jacobsapps/ObservationTestUtils

Test utility for the Swift Observation framework

ObservationTestUtils

Test utilities for writing unit tests on @Observable view models.

Testing Observation

There is a trick to testing properties on @Observable view models.

Built into the Observation framework is a global function, withObservationTracking, which by Apple's own documentation, "Tracks access to properties."

This function takes two closures:

1. apply accesses properties on an @Observable entity.
2. onChange executes when values captured by apply change. 

With this understanding in place, we can write unit tests to handle our view model being changed:

func test_listenerSentBeersSuccessfully_setsBeers() {
    sut = BeerViewModel(repository: mockBeerRepository)
    let sampleBeers = [Beer.sample()]
    mockBeerRepository.beersPublisher.send(.success(sampleBeers))
    let exp = expectation(description: #function)
    withObservationTracking {
        _ = sut.beers
    } onChange: {
        exp.fulfill()
    }
    waitForExpectations(timeout: 1.0)
    XCTAssertEqual(sampleBeers, sut.beers)
}

The withObservationTracking Helper Function

This is lovely and all, but we aren't exactly waving goodbye to boilerplate with these tests. We're actually using even more lines than the Combine-based tests.

But through the magic of keypaths, we can ameilorate this issue.   A helper function, resplendent with doc comments, is in order:

/// Waits for changes to a property at a given key path of an `@Observable` entity.
///
/// Uses the Observation framework's global `withObservationTracking` function to track changes to a specific property.
/// By using wildcard assignment (`_ = ...`), we 'touch' the property without wasting CPU cycles.
///
/// - Parameters:
///   - keyPath: The key path of the property to observe.
///   - parent: The observable view model that contains the property.
///   - timeout: The time (in seconds) to wait for changes before timing out. Defaults to `1.0`.
///
func waitForChanges<T, U>(to keyPath: KeyPath<T, U>, on parent: T, timeout: Double = 1.0) {
    let exp = expectation(description: #function)
    withObservationTracking {
        _ = parent[keyPath: keyPath]
    } onChange: {
        exp.fulfill()
    }
    waitForExpectations(timeout: timeout)
}

Here, our BeerViewModel serves as T and key paths for both beers and showAlert work for U.

Now our success case is a beautiful 5 lines of code:

func test_listenerSentBeersSuccessfully_setsBeers() {
    sut = BeerViewModel(repository: mockBeerRepository)
    let sampleBeers = [Beer.sample()]
    mockBeerRepository.beersPublisher.send(.success(sampleBeers))
    waitForChanges(to: \.beers, on: sut)
    XCTAssertEqual(sampleBeers, sut.beers)
}