Swift - Protocol can only be used as a generic constraint because it has Self or associated type requirements

You have to turn these requirements around;

Instead of injecting a MicroServiceProvider into each request, you should write a generic MicroService 'Connector' Protocol that should define what it expects from each request, and what each request expects it to return.

You can then write a TestConnector which conforms to this protocol, so that you have complete control over how your requests are handled. The best part is, your requests won't even need to be modified.

Consider the following example:

protocol Request {    // What type data you expect to decode and return    associatedtype Response    // Turn all the data defined by your concrete type     // into a URLRequest that we can natively send out.    func makeURLRequest() -> URLRequest    // Once the URLRequest returns, decode its content    // if it succeeds, you have your actual response object     func decode(incomingData: Data?) -> Response?}protocol Connector {    // Take in any type conforming to Request,     // do whatever is needed to get back some potential data,     // and eventually call the handler with the expected response    func perform<T: Request>(request: T, handler: @escaping (T.Response?) -> Void)}

These are essentially the bare minimum requirements to setup such a framework. In real life, you'll want more requirements from your Request protocol (such as ways to define the URL, request headers, request body, etc).

The best part is, you can write default implementations for your protocols. That removes a lot of boilerplate code! So for an actual Connector, you could do this:

extension Connector {    func perform<T: Request>(request: T, handler: @escaping (T.Response?) -> Void) {        // Use a native URLSession        let session = URLSession()        // Get our URLRequest        let urlRequest = request.makeURLRequest()        // define how our URLRequest is handled        let task = session.dataTask(with: urlRequest) { data, response, error in            // Try to decode our expected response object from the request's data            let responseObject = request.decode(incomingData: data)            // send back our potential object to the caller's completion block            handler(responseObject)        }        task.resume()    }}

Now, with that, all you need to do is implement your ProfilePictureRequest like this (with extra example class variables):

struct ProfilePictureRequest: Request {    private let userID: String    private let useAuthentication: Bool    /// MARK: Conform to Request    typealias Response = UIImage    func makeURLRequest() -> URLRequest {        // get the url from somewhere        let url = YourEndpointProvider.profilePictureURL(byUserID: userID)        // use that URL to instantiate a native URLRequest        var urlRequest = URLRequest(url: url)        // example use: Set the http method        urlRequest.httpMethod = "GET"        // example use: Modify headers        if useAuthentication {            urlRequest.setValue(someAuthenticationToken.rawValue, forHTTPHeaderField: "Authorization")        }        // Once the configuration is done, return the urlRequest        return urlRequest    }    func decode(incomingData: Data?) -> Response? {        // make sure we actually have some data        guard let data = incomingData else { return nil }        // use UIImage's native data initializer.        return UIImage(data: data)    }}

If you then want to send a profile picture request out, all you then need to do is (you'll need a concrete type that conforms to Connector, but since the Connector protocol has default implementations, that concrete type is mostly empty in this example: struct GenericConnector: Connector {}):

// Create an instance of your request with the arguments you desirelet request = ProfilePictureRequest(userID: "JohnDoe", useAuthentication: false)// perform your request with the desired ConnectorGenericConnector().perform(request) { image in     guard let image = image else { return }    // You have your image, you can now use that instance whichever way you'd like    ProfilePictureViewController.current.update(with: image)}

And finally, to set up your TestConnector, all you need to do is:

struct TestConnector: Connector {    // define a convenience action for your tests    enum Behavior {        // The network call always fails        case alwaysFail        // The network call always succeeds with the given response        case alwaysSucceed(Any)    }    // configure this before each request you want to test    static var behavior: Behavior    func perform<T: Request>(request: T, handler: @escaping (T.Response?) -> Void) {        // since this is a test, you don't need to actually perform any network calls.        // just check what should be done        switch Self.behavior {        case alwaysFail:            handler(nil)        case alwaysSucceed(let response):            handler(response as! T)        }    }}

With this, you can easily define Requests, how they should configure their URL actions and how they decode their own Response type, and you can easily write mocks for you connectors.

Of course, keep in mind that the examples given in this answer are quite limited in how they can be used. I would highly suggest you to take a look at this library I wrote. It extends this example in a much more structured way.