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.

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() {  }
}