XCTest - Assert notification is (not) posted when using a custom notification center.

CustomNotificationCenterTestCase.swift

import XCTest

@MainActor
final class CustomNotificationCenterTestCase: XCTestCase {

  func test_notification_isPosted() {
    // Given
    let notificationCenter = NotificationCenter()
    let notificationExpectation = XCTNSNotificationExpectation(
      name: .fooBar,
      object: nil,
      notificationCenter: notificationCenter,
    )

    // When
    notificationCenter.post(
      name: .fooBar,
      object: self,
    )

    // Then
    wait(for: [notificationExpectation], timeout: 0.1)
  }

  func test_notification_isNotPosted() {
    // Given
    let notificationCenter = NotificationCenter()
    let notificationExpectation = XCTNSNotificationExpectation(
      name: .fooBar,
      object: nil,
      notificationCenter: notificationCenter,
    )
    notificationExpectation.isInverted = true

    // When
    // Run code that should not trigger a notification.
    if false {
      notificationCenter.post(
        name: .fooBar,
        object: nil,
      )
    }

    // Then
    wait(for: [notificationExpectation], timeout: 0.1)
  }
}

// MARK: - Helper

private extension Notification.Name {
  static let fooBar = Notification.Name(rawValue: "fooBar")
}