Testable Asynchronous Design Pattern in Swift - ios

Question

I'm learning Test Driven Development in Swift. I hit a wall when I realized the delegate pattern I regularly use for asynchronous requests is difficult to test. I've learned that if something's difficult to test, the design pattern behind the implementation could probably be better. This is confusing me because I think the delegate pattern I'm using is common and I'm wondering how others have dealt with this issue.
The pattern:
I wrote a service, which executes an asynchronous request in a static function which takes a delegate instance. The delegate instance conforms to a protocol which requires implementation of a success and failure method. I've contrived an example which hits Google.com. Please ignore the Type safety issues in this example. The actual code I'm running to hit an endpoint and parse JSON is safer. I just wanted to come up with a very small snippet of code to depict the issue that's causing difficulty while testing:
protocol GoogleServiceDelegate {
func gotGoogle(str: String);
func gotError(str: String);
}
struct GoogleService {
static func getGoogle(delegate: GoogleServiceDelegate) {
let url: NSURL! = NSURL(string: "http://google.com")
NSURLSession.sharedSession().dataTaskWithURL(url) { data, response, error in
if let data = data {
let str: NSString! = NSString(data: data, encoding: NSUTF8StringEncoding)
delegate.gotGoogle(str as String)
} else {
delegate.gotError("\(error)")
}
}
}
}
Here's the test which illustrates the problem:
class AsyncTestingTests: XCTestCase {
func testExample() {
let responseExpectation = expectationWithDescription("Got google response!")
struct GoogleDelegate: GoogleServiceDelegate {
func gotGoogle(str: String) {
// expectations about response
responseExpectation.fulfill()
}
func gotError(str: String) {
// expectations about error
responseExpectation.fulfill()
}
}
let myGoogleServiceDelegate = GoogleDelegate()
GoogleService.getGoogle(myGoogleServiceDelegate)
waitForExpectationsWithTimeout(5) { _ in
print("Never got a response from Google :(")
}
}
}
The problem arises at the two .fulfill() lines. I get the following error from Xcode:
Struct declaration cannot close over value 'responseExpectation' defined in outer scope
I understand the error, but am unsure what to adjust... Is there a workaround for this which I can use in the test, or is there a better (easily testable) pattern for asynchronous callbacks than what I am attempting? If you know of a better testable solution, would you mind taking the time to write down an example?

Answer 1

Yes, you can not close over variables defined outside of struct, to workaround, we need to use closures/functions and pass it to the struct. Methods in struct can invoke it when they receive the response.
func testExample() {
let responseExpectation = expectationWithDescription("Got google response!")
//Let a function capture the fulfilling of the expectation
func fullFillExpectation(){
responseExpectation.fullFill()
}
struct GoogleDelegate: GoogleServiceDelegate {
var fullFiller : (()->Void)!
func gotGoogle(str: String) {
// expectations about response via invoke the closure
fullFiller()
}
func gotError(str: String) {
// expectations about error - invoke the closure
fullFiller()
}
}
//Create the delegate with full filler function.
let myGoogleServiceDelegate = GoogleDelegate(fullFiller: fullFillExpectation)
GoogleService.getGoogle(myGoogleServiceDelegate)
waitForExpectationsWithTimeout(5) { _ in
print("Never got a response from Google :(")
}
}
}
PS: I could not test this, please test and let me know.