Escaping callback closures execution order

[kublaios]

In my use case, I want to access a variable that's assigned during Given/When steps in Then step. Checking the callback closure in Given/When initializers, I see that they are escaping. I want to understand the risks when I assume that the closures will run serially like my example below.

struct MyFeature {
    static func setupSteps(app: XCUIApplication) {
        Given("^The user is on home screen$") { _, _ in
            XCTAssertTrue(app.staticTexts["Welcome!"].exists)
        }
        var mapScreen: MapScreen!
        When("^Map button is tapped$") { _, _ in
            mapScreen = HomeScreen(app: app).tapMapButton()
        }
        Then("^Map is visible$") { _, _ in
            XCTAssertTrue(mapScreen.verifyMapViewVisible())
        }
    }
}

Note that this code works as expected, meaning I have mapScreen assigned by the time the execution reaches the line I use it.

[Tyler-Keith-Thompson]

The guarantee the library makes is that these will execute in the order defined in your .feature file. That means I could crash your tests by writing:

Given The user is on home screen
Then Map is visible

However, unless I maliciously manipulate your Gherkin (or you write such generic steps that accidental re-use causes issues) your code should be safe.

One way you could choose to rewrite this so that it fails, but doesn't crash is:

Given("^The user is on home screen$") { _, _ in
    XCTAssertTrue(app.staticTexts["Welcome!"].exists)
}
var mapScreen: MapScreen?
When("^Map button is tapped$") { _, _ in
    mapScreen = HomeScreen(app: app).tapMapButton()
}
Then("^Map is visible$") { _, _ in
    let screen = try XCTUnwrap(mapScreen) // any errors thrown will get caught by XCTest
    XCTAssertTrue(screen.verifyMapViewVisible())
}

[Tyler-Keith-Thompson]

One interesting note: the library doesn't guarantee thread safety. I'll be adding async closures to steps at some point. Really all that means is you're responsible for your fate in that regard. So you could also crash this simply by wrapping your assignment to mapScreen in a background thread. That'd cause a race condition.

[kublaios]

Thanks, that's all I needed to know.