How can I pass a dependancy to a URLProtocol subclass?

I have been working on an interceptor for adding auth headers to network requests in my app.

final class AuthInterceptor: URLProtocol {


    private var token: String = "my.access.token"   
    private var dataTask: URLSessionTask?
    private struct UnexpectedValuesRepresentationError: Error { }
    
    override class func canInit(with request: URLRequest) -> Bool {
        guard URLProtocol.property(forKey: "is_handled", in: request) as? Bool == nil else { return false }

        return true
        
        //return false // URL Loading System will handle the request using the system’s default behavior
    }
    
    override class func canonicalRequest(for request: URLRequest) -> URLRequest {
        return request
    }
    
    override func startLoading() {
        guard let mutableRequest = (request as NSURLRequest).mutableCopy() as? NSMutableURLRequest else { return }
        URLProtocol.setProperty(true, forKey: "is_handled", in: mutableRequest)
        mutableRequest.addValue(token, forHTTPHeaderField: "Authorization")
        
        dataTask = URLSession.shared.dataTask(with: mutableRequest as URLRequest) { [weak self] data, response, error in
            guard let self = self else { return }
            if let error = error {
                self.client?.urlProtocol(self, didFailWithError: error)
            } else if let data = data, let response = response {
                self.client?.urlProtocol(self, didLoad: data)
                self.client?.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)
            } else {
                self.client?.urlProtocol(self, didFailWithError: UnexpectedValuesRepresentationError())
            }
            self.client?.urlProtocolDidFinishLoading(self)
        }
        
        dataTask?.resume()
    }
    
    override func stopLoading() {
        dataTask?.cancel()
        dataTask = nil
    }
}

As you can see I am currently just using private var token: String = "my.access.token" to mock a token. I'd like to introduce a TokenLoader that will fetch my token from it's cache.

As the URL Loading system will initialize instances of my protocol as needed, I am not sure how I can inject this. It is currently registered using: URLProtocol.registerClass(AuthInterceptor.self)

Imagine I had an interface like this -

public typealias LoadTokenResult = Result<String, Error>
public protocol TokenLoader {
  func load(_ key: String, completion: @escaping (LoadTokenResult) -> Void)
}

I'd like to ensure this is testable so I'd expect to be able to stub this in a test case or use a spy.

How can I achieve this?

from How can I pass a dependancy to a URLProtocol subclass?