Refactoring Legacy Code: Replace Free Function in Tests, Even Swift's Own Functions

Refactoring Legacy Code is hard. There are a few safe refactorings you can do with caution. But most chirurgical cuts require you to put the code in a test harness first to guard against regression.

With C and Swift, you can create free functions as part of your app. To verify your objects use that function, you need to find a way to insert a test double.

It’s possible to inject a different implementation for C functions during tests. Swift poses a different challenge, though.

When you have a free function in Swift and want to replace the default behavior with a mock object or a stub, here’s the steps you need to take:

  1. Refactor the function to delegate to a global closure in a variable.
  2. In your test suite, copy the default closure during setUp().
  3. Replace the closure with a test double.
  4. Ensure to set the closure back to its default during tearDown().

This could look like the following:

var fetchDataFromServerBlock: (ServerConnection) -> TheData = { conn in
    // ...
    return TheData(rawData: conn.data())
}

func fetchDataFromServer(conn: ServerConnection) -> TheData {
    return fetchDataFromServerBlock(conn)
}

And in tests:

class DataManglerTests: XCTestSuite {
    var originalFetchBlock: ((ServerConnection) -> TheData)!

    func setUp() {
        super.setUp()
        originalFetchBlock = fetchDataFromServerBlock
    }
    
    func tearDown() {
        fetchDataFromServerBlock = originalFetchBlock
        super.tearDown()
    }
    
    let aTheDataStub = TheDataStub()
    
    func testManglingFetchesData() {
        let mangler = DataMangler()
        var didFetch = false
        fetchDataFromServerBlock = { _ in 
            didFetch = true
            return aTheDataStub
        }
        
        mangler.mangle()
        
        XCTAssert(didFetch)
    }
    
    class TheDataStub: TheData {
        // ...
    }
}

To make this even more robust, consider adding a defaultFetchDataFromServerBlock to switch implementations during tests:

var fetchDataFromServerBlock = defaultFetchDataFromServerBlock
let defaultFetchDataFromServerBlock: (ServerConnection) -> TheData { conn in
    // ...
    return TheData(rawData: conn.data())
}

And in tests:

class DataManglerTests: XCTestSuite {
    
    func tearDown() {
        fetchDataFromServerBlock = defaultFetchDataFromServerBlock
        super.tearDown()
    }
    
    // ...
}

Swift is merely a year old. There won’t be much legacy code written in Swift to deal with. But this technique applies to free functions in Objective-C as well: delegate to blocks and replace blocks during tests.

You can use this technique to test free functions from Swift’s standard library, too: override the method in your module, then delegate to the original with the Swift module prefix. If you want to see an example, Nikolaj Schumacher wrote a Gist to replace precondition during tests.

Check out Michael Feather’s “Working Effectively with Legacy Code” to learn more about safe refactorings and putting legacy code under test.


Picture: “Inca” by Ajar on flickr. License: CC-BY-SA.

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