When I started using Swift I decided to unit test all the code that I wrote, the intent was that coding test-first would allow me to concentrate on the language more efficiently. To aid unit testing I have started to put together a bast test class with common helper methods. The criteria for the helpers are that they

• Are reusable across many tests (obviously)
• Help me learn the intricacies of Swift¹
• Help me, help myself and stop me from repeating past mistakes

In programming for the apple watch I’ve been passing data back to to the watch in an NSData serialized representation using NSKeyedArchivers. Unit testing these is usually a snip, I serialize a class with a known value, deserialize a copy and compare the values with the expected value from the original.

To test this code I originally place a helper method into the test class to do the archive/unarchive step. As I started the second class using the same technique it mandated the code be moved into the base class and generalized.

class TKUnitTest: XCTestCase {
    func cloneViaCoding(root: T) -> T {
        var data = NSKeyedArchiver.archivedDataWithRootObject(root)
        var res = NSKeyedUnarchiver.unarchiveObjectWithData(data) as T
        return res
    }
}

By using a generic method we can provide a nice clean type safe clone method and our unit test is

 func testCloneSetNameCorrectly() {
        let o = TKMiniLineStatus(name: "name", status: "")
        let res = self.cloneViaCoding(o)
        XCTAssertEqual("name", res.name)
    }

As usual I test only one thing per test case so there will be another method to test status etc.

That’s all good. Next I wrote another class with archiving, however, on that class all of my tests failed! It took me a few minutes to spot the error, I’d forgot to extend NSCoding (which is easy to do with optional protocols). This got me to thinking about how to avoid making this mistake again (point 3 above).

A quick flick through the Swift Programming Language manual from Apple lead to the section on Type Constraint Syntax. So I extended the unit test to mandate that the type T being cloned must implement the NSCoding protocol.

class TKUnitTest: XCTestCase {
    func cloneViaCoding<T: NSCoding>(root: T) -> T {
        var data = NSKeyedArchiver.archivedDataWithRootObject(root)
        var res = NSKeyedUnarchiver.unarchiveObjectWithData(data) as T
        return res
    }
}

Now instead of the clone name test failing, leaving me head scratching as to why, I get a nice clear build error, for eaxmple this test

    func testCompileTimeSupport() {
        let tmp = ColorTool();
        let res = self.cloneViaCoding(tmp)
    }

gets this build error

Type 'ColorTool' does not conform to protocol 'NSCoding'

Sweet :-)

¹ Yes it’s a bad idea to drive class design through language features but this is a learning project