# Debugging ASWebAuthenticationSession
In this blog post, you'll learn how to debug [ASWebAuthenticationSession](https://developer.apple.com/documentation/authenticationservices/aswebauthenticationsession) and I'll share a trick on how to buy enough time to attach the Safari Web Inspector before logic executes.
You use `ASWebAuthenticationSession` to [authenticate a user through a web service](https://developer.apple.com/documentation/authenticationservices/authenticating_a_user_through_a_web_service). When the user navigates to the site’s authentication URL, the site presents the user with a form to collect credentials. After validating the credentials, the site redirects the user’s browser, typically using a custom scheme, to a URL that indicates the outcome of the authentication attempt.
```swift
// Use the URL and callback scheme specified by the authorization provider.
guard let authURL = URL(string: "https://example.com/auth") else { return }
let scheme = "exampleauth"
// Initialize the session.
let session = ASWebAuthenticationSession(url: authURL, callbackURLScheme: scheme)
{ callbackURL, error in
// Handle the callback.
}
```
`ASWebAuthenticationSession` runs in a separate process and the content executed is HTML & JavaScript. Hence you can use the Safari Web Inspector to debug the HTML and JavaScript sources. The session is presented in a modal view controller that is similar in appearance to `SFSafariViewController,` but it is not Safari itself.
As a prerequisite, you have to [enable Safari Web Inspector](https://developer.apple.com/library/archive/documentation/NetworkingInternetWeb/Conceptual/Web_Inspector_Tutorial/EnableWebInspector/EnableWebInspector.html#//apple_ref/doc/uid/TP40017576-CH3-DontLinkElementID_29) on your Mac
![](https://cdn.hashnode.com/res/hashnode/image/upload/v1678902535981/77bbd799-69a1-4de5-926c-4cd821c0b41b.png align="center")
as well as enabling it on your iOS device.
![](https://cdn.hashnode.com/res/hashnode/image/upload/v1678902670247/96fb86c1-108e-4a83-ac4d-12de429471d6.png align="center")
Once `ASWebAuthenticationSession` presents the web content you can attach to the [Safari Web Inspector](https://developer.apple.com/library/archive/documentation/NetworkingInternetWeb/Conceptual/Web_Inspector_Tutorial/DebuggingyourWebpage/DebuggingyourWebpage.html#//apple_ref/doc/uid/TP40017576-CH5-DontLinkElementID_24).
![](https://cdn.hashnode.com/res/hashnode/image/upload/v1679269861321/223304ba-e6e9-4a9c-97be-06796afd37a2.png align="center")
I often run into the tricky situation that the web content executes critical code before I can manually attach the Safari Web Inspector.
I came up with the following trick:
1. Replace the URL to be launched in `ASWebAuthenticationSession` with a different URL. The original URL gets encoded and passed as a URL query parameter.
2. Once the website is loaded I have all the time in the world to attach the Safari Web Inspector.
3. I manually trigger the navigation to the original URL through a button in the presented website of the `ASWebAuthenticationSession`.
All details are outlined below but frankly, there is an easier solution. Kudos to Markus Bösch for pointing out that you can a Web Proxy (e.g. Proxyman, Charles, or Fiddler) and set a breakpoint to pause, attach the Safari Web Inspector, and resume.
%[https://twitter.com/mboesch_ios/status/1638147053871067136]
The approach from Markus is extremely nice as it does not require to re-compile the app!
Back to my original approach: I have deployed the following HTML/JS code as a [GitHub page](https://marcoeidinger.github.io/DelayedRelay/index.html) of a [GitHub repository](https://github.com/MarcoEidinger/DelayedRelay) of mine.
```xml
Redirect
```
Replacing the URL is fairly easy if you use `ASWebAuthenticationSession` directly. But what if you use it indirectly, i.e. use a library that determines the original URL and passes it to `ASWebAuthenticationSession`?
You can use Method Swizzling to replace the initializer of `ASWebAuthenticationSession`. This works fine for a debug build but for some reason, this does not work for a release build.
```swift
import AuthenticationServices
extension ASWebAuthenticationSession {
static func useDelayRelay() {
ASWebAuthenticationSession.swizzleTheInit()
}
}
extension ASWebAuthenticationSession {
@objc static func swizzleTheInit() {
let oldSelector = #selector(ASWebAuthenticationSession.init(url:callbackURLScheme:completionHandler:))
let newSelector = #selector(ASWebAuthenticationSession.swizzledInit(url:callbackURLScheme:completionHandler:))
let oldMethod = class_getInstanceMethod(self, oldSelector)!
let newMethod = class_getInstanceMethod(self, newSelector)!
let oldImplementation = method_getImplementation(oldMethod)
let newImplementation = method_getImplementation(newMethod)
_ = method_setImplementation(oldMethod, newImplementation)
_ = method_setImplementation(newMethod, oldImplementation)
}
@objc private func swizzledInit(url: URL, callbackURLScheme: String?, completionHandler: @escaping ASWebAuthenticationSession.CompletionHandler) {
let encoded = url.absoluteURL.absoluteString.addingPercentEncoding(withAllowedCharacters: .urlHostAllowed)
let queryItems: [URLQueryItem] = [.init(name: "org", value: encoded)]
var urlComps = URLComponents(string: "https://marcoeidinger.github.io/DelayedRelay/index.html")!
urlComps.queryItems = queryItems
let new = urlComps.url!
// calling the original implementation
// note: does not work in release build configuration
self.swizzledInit(url: new, callbackURLScheme: callbackURLScheme, completionHandler: completionHandler)
}
}
```
In this video I am illustrating all steps:
%[https://youtu.be/1TBDxKBGxVk]