NimbleAssertions.md

Clearer Tests Using Nimble Assertions

When code doesn't work the way it's supposed to, unit tests should make it clear exactly what's wrong.

Take the following function which, given a bunch of monkeys, only returns the silly monkeys in the bunch:

public func silliest(monkeys: [Monkey]) -> [Monkey] {
  return monkeys.filter { $0.silliness == .verySilly }
}

Now let's say we have a unit test for this function:

func testSilliest_whenMonkeysContainSillyMonkeys_theyreIncludedInTheResult() {
  let kiki = Monkey(name: "Kiki", silliness: .extremelySilly)
  let carl = Monkey(name: "Carl", silliness: .notSilly)
  let jane = Monkey(name: "Jane", silliness: .verySilly)
  let sillyMonkeys = silliest([kiki, carl, jane])
  XCTAssertTrue(contains(sillyMonkeys, kiki))
}

The test fails with the following failure message:

XCTAssertTrue failed

The failure message leaves a lot to be desired. It leaves us wondering, "OK, so something that should have been true was false--but what?" That confusion slows us down, since we now have to spend time deciphering test code.

Better Failure Messages, Part 1: Manually Providing XCTAssert Failure Messages

XCTAssert assertions allow us to specify a failure message of our own, which certainly helps:

func testSilliest_whenMonkeysContainSillyMonkeys_theyreIncludedInTheResult() {
  let kiki = Monkey(name: "Kiki", silliness: .extremelySilly)
  let carl = Monkey(name: "Carl", silliness: .notSilly)
  let jane = Monkey(name: "Jane", silliness: .verySilly)
  let sillyMonkeys = silliest([kiki, carl, jane])
-  XCTAssertTrue(contains(sillyMonkeys, kiki))
+  XCTAssertTrue(contains(sillyMonkeys, kiki), "Expected sillyMonkeys to contain 'Kiki'")
}

But we have to write our own failure message.

Better Failure Messages, Part 2: Nimble Failure Messages

Nimble makes your test assertions, and their failure messages, easier to read:

func testSilliest_whenMonkeysContainSillyMonkeys_theyreIncludedInTheResult() {
  let kiki = Monkey(name: "Kiki", silliness: .extremelySilly)
  let carl = Monkey(name: "Carl", silliness: .notSilly)
  let jane = Monkey(name: "Jane", silliness: .verySilly)
  let sillyMonkeys = silliest([kiki, carl, jane])
-  XCTAssertTrue(contains(sillyMonkeys, kiki), "Expected sillyMonkeys to contain 'Kiki'")
+  expect(sillyMonkeys).to(contain(kiki))
}

We don't have to write our own failure message--the one provided by Nimble is already very readable:

expected to contain <Monkey(name: Kiki, sillines: extremelySilly)>,
                got <[Monkey(name: Jane, silliness: verySilly)]>

The failure message makes it clear what's wrong: we were expecting kiki to be included in the result of silliest(), but the result only contains jane. Now that we know exactly what's wrong, it's easy to fix the issue:

public func silliest(monkeys: [Monkey]) -> [Monkey] {
-  return monkeys.filter { $0.silliness == .verySilly }
+  return monkeys.filter { $0.silliness == .verySilly || $0.silliness == .extremelySilly }
}

Nimble provides many different kind of assertions, each with great failure messages. And unlike XCTAssert, you don't have to type your own failure message every time.

For the full list of Nimble assertions, check out the Nimble README. Below is just a sample, to whet your appetite:

expect(1 + 1).to(equal(2))
expect(1.2).to(beCloseTo(1.1, within: 0.1))
expect(3) > 2
expect("seahorse").to(contain("sea"))
expect(["Atlantic", "Pacific"]).toNot(contain("Mississippi"))
expect(ocean.isClean).toEventually(beTruthy())