Wait for URLProtocol to start loading requests before cancelling the …

…URLSessionTasks when running tests to achieve predictable test results and prevent tasks leaking outside the test scope (which could affect other tests).

EssentialFeed/EssentialFeedTests/Shared API Infra/Helpers/URLProtocolStub.swift

@@ -9,6 +9,7 @@ class URLProtocolStub: URLProtocol {
 		let data: Data?
 		let response: URLResponse?
 		let error: Error?
+		let shouldCancelTask: Bool
 		let requestObserver: ((URLRequest) -> Void)?
 	}
 
@@ -21,11 +22,15 @@ class URLProtocolStub: URLProtocol {
 	private static let queue = DispatchQueue(label: "URLProtocolStub.queue")
 
 	static func stub(data: Data?, response: URLResponse?, error: Error?) {
-		stub = Stub(data: data, response: response, error: error, requestObserver: nil)
+		stub = Stub(data: data, response: response, error: error, shouldCancelTask: false, requestObserver: nil)
 	}
 
-	static func observeRequests(observer: @escaping (URLRequest) -> Void) {
-		stub = Stub(data: nil, response: nil, error: nil, requestObserver: observer)
+	static func cancelIncomingTasks() {
+		stub = Stub(data: nil, response: nil, error: nil, shouldCancelTask: true, requestObserver: nil)
+	}
+
+	static func observeRequests(shouldFinish: Bool = true, observer: @escaping (URLRequest) -> Void) {
+		stub = Stub(data: nil, response: nil, error: nil, shouldCancelTask: false, requestObserver: observer)
 	}
 
 	static func removeStub() {
@@ -43,6 +48,11 @@ class URLProtocolStub: URLProtocol {
 	override func startLoading() {
 		guard let stub = URLProtocolStub.stub else { return }
 
+		if stub.shouldCancelTask {
+			task?.cancel()
+			return
+		}
+
 		if let data = stub.data {
 			client?.urlProtocol(self, didLoad: data)
 		}

EssentialFeed/EssentialFeedTests/Shared API Infra/URLSessionHTTPClientTests.swift

@@ -29,11 +29,9 @@ class URLSessionHTTPClientTests: XCTestCase {
 	}
 
 	func test_cancelGetFromURLTask_cancelsURLRequest() {
-		let exp = expectation(description: "Wait for request")
-		URLProtocolStub.observeRequests { _ in exp.fulfill() }
+		URLProtocolStub.cancelIncomingTasks()
 
-		let receivedError = resultErrorFor(taskHandler: { $0.cancel() }) as NSError?
-		wait(for: [exp], timeout: 1.0)
+		let receivedError = resultErrorFor() as NSError?
 
 		XCTAssertEqual(receivedError?.code, URLError.cancelled.rawValue)
 	}
@@ -104,8 +102,8 @@ class URLSessionHTTPClientTests: XCTestCase {
 		}
 	}
 
-	private func resultErrorFor(_ values: (data: Data?, response: URLResponse?, error: Error?)? = nil, taskHandler: (HTTPClientTask) -> Void = { _ in }, file: StaticString = #filePath, line: UInt = #line) -> Error? {
-		let result = resultFor(values, taskHandler: taskHandler, file: file, line: line)
+	private func resultErrorFor(_ values: (data: Data?, response: URLResponse?, error: Error?)? = nil, file: StaticString = #filePath, line: UInt = #line) -> Error? {
+		let result = resultFor(values, file: file, line: line)
 
 		switch result {
 		case let .failure(error):
@@ -116,17 +114,17 @@ class URLSessionHTTPClientTests: XCTestCase {
 		}
 	}
 
-	private func resultFor(_ values: (data: Data?, response: URLResponse?, error: Error?)?, taskHandler: (HTTPClientTask) -> Void = { _ in },  file: StaticString = #filePath, line: UInt = #line) -> HTTPClient.Result {
+	private func resultFor(_ values: (data: Data?, response: URLResponse?, error: Error?)?,  file: StaticString = #filePath, line: UInt = #line) -> HTTPClient.Result {
 		values.map { URLProtocolStub.stub(data: $0, response: $1, error: $2) }
 
 		let sut = makeSUT(file: file, line: line)
 		let exp = expectation(description: "Wait for completion")
 
 		var receivedResult: HTTPClient.Result!
-		taskHandler(sut.get(from: anyURL()) { result in
+		sut.get(from: anyURL()) { result in
 			receivedResult = result
 			exp.fulfill()
-		})
+		}
 
 		wait(for: [exp], timeout: 1.0)
 		return receivedResult