How would you test this generic API? - swift

Question

I've got the following APIRequest protocol:
protocol APIRequest {
associatedtype Result
var httpMethod: HTTPMethod { get }
var pathComponents: [String] { get }
func handle(responseData: Data) throws -> Result
}
Additionally, I've got the following APIClient protocol:
protocol APIClientProtocol {
func perform<T : APIRequest>(_ request: T,
completion: #escaping ((Result<T.Result, APIError>) -> Void))
}
Then I've got a class which takes an APIClientProtocol and make the request. IE:
final class DataSource {
let client: APIClientProtocol
init(client: APIClientProtocol) {
self.client = client
}
func fetchThing(completion: #escaping (Result<Thing, APIError>) -> Void) {
let thingRequest = ThingRequest() // This is an APIRequest
client.perform(thingRequest, completion: { result in
switch result {
case .success(let thing):
completion(.success(thing))
case .failure(let error):
completion(.failure(error))
}
}
}
Now I want to write a test for DataSource but I need to mock APIClientProtocol in order to do that. How can I mock it?

Answer 1

Using a mock of URLProtocol is thinking too deep.
There are two basic ways to mock API.
Create a full mock server and connect to it instead of your real server. This way you can create integration tests.
Just mock the response directly
class APIClientProtocolMock: APIClientProtocol {
func perform<T : APIRequest>(
_ request: T,
completion: #escaping ((Result<T.Result, APIError>) -> Void)
) {
// replace with a background queue depending on your implementation
DispatchQueue.main.async {
completion(.success(/* put your mock response here */))
}
}
}
To be honest, I am not a big friend of unit testing on FE with extensive mocking because e2e tests (UI tests) do a much better job and you don't have to create hundreds of unnecessary protocols for mocking to work.
A real example:
protocol APIRequest {
associatedtype Result
var httpMethod: String { get }
var pathComponents: [String] { get }
func handle(responseData: Data) throws -> Result
}
protocol APIClientProtocol {
func perform<T : APIRequest>(
_ request: T,
completion: #escaping ((Result<T.Result, Error>) -> Void)
)
}
struct MockAPIRequest: APIRequest {
typealias Result = String
let httpMethod: String = "POST"
let pathComponents: [String] = ["url"]
func handle(responseData: Data) throws -> Result {
return "success"
}
}
struct SecondMockAPIRequest: APIRequest {
typealias Result = String
let httpMethod: String = "POST"
let pathComponents: [String] = ["url"]
func handle(responseData: Data) throws -> Result {
return "failed"
}
}
class MockAPIClientProtocol: APIClientProtocol {
var mockData: [String: Data] = [:]
// you can load mock data dynamically
func perform<T : APIRequest>(_ request: T, completion: #escaping ((Result<T.Result, Error>) -> Void)) {
let data = mockData[request.pathComponents.joined(separator: "/")] ?? Data()
completion(.success(try! request.handle(responseData: data)))
}
// you can add implementation for a specific type
func perform(_ request: SecondMockAPIRequest, completion: #escaping ((Result<SecondMockAPIRequest.Result, Error>) -> Void)) {
let data = mockData[request.pathComponents.joined(separator: "/")] ?? Data()
completion(.success(try! request.handle(responseData: data)))
}
}
let client = MockAPIClientProtocol()
client.mockData["url"] = Data()
client.perform(MockAPIRequest()) { result in
print(result) // success
}
client.perform(SecondMockAPIRequest()) { result in
print(result) // failed
}