Hey there, curious minds! Welcome to the inaugural post of an enlightening series where we’ll venture into the development of a bulletproof networking layer for iOS apps. As I unravel this complex subject, I’m learning right alongside you—I’ve never built robust and tested from scratch before, so consider this a shared journey of discovery and improvement.
The plan is to break down this endeavor into a series of manageable posts, with the possibility of adding or adjusting topics as we go along. Each post will focus on a different facet of building a rock-solid network layer. So, if you’re passionate about iOS development and always on the lookout for ways to make your code more resilient, you’ll find value here.
Today’s agenda brings us to the URLProtocol. We’ll delve into why URLProtocol should be your testing ally, making your network interactions not just functional but absolutely foolproof.
Stay tuned for upcoming posts. Trust me, this is a journey you won’t want to miss.
Why URLProtocol?
The efficacy of a network layer hinges not just on its operational robustness but also on its testability. In iOS development, URLProtocol serves as an often underutilized, yet incredibly effective tool for this purpose. Here’s why I consider it a go-to solution for testing networking code:
1. API-agnostic Testing
While some testing strategies bind closely to URLSession APIs, requiring complex mock setups and frequent updates, URLProtocol offers a more resilient approach. It allows defining mock behaviors at a lower level, without getting entangled with the intricacies of URLSession methods. This not only simplifies the testing process but also makes tests future-proof against API changes.
Before we dive deeper into a strategy using the URLProtocol I want to illustrate the tight coupling that can occur when directly using URLSession for testing. A naive approach might involve subclassing URLSession or directly using it in test cases, which can create cumbersome and rigid tests. Here’s an example using a mock URLSession subclass:
class URLSessionMock: URLSession {
var data: Data?
var response: URLResponse?
var error: Error?
override func dataTask(
with request: URLRequest,
completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void
) -> URLSessionDataTask {
return URLSessionDataTaskMock { [weak self] in
completionHandler(self?.data, self?.response, self?.error)
}
}
}
class URLSessionDataTaskMock: URLSessionDataTask {
private let closure: () -> Void
init(closure: @escaping () -> Void) {
self.closure = closure
}
override func resume() {
closure()
}
}To use this mock in tests, you would then do something like:
let mockSession = URLSessionMock()
mockSession.data = someData
mockSession.response = someResponse
mockSession.error = someError
let client = MyNetworkClient(session: mockSession)
// test various casesThe problem here is twofold:
- Creating and Maintaining Mock Classes: When you’re closely tied to URLSession APIs, your mock classes need to reflect URLSession’s structure and behavior. This could lead to unwieldy code, making it difficult to pinpoint issues solely related to your app’s logic. The mock classes essentially become a second version of URLSession, doubling the potential for errors and making the test suite less maintainable.
- Updating Mock Classes for API Changes: In our development journey, we sometimes encounter API changes that don’t just affect our main code but also shake up our testing infrastructure. Take, for instance, the warning:
"Please use -[NSURLSession dataTaskWithRequest:] or other NSURLSession methods to create instances"that we received when directly initiating a subclass of URLSessionDataTask. This warning was more than just a heads-up; it was a signal that our closely tied mock classes needed an update. Such a situation exemplifies the risk of high maintenance overhead. When your test suite is closely tied to a specific API, especially an Apple-provided one like URLSession, any update to that API can essentially become a breaking change for your tests. This makes your test suite fragile and increases the time you spend on maintenance.
Both of these implications could result in a slower development cycle, less robust testing, and ultimately, a decrease in the quality of the application itself. With URLProtocol, these issues become less of a concern, making it a more resilient choice for testing network interactions.
2. Reduced Code Noise
URLProtocol allows for a streamlined testing approach by focusing on network behavior, eliminating the need for intricate, API-specific test setups. This enhances test readability and maintainability.
With API-specific testing methods, you might end up with lengthy and hard-to-read tests. For example, using completion handlers and expectations can clutter your test functions:
func test_apiCallCompletionDataHandler() {
let expectation = expectation(description: "API request completed")
var receivedResponseData: Data?
let mockSession = URLSessionMock()
mockSession.data = someTestData // Set your mock data here
mockSession.dataTask(with: url) { data, _, _ in
receivedResponseData = data
expectation.fulfill()
}.resume()
waitForExpectations(timeout: 1, handler: nil)
XCTAssertEqual(receivedResponseData, someTestData)
}While this method might seem functional, the multiple properties (data, response, error) in the mock class itself become other vectors that need individual testing. It not only complicates the tests but also increases the potential for errors or omissions. In contrast, URLProtocol’s streamlined approach allows us to focus on what truly matters: the behavior we’re trying to test.
3. Operational Reliability
In software development, placing your trust in a well-established API is akin to letting Dad handle the BBQ grill—there’s a certain level of expertise and reliability you can count on. URLProtocol isn’t just any API; it’s Apple-endorsed. This recommendation to use it for testing carries weight, especially in a field where libraries come and go like fashion trends.
Using an Apple-recommended API for network testing provides several key advantages:
- Compatibility: It’s reasonable to assume that URLProtocol will continue to be compatible with future iOS updates.
- Flexibility: URLProtocol is designed to be subclassed and customized, making it easier to control network behavior in a test environment. URLSession, on the other hand, is more rigid and has a steeper learning curve for mocking.
- Robustness: Because URLProtocol is designed for testability, changes in its API are less likely to break your tests. Conversely, as you’ve seen, a simple change in URLSession can lead to breaking changes in your mock classes.
- Community Support: With a broad developer base, you’ll find ample tutorials, community solutions, and Stack Overflow threads.
- Deprecation Risks: Less worry about the API being deprecated overnight. Remember, Dad’s grilling techniques are timeless, just like URLProtocol.
4. Exhaustive Test Coverage
URLProtocol enables us to stub different types of responses—successful, erroneous, or even missing—thus facilitating robust unit tests that cover a gamut of real-world scenarios.
// Simulate Success
URLProtocolStub.stub(url: testURL, data: someData, response: someResponse, error: nil)
// Perform Test
// Simulate Server Error
let serverErrorResponse = HTTPURLResponse(url: testURL, statusCode: 500, httpVersion: nil, headerFields: nil)
URLProtocolStub.stub(url: testURL, data: nil, response: serverErrorResponse, error: nil)
// Perform Test
// Simulate No Internet
URLProtocolStub.stub(url: testURL, data: nil, response: nil, error: someError)
// Perform TestThis way, the test suite becomes a true reflection of the range of scenarios the app could encounter, making it bulletproof for the real world.
5. Decoupling Tests from Production
URLProtocol approach provides a robust framework for testing without tangling up production code with test implementations. Here’s why that’s a breath of fresh air:
- Resilience to Change: Tests remain unaffected when there are modifications in the app’s actual network layer, ensuring the test suite remains robust over time. However, if there is a need to switch to a completely different networking stack that doesn’t use URLSession, then URLProtocol-based tests will need to be adapted or replaced.
- No Mocking of URLSession: By not requiring a mock URLSession client, URLProtocol eliminates an additional layer of complexity, making the test setup leaner and less prone to errors.
Like a well-seasoned chef, URLProtocol knows that sometimes less is more, reducing the opportunity cost of crafting your test suite.
6. Custom Protocols and Beyond
When it comes to networking, HTTP and HTTPS are the reigning champions. But what if your app walks the less-traveled road? Maybe it’s an IoT app that needs to communicate over MQTT, or perhaps you’re implementing a peer-to-peer feature using a custom protocol. Here’s where URLProtocol shines—it doesn’t judge; it just adapts.
Suppose you need to implement a custom FTP protocol. With URLProtocol, you can register your own custom protocol class:
class CustomFTPProtocol: URLProtocol {
override class func canInit(with request: URLRequest) -> Bool {
return request.url?.scheme == "ftp"
}
// Implementation for handling FTP logic
}This allows your test suite to be as versatile as your app’s networking needs, ensuring you’re not just limited to the HTTP/HTTPS protocols. Like a Swiss Army knife, URLProtocol ensures you’re prepared for whatever networking challenges lie ahead.
Conclusion
In this first entry of my deep-dive-learning series into crafting a bulletproof networking layer, I’ve unpacked the multitude of reasons why URLProtocol is my go-to for testing. From its ability to offer API-agnostic testing to its operational reliability, URLProtocol isn’t just another tool in the shed—it’s a strategic ally in building a resilient and flexible network layer.
Look forward to my upcoming posts where I’ll be sharing practical examples, diving into the details of URLProtocol, URLSession, and unit testing, and discussing the art of crafting a reusable networking layer.
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