A simple MockURLProtocol for Unit/Integration tests relying on URLSession.

MockURLProtocol.swift

//
//  MockURLProtocol.swift
//

/// Inspired by [Testing Tips & Tricks](https://developer.apple.com/videos/play/wwdc2018/417).
final class MockURLProtocol: URLProtocol {
    static nonisolated(unsafe) var responseBuilders: [(URLRequest) throws -> (Data, HTTPURLResponse, URLRequest?)] = []

    override static func canInit(with request: URLRequest) -> Bool {
        !responseBuilders.isEmpty
    }

    override static func canonicalRequest(for request: URLRequest) -> URLRequest {
        request
    }

    override func startLoading() {
        do {
            let responseBuilder = Self.responseBuilders.removeFirst()

            let (data, response, request) = try responseBuilder(request)

            client?.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)

            if let request {
                client?.urlProtocol(self, wasRedirectedTo: request, redirectResponse: response)
            }

            client?.urlProtocol(self, didLoad: data)

            client?.urlProtocolDidFinishLoading(self)
        } catch {
            client?.urlProtocol(self, didFailWithError: error)
        }
    }

    override func stopLoading() {}
}

// MARK: Snipet of a simple HTTP Client

public extension HTTP {
    private final class DoNotFollowRedirectsDelegate: NSObject, URLSessionTaskDelegate {
        func urlSession(
            _ session: URLSession,
            task: URLSessionTask,
            willPerformHTTPRedirection response: HTTPURLResponse,
            newRequest request: URLRequest
        ) async -> URLRequest? {
            nil
        }
    }

    final class Client: Sendable {
        // ...

        private func send(_ request: HTTP.Request) async throws -> SendResult {
            // ...

            var (data, httpURLResponse) = try await URLSession.shared.data(
                for: urlRequest, delegate: request.followRedirects ? nil : DoNotFollowRedirectsDelegate()
            )

            // ...
        }
    }
}

// MARK: Usage (with automatic redirect)

let configuration = URLSessionConfiguration.ephemeral
configuration.protocolClasses = [MockURLProtocol.self]

let session = URLSession(configuration: configuration)
let httpClient = HTTP.Client(session: session)

let redirectingResponse = HTTPURLResponse()
let redirectingResponseData = Data("Some Redirecting Body".utf8)

let response = HTTPURLResponse()
let responseData = Data("Some Body".utf8)

MockURLProtocol.responseBuilders = [
    { request in
        (redirectingResponseData, redirectingResponse, request)
    },
    { request in
        (responseData, response, nil)
    },
]

let url = URL(string: "https://example.ios")!

let request = HTTP.Request(
    url: url,
    method: .post,
    body: Data(),
    headers: [
        .userAgent("Some User-Agent"),
        .accept(.json)
    ],
    followRedirects: true
)

let httpResponse = try await httpClient.send(
    request,
    tags: ["My Tag": "Hello World!"]
)

#expect(httpResponse.body == responseData)

// MARK: Usage (without automatic redirect)

let configuration = URLSessionConfiguration.ephemeral
configuration.protocolClasses = [MockURLProtocol.self]

let session = URLSession(configuration: configuration)
let httpClient = HTTP.Client(session: session)

let redirectingResponse = HTTPURLResponse()
let redirectingResponseData = Data("Some Redirecting Body".utf8)

let response = HTTPURLResponse()
let responseData = Data("Some Body".utf8)

MockURLProtocol.responseBuilders = [
    { request in
        (redirectingResponseData, redirectingResponse, request)
    },
    { request in
        (responseData, response, nil)
    },
]

let url = URL(string: "https://example.ios")!

let request = HTTP.Request(
    url: url,
    method: .post,
    body: Data(),
    headers: [
        .userAgent("Some User-Agent"),
        .accept(.json)
    ],
    followRedirects: false
)

let httpResponse = try await httpClient.send(
    request,
    tags: ["My Tag": "Hello World!"]
)

#expect(httpResponse.body == redirectingResponseData)