Testing completion handler APIs with Swift Testing – Donny Wals

The Swift testing framework is an extremely useful gizmo that enables us to put in writing extra expressive assessments with handy and fashionable APIs.

That is my first publish about Swift Testing, and I’m primarily writing it as a result of I wished to put in writing about one thing that I encountered not too way back once I tried to make use of Swift testing on a code base the place I had each async code in addition to older completion handler based mostly code.

The async code was very simple to check attributable to how Swift Testing is designed, and I can be writing extra about that sooner or later.

The completion handler base code was somewhat bit tougher to check, primarily as a result of I used to be changing my code from XCTest with take a look at expectations to regardless of the equal could be in Swift testing.

Understanding the issue

Once I began studying Swift testing, I truly checked out Apple’s migration doc and I discovered that there’s an one thing that’s presupposed to be analogous to the expectation object, which is the affirmation object. The examples from Apple have one little caveat in there.

The Swift Testing instance seems somewhat bit like this:

// After
struct FoodTruckTests {
  @Take a look at func truckEvents() async {
    await affirmation("…") { soldFood in
      FoodTruck.shared.eventHandler = { occasion in
        if case .soldFood = occasion {
          soldFood()
        }
      }
      await Buyer().purchase(.soup)
    }
    ...
  }
  ...
}

Now, as you may see within the code above, the instance that Apple has exhibits that we have now a perform and a name to the affirmation perform in there, which is how we’re supposed to check our async code.

They name their outdated completion handler based mostly API and within the occasion handler closure they name their affirmation closure (known as soldFood within the instance).

After calling setting the occasion handler they await Buyer().purchase(.soup).

And that is actually the place Apple needs us to pay shut consideration as a result of within the migration doc, they point out that we need to catch an occasion that occurs throughout some asynchronous course of.

The await that they’ve as the ultimate line of that affirmation closure is basically the important thing a part of how we must be utilizing affirmation.

Once I tried emigrate my completion handler based mostly code that I examined with XCTestExpectation, I did not have something to await. My authentic testing code regarded somewhat bit like this:

func test_valueChangedClosure() {
  let anticipate = expectation(description: "Anticipated synchronizer to finish")

  let synchronizer = Synchronizer()
  synchronizer.onComplete = {
    XCTAssert(synchronizer.newsItems.rely == 2)
    anticipate.fulfill()
  }

  synchronizer.synchronize()
  waitForExpectations(timeout: 1)
}

Primarily based on the migration information and skimming the examples I although that the next code could be the Swift Testing equal:

@Take a look at func valueChangedClosure() async {    
  await affirmation("Synchronizer completes") { @MainActor affirm in
    synchronizer.onComplete = {
      #anticipate(synchronizer.newsItems.rely == 2)
      affirm()
    }

    synchronizer.synchronize()
  }
}

My code ended up trying fairly just like Apple’s code however the important thing distinction is the final line in my affirmation. I’m not awaiting something.

The outcome when working that is all the time a failing take a look at. The take a look at just isn’t ready for me to name the affirm closure in any respect. That await proper on the finish in Apple’s pattern code is just about wanted for this API to be usable as a alternative of your expectations.

What Apple says within the migration information once you rigorously learn is definitely that the entire confirmations must be known as earlier than your closure returns:

Confirmations perform equally to the expectations API of XCTest,
nonetheless, they don’t block or droop the caller whereas ready for a
situation to be fulfilled. As a substitute, the requirement is anticipated to be confirmed (the equal of fulfilling an expectation) earlier than affirmation() returns

So each time that affirmation closure returns, Swift Testing expects that we have now confirmed all of our confirmations. In a conventional completion handler-based setup, this may not be the case since you’re not awaiting something as a result of you do not have something to await.

This was fairly tough to determine.

Write a take a look at for completion handler code

The answer right here is to not use a affirmation object right here as a result of what I assumed would occur, is that the affirmation would act somewhat bit like a continuation within the sense that the Swift take a look at would anticipate me to name that affirmation.

This isn’t the case.

So what I’ve actually discovered is that one of the simplest ways to check your completion handler-based APIs is to make use of continuations.

You should use a continuation to wrap your name to the completion handler-based API after which within the completion handler, do your whole assertions and resume your continuation. It will then resume your take a look at and it’ll full your take a look at.

Right here’s what that appears like for instance:

@Take a look at func valueChangedClosure() async {
    await withCheckedContinuation { continuation in
        synchronizer.onComplete = {
            #anticipate(synchronizer.newsItems.rely == 2)
            continuation.resume()
        }

        synchronizer.synchronize()
    }
}

This method works very nicely for what I wanted, and it permits me to droop the take a look at whereas my callback based mostly code is working.

It is the only method I might give you, which is often an excellent signal. However when you’ve got another approaches that you simply want, I might love to listen to about them, particularly when it pertains to testing completion handler APIs. I do know this isn’t a full-on alternative for all the pieces that we are able to do with expectations, however for the completion handler case, I feel it is a fairly good alternative.

When to not use continuations for testing completion handler code

The method of testing outlined above assumes that our code is considerably freed from sure bugs the place the completion handler is rarely known as. Our continuation does not do something to stop our take a look at from hanging without end which might (let’s be sincere, will) be a difficulty for sure eventualities.

There are code snippets on the market that may get you the flexibility to deal with timeouts, just like the one discovered on this gist that was shared with me by Alejandro Ramirez.

I have not carried out intensive testing with this snippet but however a few preliminary assessments look good to me.