Testing completion handler APIs with Swift Testing – Donny Wals

Understanding the issue

After I began studying Swift testing, I truly checked out Apple’s migration doc and I discovered that there’s an one thing that’s imagined 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 just a little 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’ll be able to see within the code above, the instance that Apple has exhibits that we’ve got a operate and a name to the affirmation operate 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They name their outdated completion handler based mostly API and within the occasion handler closure they name their affirmation closure (referred to as soldFood within the instance).

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

And that is actually the place Apple desires 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 absolutely the important thing a part of how we must be utilizing affirmation.

After 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 just a little 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 verify in
    synchronizer.onComplete = {
      #anticipate(synchronizer.newsItems.rely == 2)
      verify()
    }

    synchronizer.synchronize()
  }
}

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

The end result when working that is all the time a failing check. The check shouldn’t be ready for me to name the verify 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 substitute of your expectations.

What Apple says within the migration information if you fastidiously learn is definitely that the entire confirmations must be referred to as earlier than your closure returns:

Confirmations operate 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 predicted 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’ve got confirmed all of our confirmations. In a conventional completion handler-based setup, this would possibly 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 check 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 just a little bit like a continuation within the sense that the Swift check would anticipate me to name that affirmation.

This isn’t the case.

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

You need to 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. This can then resume your check and it’ll full your check.

Right here’s what that appears like for example:

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

        synchronizer.synchronize()
    }
}

This strategy works very properly for what I wanted, and it permits me to droop the check whereas my callback based mostly code is working.

It is the best strategy I might give you, which is normally a great signal. However if in case you have every other approaches that you just desire, I’d love to listen to about them, particularly when it pertains to testing completion handler APIs. I do know this isn’t a full-on substitute for every part that we are able to do with expectations, however for the completion handler case, I feel it is a fairly good substitute.

When to not use continuations for testing completion handler code

The strategy of testing outlined above assumes that our code is considerably freed from sure bugs the place the completion handler is rarely referred to as. Our continuation would not do something to forestall our check from hanging without end which might (let’s be trustworthy, will) be a difficulty for sure situations.

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

I have never accomplished in depth testing with this snippet but however a few preliminary assessments look good to me.