Generating dynamic test cases with XCTest

While working on the article about resizable fonts it became clear that getting a preview of every font size without needing to use the accessibility inspector was going to be a pain.

Instead, it would be great to use UI tests and dynamically re-launch the app with a different size for each variation and save a screenshot in the build results. You can see the full example in the sample app for font size testing or check out the simpler example below.

Originally, I referenced this great article by Brian Coyner from 2015, but it seemed to be out of date a bit, and I was hopeful that with some of the recent work done on XCTest it would be possible to update things just a bit.

Here's a quick example:

fileprivate let _widgetSizes: [CGFloat] = [
    0.0, 1.0, 2.0, 4.0, 8.0,
]

struct WidgetExpander {
    var widgetSize: CGFloat

    var isValid: Bool {
        return true // ;) 
    }
}

class WidgetExpanderTest: XCTestCase {

    var widgetSize: CGFloat!

    override class var defaultTestSuite: XCTestSuite {
        let suite = XCTestSuite(forTestCaseClass: WidgetExpanderTest.self)

        _widgetSizes.forEach { size in
            // Generate a test for our specific selector
            let test = WidgetExpanderTest(selector: #selector(verifyWidget))

            // Each test will take the size argument and use the instance variable in the test
            test.widgetSize = size

            // Add it to the suite, and the defaults handle the rest
            suite.addTest(test)
        }
        return suite
    }

    // NOTE: Don't name this method with "test" or it'll 
    // still get picked up by the automatic test case generation.
    @objc func verifyWidget() {
        XCTAssertTrue(WidgetExpander(size: widgetSize).isValid)
    }
}

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