XCTWaiter.wait() timeout seems to take longer sometimes

Question

To add delays in my tests I implemented this:

func execute(after: TimeInterval, testBlock: () -> Void) {
    let result = XCTWaiter.wait(for: [expectation(description: "Delayed Test")], timeout: after)

    if result == XCTWaiter.Result.timedOut {
        testBlock()
    } else {
        XCTFail("Delay interrupted.")
    }
}

Then I wrote a test:

func testExecute() {
    var i = 1

    DispatchQueue.main.asyncAfter(deadline: .now() + 0.40) {
        i = 2
    }

    execute(after: 0.20) {
        XCTAssert(i == 1)
    }

    execute(after: 0.15) {
        XCTAssert(i == 1)   // Fails once every three or four runs.
    }

    execute(after: 0.06) {  // Never fails.
        XCTAssert(i == 2)
    }
}

Why does this second XCTAssert() fail regularly?

This is the only thing running on my simulator. You'd expect some jitter, but shouldn't that stay with 1 or 2 times the system clock period of 1/60s?

Answer 1

It turns out that delays may take up to considerably longer (up to 200ms in this 2011 experiment: http://atastypixel.com/blog/experiments-with-precise-timing-in-ios/).

Must take sufficient margins when using this execute(after:testBlock:) function.