Test Double in Swift - Dummy

Dummy objects are passed around but never actually used. Usually, they are just used to fill parameter lists. e.g. Type attributes or method parameters.

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Sometimes we need to pass an object just to satisfy language syntax, but we don’t care about its value. We can use dummy arguments whenever a method of the sut(system under test) take objects as arguments and those objects are not relevant to the test.

The simplest implementation of Dummy Object is to pass a null as the parameter. The biggest disadvantage of using null is that it is not very descriptive. Clearly communicated to your reads which arguments are dummy arguments. For a string, this can be done by setting the value to "Dummy" or "", for numbers, 0 is often clear enough. For other types, extract a variable and give it a name starting with dummy

String arguments

class Greet {
    func greeting(_ name: String) -> String {
        return "Hello, \(name)"
    }
}

func test_greeting_shouldStartWithHello() {
    let sut = Greet()

    let result = sut.greeting("Dummy")

    XCTAssertTrue(result.hasPrefix("Hello,"))
}

Type in Spy

sut never care the DummyURLSessionDataTask in Spy, it’s just satisfy the resume() called by session’s task in production code.

protocol URLSessionProtocol {
    func dataTask(
           with url: URL,
           completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void
    ) -> URLSessionDataTask
}

var session: URLSessionProtocol = URLSession.shared

let dataTask = session.dataTask(with: url) {
    [weak self] (data: Data?, response: URLResponse?, error: Error?)
    -> Void in
        doSomething()
}
dataTask?.resume()

class SpyURLSession: URLSessionProtocol {
    var dataTaskCallCount = 0
    var dataTaskArgsURL: [URL] = []
    var dataTaskArgsCompletionHandler: [(Data?, URLResponse?, Error?) -> Void] = []

    func dataTask(
            with url: URL,
            completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void
    ) -> URLSessionDataTask {
        dataTaskCallCount += 1
        dataTaskArgsURL.append(url)
        dataTaskArgsCompletionHandler.append(completionHandler)
        return DummyURLSessionDataTask()
    }
}

private class DummyURLSessionDataTask: URLSessionDataTask {
    override func resume() {  }
}