Preventing URLSession redirect in Swift

Question

I need to fetch a redirecting URL but prevent redirection in Swift. From other posts and Apple docs I understand I must implement the delegate method URLS

Answer 1

I also found the solution helpful, but I am using Swift 4.2 in my current project.

So, here is an adapted shorter version of the solution above, that also works with Swift 4.2 and Xcode 10.

import Foundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class MySession: NSObject, URLSessionTaskDelegate {

    func urlSession(_ session: URLSession, task: URLSessionTask, willPerformHTTPRedirection response: HTTPURLResponse, newRequest request: URLRequest, completionHandler: @escaping (URLRequest?) -> Void) {
        completionHandler(nil)
    }
}

func getDataFromServerWithSuccess(myURL: String, noRedirect: Bool) {
    let myDelegate: MySession? = noRedirect ? MySession() : nil

    let session = URLSession(configuration: URLSessionConfiguration.default, delegate: myDelegate, delegateQueue: nil)
    let loadDataTask = session.dataTask(with: URL(string:myURL)!) { (data, response, error) in

        // OMITTING ERROR CHECKING FOR BREVITY
        if let data = data {
            if let dataString = String(bytes: data, encoding: .utf8) {
                print(dataString)
                if dataString.contains("Bitly") == true {
                    print("success: redirection was prevented")
                } else {
                    print("failure: redirection went through")
                }
            }
        }
    }
    loadDataTask.resume()
}

getDataFromServerWithSuccess(myURL: "http://bitly.com/filmenczer", noRedirect: true)

Answer 2

You have two things standing in your way with your current implementation.

1. You never set the delegate property on the NSURLSession instance that you're using to make the request. Without the delegate property set, your delegate methods won't ever be called. Instead of getting NSURLSession.sharedSession(), look at the NSURLSession(configuration:delegate:delegateQueue:) initializer. The first and last parameters can be NSURLSessionConfiguration.defaultSessionConfiguration() and nil, respectively, see below for more about the delegate.

   Note that when you use the variant of session.dataTaskWithURL that has a completion handler, delegate methods that handle response and data delivery will be ignored, but authentication and redirection handlers are still used.

2. You'll have to refactor somewhat to use MySession as a delegate, since you're using class methods to make the request. You need an instance to use as the session's delegate.

I took a short and incomplete route to having the delegate pick up on the redirect with this alternate code—you'll need to refactor as in #3 to make sure you can still call your callback:

class func getDataFromServerWithSuccess(myURL: String, success: (response: String!) -> Void) {
    let delegate = MySession()
    var session = NSURLSession(configuration: NSURLSessionConfiguration.defaultSessionConfiguration(), delegate: delegate, delegateQueue: nil)
    let task = session.dataTaskWithURL(NSURL(string: myURL)!) {
        // ...
    } 
    task.resume()
}

Hope that helps!

Answer 3

Minor adjustments to the solutions proposed above. This works with Swift 2 in XCode 7.

import UIKit
import Foundation
import XCPlayground

XCPSetExecutionShouldContinueIndefinitely(true)

class MySession: NSObject, NSURLSessionDelegate {

    // to prevent redirection
    func URLSession(session: NSURLSession, task: NSURLSessionTask, willPerformHTTPRedirection response: NSHTTPURLResponse, newRequest request: NSURLRequest, completionHandler: (NSURLRequest!) -> Void) {
    completionHandler(nil)
    }

    // fetch data from URL with NSURLSession
    class func getDataFromServerWithSuccess(myURL: String, noRedirect: Bool, success: (response: String!) -> Void) {
    var myDelegate: MySession? = nil
    if noRedirect {
        myDelegate = MySession()
    }
    let session = NSURLSession(configuration: NSURLSessionConfiguration.defaultSessionConfiguration(), delegate: myDelegate, delegateQueue: nil)
    let loadDataTask = session.dataTaskWithURL(NSURL(string: myURL)!) { (data: NSData?, response: NSURLResponse?, error: NSError?) -> Void in
        // OMITTING ERROR CHECKING FOR BREVITY
        success(response: NSString(data: data!, encoding: NSASCIIStringEncoding) as! String)
    }
    loadDataTask.resume()
    }

    // extract data from redirect
    class func getRedirectionInfo(url: String) {
    getDataFromServerWithSuccess(url, noRedirect: true) {(data) -> Void in
        if let html = data {
            if html.rangeOfString("<html>\n<head><title>Bitly</title>", options: .RegularExpressionSearch) != nil {
                print("success: redirection was prevented")
            } else {
                print("failure: redirection went through")
            }
        }
    }
    }
}

MySession.getRedirectionInfo("http://bit.ly/filmenczer")