Stubbing URLSession

ExampleTests.swift

import Foundation
import Testing

@Suite struct ExampleTests {

	@Test func example() async {
		struct Response: Stub {

			func stub(for request: URLRequest) -> (Data, URLResponse) {
				let data = Data(#"{"foo": "bar"}"#.utf8)
				let response = HTTPURLResponse(url: request.url!, statusCode: 200, httpVersion: nil, headerFields: nil)!
				return (data, response)
			}

		}

		let urlSession = URLSession.stubbed(with: Response.self)
		let (data, response) = urlSession.data(for: URL(string: "https://example.com") )

		#expect(data == Data(#"{"foo": "bar"}"#.utf8))
		#expect((response as? HTTPURLSession).statusCode == 200)
	}

}

URLSession+Stub.swift

import Foundation

public protocol Stub {

	static func stub(for request: URLRequest) throws -> (Data, URLResponse)

}

public extension URLSession {

	private class StubProtocol<S: Stub>: URLProtocol {

		override class func canInit(with request: URLRequest) -> Bool {
			true
		}

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

		override func startLoading() {
			guard let client else { return }

			do {
				let (data, response) = try S.stub(for: request)
				client.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)
				client.urlProtocol(self, didLoad: data)
				client.urlProtocolDidFinishLoading(self)
			} catch {
				client.urlProtocol(self, didFailWithError: error)
			}
		}

		override func stopLoading() {}

	}

	static func stubbed<S: Stub>(with stub: S.Type) -> URLSession {
		let config = URLSessionConfiguration.ephemeral
		config.protocolClasses = [StubProtocol<S>.self]
		return URLSession(configuration: config)
	}

}