Async unit testing

My notes on async unit testing

  • The Expectations pattern is based on the usage of XCTestExpectation.
  • Expectation is an expected outcome in an asynchronous test.

The pattern can be summarized into four steps:

  • Create an instance of XCTestExpectation
  • Fulfil the expectation when async operation has finished
  • Wait for the expectation to be fulfilled
  • Assert the expected result

Callback based Async unit tests:

// simple example:
func testAsyncFunction() {
    let asyncDone = self.expectation(description: "Async function") // expectation is in the XCTestCase
    // ...
    someAsyncFunction(...) {
        // ...
        asyncDone.fulfill() // call this to indicate the test was successful
    wait(for: [asyncDone], timeout: 10) // Add after work has been called
    /* Test the results here */

Inverted Expectations

Inverted Expectations allows us to verify that something did not happen

func testInverteExpectation() {
    let expectation = XCTestExpectation(description: "Async function completes successfully")
    asyncFunction { result in
    wait(for: [expectation], timeout: 5.0)



  • Look into: XCTNSPredicateExpectation