove an item from cart
   let afterRemoving = UserDefaults.savedCartItems
   XCTAssertTrue(beforeRemoving?.count != afterRemoving?.count, "Could not remove item from cart") // 4
}
}

Please have a look at userDefaultTestKey constant, in the Unit Test case we are using a different key and protocol UserDefaultKey allows us to do so.

in setUp() we are setting a new Key for UserDefaults and in tearDown() we are removing.

Here in the above code, have a look at steps 1, 2, 3 & 4.

In step 1, we are fetching storedIDs from a fake model.

In step 2, we are storing storedIDs an array named beforeRemoving.

In step 3, remove an item.

In step 4, we need to verify if the count is different or not. Here we can check for contains(object: ) too.

That’s it. Now you can test your UserDefaults without altering actual data saved in UserDefault.

Enjoy!!!