State of Mocking in Swift Tests

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Mocking and stubbing in tests is useful to verify behavior and replace collaborating objects with simpler alternatives. Since Swift is statically typed, you can’t use a mocking library anymore. But it’s trivially simple to write simple mock objects as subclasses in place:

class Bar { func bar() {} }

func foo(bar: Bar) {
    bar.bar()
}
class FooTest: XCTestCase {
    func testFoo() {
        class MockBar: Bar { /* ... */ }
        let barDouble = MockBar()
    
        let baz = foo(barDouble)
    
        XCTAssertNotNil(baz)
    }
}

You can declare subclasses in the scope of the test method and override only the minimum necessary parts.

I like to take the additional safety measure to exclusively descend from Null-Objects in such cases. If foo() called another method on its Bar parameter than expected, we end up with the real behavior and real side-effects. The Null-Object pattern ensures the parent class does nothing for every method, not just the one we override in a specific test.

strawman
Photo Credit: 稻田 ricefilm Strawman by earl258. License: CC-BY-NC 2.0

Using subclasses private to a method is clever, but it doesn’t cut it. You can write stubs easily, but you cannot write mock objects which record incoming calls for verification without a lot of hassle.

XCTestExpectation would work to verify calls – but it’s meant for asynchronous tests. Calling waitForExpectationsWithTimeout(0) just to verify if a synchronous expectation is fulfilled is overkill. (Using larger timeouts is a bad idea as it slows down your test suite without any benefit.)

Also, I get segmentation faults all the time:

func testFoo() {
    let expectation = expectationWithDescription("called")
    class MockBar: Bar {
        private override func bar() {
            expectation.fulfill() // comment-out this line to not raise segfaults
        }
    }

    let barDouble = MockBar()

    foo(barDouble)

    waitForExpectationsWithTimeout(0) { error in }
}

Same for booleans, which otherwise sound like a reasonable alternative:

func testFoo() {
    var expectation = false
    
    class MockBar: Bar {
        private override func bar() {
            expectation = true
        }
    }

    let barDouble = MockBar()

    foo(barDouble)

    XCTAssertTrue(expectation)
}

So we end up with the more clumsy variation of boolean properties:

func testFoo() {
    class MockBar: Bar {
        var didCallBar = false
        private override func bar() {
            didCallBar = true
        }
    }
    
    let barDouble = MockBar()
    
    foo(barDouble)
    
    XCTAssertTrue(barDouble.didCallBar)
}

This sucks, but it works.

Meanwhile, I filed a Radar and will now figure out if this is worth changing structs to classes in production code. Because structs cannot be sub_classed_.

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