Testable NSURLSession using the 'Subclass and Override Method' technique.

NSURLSessionTestable.swift

class NSURLSessionTestable: NSURLSession {

    var response: NSURLResponse?
    var data: NSData?
    
    override func dataTaskWithRequest(request: NSURLRequest, completionHandler: (NSData?, NSURLResponse?, NSError?) -> Void) -> NSURLSessionDataTask {
        
        completionHandler(nil, self.response, nil)
    
        return NSURLSessionDataTaskNoOp()
    }
    

}

class NSURLSessionDataTaskNoOp: NSURLSessionDataTask {
    
    override func resume() { }
    
}

/// EXAMPLE OF USAGE IN A TEST

class URLSessionHTTPClientTests: XCTestCase {

    let session = NSURLSessionTestable()
    var client: URLSessionHTTPClient!
    
    override func setUp() {
        super.setUp()
        client = URLSessionHTTPClient(session: session)
    }
    
    func testSessionNullURLResponseClientReturnsOfflineError() {
     
        session.response = nil
        session.data = nil
        
        client.GET("http://www.apple.com") { (URL, error) in
            completedError = error
        }
        XCTAssertEqual(completedError, .Offline)
        
    }
    
}