An example of tests using the mock

ExampleServiceTest.swift

import Testing
@testable import ExampleApp

@Suite("Example Service Test") struct ExampleServiceTest {
  
  @Test("Restores from UserDefaults") func restoreValues() throws {
    // setup
    let classToBeStored = SomeClass(foo: "bar")
    let dataToStore = try? JSONEncoder().encode(classToBeStored)
    let mockUserDefault = MockUserDefault()
    mockUserDefault.set(dataToStore, forKey: "someKey")
    
    // test
    let service = ExampleService(userDefaults: mockUserDefaults)
    #expect(service.aStoredClass == classToBeStored)
  }
  
  @Test("Uses default when note stored") func usesDefaults() throws {
    // setup
    let mockUserDefault = MockUserDefault()
    
    // test
    let service = ExampleService(userDefaults: mockUserDefaults)
    #expect(service.aStoredClass == SomeClass())
  }
  
  @Test("Saves values when doSomething method called") func savesValues() throws {
    // setup
    let classToBeStored = SomeClass(foo: "bar")
    let dataToStore = try? JSONEncoder().encode(classToBeStored)
    let mockUserDefault = MockUserDefault()
    mockUserDefault.set(dataToStore, forKey: "someKey")
    
    // test
    let service = ExampleService(userDefaults: mockUserDefaults)
    let newValue = SomeClass(foo: "Buzz")
    service.doSomething(newValue: newValue)
    
    let savedValue = try? JSONDecoder().decode(SomeClass.self, from: mockUserDefaults.persistedValue!)
    #expect(savedValue == newValue)
  }
}