Testing UserDefaults values in UiTests or UnitTests - xcode

Question

I am saving an Int in UserDefaults and this is reduced by one by clicking a button. I don't know if that is important but I have added an extension to UserDefaults to load an initial value if the app starts for the first time:
extension UserDefaults {
public func optionalInt(forKey defaultName: String) -> Int? {
let defaults = self
if let value = defaults.value(forKey: defaultName) {
return value as? Int
}
return nil
}
}
The UserDefaults are used as ObservableObject and accessed as EnvironmentObject within the app like this:
class Preferences: ObservableObject {
#Published var counter: Int = UserDefaults.standard.optionalInt(forKey: COUNTER_KEY) ?? COUNTER_DEFAULT_VALUE {
didSet {
UserDefaults.standard.set(counter, forKey: COUNTER_KEY)
}
}
}
I am now trying to test that the value in the UserDefaults decreases when the button is clicked.
I am trying to read the UserDefaults in the test with:
XCTAssertEqual(UserDefaults.standard.integer(forKey: "COUNTER_KEY"), 9)// default is 10
I have tried it with normal UnitTests where the methods behind the Button are called and with UITests but both do not work. In the UnitTests I get back the COUNTER_DEFAULT_VALUE and in the UiTests I get back 0.
I am trying to access the UserDefaults directly in the test, instead of using the Preferences object, because I have not found a way to access that as it is an ObservableObject.
I have checked in the Emulator that the UserDefaults are saved/loaded correctly when using the app. Is it not possible to access the UserDefaults in the tests or am I doing it wrong?

Answer 1

The key to success is Dependency injection. Instead of directly accessing the shared user defaults object (UserDefaults.standard), declare an object of type UserDefaults within the class:
let userDefaults: UserDefaults
In the view where you declare the model, you are free to use the shared object:
#StateObject var model = Preferences(userDefaults: UserDefaults.standard)
But inside your test, create an dedicated UserDefaults object and pass it to the initializer like so:
let userDefaults = UserDefaults(suiteName: #file)!
userDefaults.removePersistentDomain(forName: #file)
let model = Preferences(userDefaults: userDefaults)
The benefit is clear: You control the state of UserDefaults. And that means the code works in every environment. To keep it simple, I haven't incorporated your extension, yet. But I'm sure you will manage to get it working.
TL;DR
Please see my working example below:
ContentView.swift
import SwiftUI
import Foundation
class Preferences: ObservableObject {
let userDefaults: UserDefaults
#Published var counter: Int {
didSet {
self.userDefaults.set(counter, forKey: "myKey")
}
}
init(userDefaults: UserDefaults) {
self.userDefaults = userDefaults
self.counter = userDefaults.integer(forKey: "myKey")
}
func decreaseCounter() {
self.counter -= 1
}
}
struct ContentView: View {
#StateObject var model = Preferences(userDefaults: UserDefaults.standard)
var body: some View {
HStack {
Text("Value: \(self.model.counter)")
Button("Decrease") {
self.model.decreaseCounter()
}
}
}
}
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView()
}
}
InjectionTests.swift
import XCTest
#testable import Injection
class InjectionTests: XCTestCase {
func testPreferences() throws {
// arrange
let userDefaults = UserDefaults(suiteName: #file)!
userDefaults.removePersistentDomain(forName: #file)
let model = Preferences(userDefaults: userDefaults)
// act
let valueBefore = userDefaults.integer(forKey: "myKey")
model.decreaseCounter()
let valueAfter = userDefaults.integer(forKey: "myKey")
// assert
XCTAssertEqual(valueBefore - 1, valueAfter)
}
}