---
title: Testing asynchronous code
framework: testing
role: article
role_heading: Article
path: testing/testing-asynchronous-code
---

# Testing asynchronous code

Validate whether your code causes expected events to happen.

## Overview

Overview The testing library integrates with Swift concurrency, meaning that in many situations you can test asynchronous code using standard Swift features.  Mark your test function as async and, in the function body, await any asynchronous interactions: @Test func priceLookupYieldsExpectedValue() async {   let mozarellaPrice = await unitPrice(for: .mozarella)   #expect(mozarellaPrice == 3) } In more complex situations you can use Confirmation to discover whether an expected event happens. Confirm that an event happens Call confirmation(_:expectedCount:isolation:sourceLocation:_:) in your asynchronous test function to create a Confirmation for the expected event. In the trailing closure parameter, call the code under test. Swift Testing passes a Confirmation as the parameter to the closure, which you call as a function in the event handler for the code under test when the event you’re testing for occurs: @Test("OrderCalculator successfully calculates subtotal for no pizzas") func subtotalForNoPizzas() async {   let calculator = OrderCalculator()   await confirmation() { confirmation in     calculator.successHandler = { _ in confirmation() }     _ = await calculator.subtotal(for: PizzaToppings(bases: []))   } } If you expect the event to happen more than once, set the expectedCount parameter to the number of expected occurrences. The test passes if the number of occurrences during the test matches the expected count, and fails otherwise. You can also pass a range to confirmation(_:expectedCount:isolation:sourceLocation:_:) if the exact number of times the event occurs may change over time or is random: @Test("Customers bought sandwiches") func boughtSandwiches() async {   await confirmation(expectedCount: 0 ..< 1000) { boughtSandwich in     var foodTruck = FoodTruck()     foodTruck.orderHandler = { order in       if order.contains(.sandwich) {         boughtSandwich()       }     }     await FoodTruck.operate()   } } In this example, there may be zero customers or up to (but not including) 1,000 customers who order sandwiches. Any range expression which includes an explicit lower bound can be used:  |   |   |   |   |  Confirm that an event doesn’t happen To validate that a particular event doesn’t occur during a test, create a Confirmation with an expected count of 0: @Test func orderCalculatorEncountersNoErrors() async {   let calculator = OrderCalculator()   await confirmation(expectedCount: 0) { confirmation in     calculator.errorHandler = { _ in confirmation() }     calculator.subtotal(for: PizzaToppings(bases: []))   } }

## See Also

### Confirming that asynchronous events occur

- [confirmation(_:expectedCount:isolation:sourceLocation:_:)](testing/confirmation(_:expectedcount:isolation:sourcelocation:_:)-5mqz2.md)
- [confirmation(_:expectedCount:isolation:sourceLocation:_:)](testing/confirmation(_:expectedcount:isolation:sourcelocation:_:)-l3il.md)
- [Confirmation](testing/confirmation.md)